Two videos of a Fullerscopes MkIV mounting with added Goto

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Just discovered these videos on YouTube:

The author of these videos has some fascinating content if you delve deeper on YouTube.
Including his restoration of a Merz triplet 90mm f/11 Apo photographic refractor
Easily one of the best amateur astronomy videos I have ever seen! Highly recommended viewing.
An English voice-over is an option with details about Fraunhofer and visit to the Anaformose shop.
Beautifully illustrated and great video content.

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The Scale of Ambition?

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Many people like to set new year resolutions for themselves. A challenge, usually involving a difficult change of behaviour or breaking an unhealthy or unwanted habit. It occurred to me that I might have some unique insights into making resolutions regarding amateur astronomy.

The best telescope is the one which gets used. Many amateurs must aspire to a larger instrument. Only to be defeated by the mental and physical hurdles of actually setting it up outside.

For over 50 years I have been building white elephants in the name of ATM [amateur telescope making.] Often my ambitions exceeded all common sense where my absence of clairvoyance really mattered. 

I once made a 16" mirror for a large, classical Dobsonian shortly after discovering this class of instrument in American publications. I even went so far as to buy a 20" diameter PVC tube for it. Which weighed at least as much as the full thickness mirror blank! The problem then proved to be a lack of storage height for an F5 colossus of such an aperture!  Having perfected the plate glass mirror at F5 I was then forced to regrind and deepen it to F4. A far more difficult and demanding surface. 

This also coincided with the arrival of summer and extreme heat in in my concrete roofed, optical workshop. My pitch laps were literally flowing off the tools overnight. I eventually cured the heat problem with a layer of recycled aluminium sheeting. Fixed on top of the cast concrete roof it was highly effective but far too late by then. I never finished that plate glass mirror surface at F4 to my satisfaction.

I found a 24" cast iron lathe faceplate in a scrap yard and dragged it home somehow. This became a disk type, equatorial mounting. A solid steel TV stand acted as the PA axis with 4" steel rollers supporting the rim. Despite ball bearings all round, the whole thing weighed a ton and the friction and momentum was far too high for manual slewing.

I made a 5" F/15 achromatic lens from Schott BK7 and F2. Being so long is was quite a struggle to mount and far too heavy in a 6" PVC drainpipe. So I built a scaffolding pipe tripod and offset and cantilevered, Dobsonian/Berry fork mounting for it. It was never very successful but showed me the planets, Moon and a bright comet. Later I made a rolled aircraft ply tube for it and that transformed its weight. But I was experimenting with plywood, equatorial mountings with Dobsonian PTFE/Teflon bearings at the time. Probably trying to be too clever [again!]

The friction was far too high and the steel pier impossibly heavy to lift out of the ground once filled with concrete.  So it stayed put and rocked no matter how I rammed huge stones around the base.

Later on I tried to make an all brass refractor using huge, artillery, shell casings and my home made 5" achromat. These casings quickly proved to be so heavy I could never have lifted the complete OTA. I finally found a 6" steel, spiral ventilation tube for the lens. This was arguably the best tube option at the time but the MkIV mounting still struggled with backlash. Nor was the spiral tube very pretty. I later discovered straight seamed, industrial extractor tubing and have used that on my 7" and 10" telescopes. Provided the seam goes into the gaps for the hinges the plywood packed rings will fit snugly.

I made a 7" f/12 refractor with an iStar achromat lens. This proved to be so long and heavy that my Fullerscopes MkIV mounting could not possibly cope. I had welded up a massive steel pier for the MkIV years earlier but it did not help. Moving the whole thing around on a sloping, undulating and often soggy lawn was an exhausting nightmare! The risk of tipping was very real regardless of which wheels I applied to the radiating legs!

I tried a folded 7" refractor based on an aluminium tubing, shelf building system. That was hardly lighter than the straight tube and only 3/4 as long. Still the MkIV complained.

So then I started on a huge equatorial mounting which would support the refractor and quite possibly my 10" F/8 reflector simultaneously. The latter had proved another white elephant thanks to beam torsional flexure!

This quickly lead to the realization that I had no clear sky views. Just moving the telescope down the rutted, gravel drive to clear southerly skies was nearly impossible. So I decided to build a platform for the mounting to go on top of a raised pier. The pyramidal pier is massive and built from full lengths of 4"x4" x 15' tall!

And so it goes on. A 6" f/12 would have fitted into 2.7m commercial dome. My 7" f/12 is a foot longer than a 6" and has no chance of fitting inside. So I am having to build a 10' plywood dome, from scratch, to go up on the platform. I even looked at a 14' GRP dome intended for rearing calves. But each of the three fiberglass segments weighed 160lbs! I would still go that way if I had clear skies at ground level... but I haven't.

If you find a chain hoist essential to your amateur astronomical pursuits you may well have crossed an important line. My ambition always far exceeds common sense by a considerable margin. Let my example be a warning to those who would imitate me! Though I should admit it has been a fascinating, if occasionally frustrating  journey. Almost always done on a shoestring from scrap materials. Success was always going to be dependent on my ability to convert the rather limited raw materials available.

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Mounting update: Whoops [again!!]

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Sunday: I was just lifting the mounting off a reinforced, B&D workbench with the chain hoist. When the steel strap I hook over the ceiling joists snapped without warning. A loud crash and the mounting toppled sideways off the bench to lodge horizontally on a stack of timber off-cuts. So now I had to reach underneath the precariously balanced mounting to extract the hoist hook from the lifting strop. Which I had wrapped around the PA assembly for the lift.

Having arranged another strop above the mounting I could gently lift it clear with the hoist. That's the second time the mounting has exceeded the limits of supposedly strong, supporting devices! I have now added a thick plate of aluminium to the top of an old plastic beer crate to support the mounting rather nearer the floor.

The mounting was becoming covered in condensation, from my breath, as I removed the Dec wormwheel in the unheated workshop. I needed to turn the brass, spacing collar down a little to center the worm on its wheel. On a proper wormwheel the "teeth" are cut as indentations with a radius equal to the worm.

It is obvious that there is only one possible position where the worm fits intimately in the slanting curves cut into the wormwheel rim. A smooth cylinder could be made to rotate in a matching, curved pulley rim but there would be no drive. By adding a helix to both the worm and wheel rim the worm can drive the wheel, albeit slowly. Another advantage of closely matching teeth is that the worm locks the wheel against all free rotation. Any unwanted freedom is called backlash. This can occur when the drive is reversed during guiding. Or if there is any imbalance in the instrument on the mounting.

Here is an excellent series of explicit instructions for making a wormwheel on a mini lathe. This limits wormwheel diameters to only [about] 7" maximum diameter.  The same as the Fullerscopes MkIV. Those with larger lathes can obviously make larger wheels.  One can save a lot of money making your own but it does require an awful lot of time and effort to do it correctly:

Step by step Telescope worm wheels and gears on a mini-lathe

And another website offering advice on DIY wormwheels:

Making Wormgears

The image shows the latest arrangement of the Dec stepper motor drive cables.

I decided the short motor cable was far more fragile than the long, sleeved cable connected to the AWR Microstep Drive box.

The initial cable was also rather long. So I folded the motor cable and zip tied it to the edge of the 10mm thick, motor support plate.

Then I P-clipped the heavier connecting cable to act as a strain relief device. This much sturdier cable goes through the large hole in the mounting's fork face plate and thence off to the AWR electronics. AWR were very generous with these connecting cables to allow for a very tall pier for the refractor and/or some distance to the drive electronics.

It is important that no load is placed on the initial, motor cable as the mounting moves the telescope around the sky. So I have added a slack loop to allow for 180° of Declination axis rotation. Not to mention catering for similar PA movements.

I used to park the folded refractor horizontally with the wights up on the north side of the pier. This was only to reduce wind loading on the temporary pier. I doubt I shall have to resort to such "gymnastics" with the telescope housed in its dome.

I don't have a name [yet] for the black, 9 pin, latching plug and socket. The AWR guide mentions a rectangular 'Quickmate' system, but I haven't found any images online to confirm this. I thought I might be able to shorten the first motor cable myself, to tidy things up, if I can source a spare socket. This will save me purchasing a complete, but shorter motor cable from AWR. Though there could be some soldering or crimping of the wires to the pins and sleeves involved. I haven't discovered if the plugs and sockets can be disassembled. Or even re-wired after use without destroying anything. Only six of the 9 pins available are actively used.

Click on any image for an enlargement.

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Mounting update 2: Drive Slew Testing.

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Sunday: Dry and 42F: Finally a chance to dismantle the big mounting. I also stripped the stepper motor and worm assemblies and tidied up the motor backing plates. I had left them rectangular and raw after rough shaping. Which prevented the assemblies from being released without withdrawing the axes shafts.

Thanks to fixing the front plate I can now fine adjust the polar altitude angle with the turnbuckle for the first time. The large round aperture in the front plate allows my hand to reach the turnbuckle adjuster. No great effort is required provided I have slackened off the altitude pivot nuts first. I deliberately chose a bulky turnbuckle design to make it easier to turn without tools.

The image shows the PA drive. With 1:2.4 ratio, [14:34] timing pulleys, toothed belt with urethane, jockey pulley, 287t x 11" Ø wormwheel driven by the large, brass, single start worm. The large stepper motor lives in the aluminium box behind the small pulley. The steel flanges on the pulleys are proving rather rust prone. As are the mounting's flange bearings.

Monday is cool but bright. I spent a couple of hours replacing all the worm assembly fixings with stainless steel, hex socket head screws. Added stainless steel spring washers beneath all the nuts to avoid them loosening with motor vibration.

The worm assemblies feel very solid and secure. While [hopefully] looking quite 'tidy' thanks to the sturdy 10mm sections of aluminium plate and the square al. tubing, stepper motor housings. My usual 'belt and braces' approach helps to ensure rigidity. I still have to smooth the edges of the newly cut motor plates. A bench, disk sander would have been handy but an expensive investment for a decent one. Not sure how to apply worm mesh, fine adjustment yet.

The single, tiny grub screw on the Beacon Hill, Declination worm housing could not hold the journal bearing in place. So I drilled the housing close to the bearing. Then added two screws with washers both sides and doubled lock nuts for security. The bearing can no longer work its way steadily outwards during slews. Over-tightening the grub screw merely locked the ball bearing solid so that it could not rotate. Beacon Hill was very mean with the worm shaft overhangs.

The Declination drive, stepper motor is visible just below in its box section housing. It was sheer luck that I found the square tubing at the scrap yard just when I needed it. The clearance between the motor and housing is ideal. The height of the square tubing is also perfect for presenting the worm to the wormwheel.

Wednesday, 45F, mild, dark and windy with showers. Spent a couple of hours in the workshop balancing and testing the mounting and AWR drives. Before balancing, the RA motor was apt to complain about having to work 'uphill.' I also fastened off the stepper motor cables with zip ties to provide some strain relief. The Dec motor cable will need to be 'dressed' to allow rotation without tension. This should not be too much of a problem because the telescope(s) would be upside down if allowed completely free reign.

I have photographed and printed off the AWR IH2 menu in several sizes to have reminders of the cascaded steps required. Having not used the drive system for months I had forgotten much of it. When I finally have the observatory set up properly I shall be pointing to objects for Goto slews on the computer screen.



Thursday and Friday: Testing drive slews with the AWR IH2 handset.

With the mounting raised on a bench the sheer scale and mass of it is quite overwhelming. The buzzes occur when starting and stopping the motors. I wasn't sure whether to shim the motors for better conduction or leave them more fully exposed to the air. I ought to have used a more neutral background for the video but had no other, lightweight tarpaulins left to quickly hide the workshop clutter. White always confuses the camera. PS: I have invested in sheet of neutral grey cloth for pocket change at a charity/thrift shop.

Dressing the divided, stepper motor, drive cables is proving slightly awkward. Both cables presently exit the access hole provided for PA altitude adjustment. The RA cable need only be quite short but the Declination cable must not hinder rotation of the PA over a rather large circle.

I have already managed to stall the drive during slews when I allowed the Dec cable to become tight. With only short, primary motor cables, from AWR, this places a rather large plug and socket dangling in mid air not far from the motor.

Unable to obtain them locally, I have ordered a boxed assortment of Nylon P-clips online. This will improve my chances of sorting out the cable problem neatly. There is no need for the main cables to be readily removable because the mounting will be permanently installed on the pier. So the heavier, main sections of cable can be clipped neatly onto the mounting. With plugs easily removable from the drive boxes at the far end when desired.

Click on any image for an enlargement.

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Mounting update: Descent and Drives:

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Sunday: First of three days, in series, with all day rain and wind forecast.

Tuesday 28th Nov: A wet start but a pause allowed me to rig up the chain hoist to bring the heavy mounting down. It was easiest to lower it down through the stairwell with the ladder removed. The mounting had been sitting on the observatory floor gathering rust and lacewings under its multilayered covering.

I need to construct fine adjustment for the drive worms. This will give me something to get on with when the weather isn't ideal for outdoor self-amusement. Snail cam adjustment might work too. A snail seems appropriate in conjunction with a worm.

Wednesday: My attempts to drill and screw the front plate of the mounting's support fork to the tines were thwarted by non-compatibility between my M5 0.8 taps and the same sized screws. The taps are clearly marked but would not fit any of my M5 nuts. The pitch looks correct when the screws and taps are held up to the light together. The screws match the nuts but the taps are too large in diameter! Without fixing screws the 10mm thick front plate slides upwards when I try to increase polar altitude with the turnbuckle. There isn't room for 6mm screws. Grr?

Thursday: Ordered a cheap set of taps and dies online. They should be okay for cutting threads in aluminium. My own hotch-potch collection of taps and dies were picked up over many years at flea markets.

I bought a [supposed] quality 'Dormer' tap on eBay and it snapped while cutting a thread in an aluminium casting. First time ever in decades of threading aluminium, brass and steel. I could have bought a set of three, no-name taps online but decided against it. No doubt they are all made in China these days . The vendors are just trying their luck in pricing to see what the market will bear.

The new M5 0.8 tap proved to be very hard work. Not so much cutting as rolling the thread! I followed up with my original tap and it cut cleanly to full depth. I now have four screws holding the front plate between the massive PA support, fork blades. I counter-bored the holes for sinking the socket head screws flush with the surface.

Click on any image for an enlargement.

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Dome Build: Vertical strut considerations.

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Thursday: Another wet and windy day forecast. If it stops raining long enough I can start adding vertical struts to the sides of the ribs. No glue. Just screws for the moment. The tarpaulins have to come off to add more structure. The table saw will be used to cut the struts on the diagonal.

The dihedral geometry of the outside angles should take care of itself. Provided [of course] I get the strut material dimensions correct and lay the hypotenuse side of the triangular cross section on the ribs. The struts need to "lean away" from the ribs. To allow the covering panels [trapezium facets] to lie flat and well supported on all four edges.

I still haven't decided on the best method of cutting the plywood ribs away to match the 'tiers' of covering panels. It may be best to mark the ribs using the vertical struts as straight edges. Then remove the vertical struts again while I cut the paired ribs with a jig saw. Only adding the vertical struts after the ribs are modified.

The problem is the horizontal ribs will get in the way of the body of the jigsaw. So that needs serious thought. Particularly considering the sheer number of straight cuts to be made. [4x32 = 128!] I don't want to have to remove all the horizontal ribs as well. That would add considerably to the work to be done when all the horizontal struts must be multiplied by 16. Or  [rather] 15 with the strut-less observation slit included. 

The quickest way may be to separate the gores/segments after marking. That would allow the jigsaw to work from the sides of the ribs unhindered by horizontal struts. Though the straight lines would need to be carried over to the bare sides for the saw blade to run along. A small hole drilled at either end of the original lines would do. This would provide an accurate guide for re-drawing the lines on the bare [joining] sides of the ribs once the segments are separated. I can run a coarse sanding disk along the sawn edges of the ribs to tidy up once all the vertical struts are all re-fixed in place and the gores re-united.

I know I tend to talk each step of the construction [almost] to death. But the sheer number of elements [and cuts] involved must be taken into account. The potential for massive time-wasting is enormous. Getting something wrong 128 times and having to start all over again doesn't even bare thinking about! Most of the endless text is for my own benefit and to remind myself why I chose to do something that way.

Better [surely] to waffle on [and on] and do it right? Rather than barging on and making endless [creative] mistakes. Improvising on my projects could have very serious financial consequences if I do screw up. Wood and birch plywood, in particular, doesn't grow on trees!

It soon became obvious that the tarpaulins weren't doing much good as they flapped wildly in the wind. I had criss-crossed the dome with cords but it wasn't really helping. Then it started spitting with rain so I did a quick removal of the tarpaulins and moved the segments back into the octagon. Where they were covered in several smaller tarpaulins. I'll have to enquire whether dome building and dismantling can be added to the Winter Olympics.

Now I have added a hefty but inexpensive, cast iron, 30cm/ 12" disk sander to my toolbox. This will be handy for quickly trimming the compound miters on the ends of the vertical struts when I'm building the dome. The struts should all be the same length. Making me wonder whether I should deliberately make them so. Then adjust the horizontal struts up and down to achieve a perfectly uniform structure. The more uniform the skeleton the more uniform the plywood cladding panels can be. Mass production with accurate machines has enormous benefits for time saving. Making lots of  components individually can introduce endless errors and prolong the build for months.

Click on any image for an enlargement.

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Dome build: Standing up for itself.

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Tuesday: The stands arrived in the post. They work okay but lack lateral rigidity. So I'm going to add several, simple, timber stands as well. Hopefully to reduce the risk of the whole lot moving sideways [and collapsing] in a high wind. These will also provide more even support without having to buy more adjustable stands.

Eight stands meant they were separated by a meter around the perimeter of the base ring. Adding three timber props will narrow the spacing. As can be seen by the first assembly up on the stands. [Left.]

I am also experimenting with a central pole and support disk to ensure the dome is the correct height. The pole must not protrude or it will pierce the tarpaulins. The pole's disk height adjustment is surprisingly sensitive due to the natural stiffness of the dome. I have now arranged the slit on the far side [East] where we usually get the least wind. It is also much handier for the workshop door. Which should save quite a lot of walking. 

The stands do indeed provide a comfortable working height inside. I can get a much better sense of scale now. The downside is the ease with which cats, foxes, pheasants and blackbirds can hide under there. Though probably not all at the same time!

The timber props cured the previous wobbles when using the stands alone. I may make another timber prop to allow them to stand at 90° to each other around the ring. Adding 1"x2" to each side of the 2"x6" uprights would stiffen the structures considerably. Now guess what? We shall have 40mph winds on Thursday with lots of rain for the next couple of days. I had better make that fourth timber prop and rig up some guy lines! Or, I may take the dome down and pack it away in the octagon for a couple of days. It's only a matter of removing the 48+ spring clamps. Assembly and dismantling of the gores/segments takes less than a 1/4 of an hour each time now thanks to my considerable practice. 

Click on any image for an enlargement.

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Dome build: A New start.

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More online research showed that the affordability of telescopic lift hire made building the dome "upstairs" an increasingly foolish and probably hazardous project. Particularly as we hurtle towards a Danish winter. Not only is the woodwork almost constantly wet and slippery, up there, but covering the dome would be ridiculously difficult. Not to mention the wind catching whatever covering I used to defend the construction from the frequently inclement weather.

Now that the decision to work on the ground has finally been made, I might as well treat the job seriously. So I have ordered eight [fairly] inexpensive trailer/caravan stabilizers. These come in sets of four so I shall have eight to play with. Each stand can rest on a 8" square of 2" thick timber as a load spreader on the increasingly muddy lawn.

The fine height adjustment of each "foot" will effortlessly undo the previously irritating slopes and hollows on the dome construction site. Moreover the inside of the dome will become readily accessible, through the observation slit, without the need for endless crouching and a safety helmet.

These pyramid stands have an aluminium base of about 160x160x220mm [6.3" x 6.3" x 8.75"] and height adjustable from about 270-430mm [10" -17"] in Olde Money. Claimed load capacity varies between 750kg and 1ton each, depending on the supplier. So eight will be more than adequate for supporting a 10' plywood dome evenly at a comfortable working height.

I can add scraps of ply to the top plates to spread the load into the base ring more evenly. These stands will also lift the base ring above the inevitable winter mud. Perhaps a car trailer of gravel over the entire working area would be sensible? It has certainly made the octagon area a pleasant and practical walking surface. The stand supplier has promised a Monday delivery. Though more likely a Monday dispatch.

A clear and bright afternoon gave me a chance to retrieve the now much heavier, 32mm / 1.3" thick, double base ring. I used the boat winch to lower the ring back down to the ground after it had been exposed to heavy rain. It was largely my inability to properly and safely protect the ring from the foul weather which had tipped me into working on the ground.   

Sunday: Found a local stockist of the stands. They really are quite lightweight but rated more than adequate to the task. I was tempted to bring four more home but decided to see if eight were enough for total stability first.  The package has been dispatched [on a Sunday] so I'm now at the mercy of the post office.

Monday: I was bored waiting for the post. So I rebuilt the dome skeleton with the ring resting on wooden blocks. Then covered it with lightweight tarpaulins just as the light went. It will all have to come apart again to go on the stands. If/when they arrive.

Click on any image for an enlargement.

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Dome build: Base ring progress at last.

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The image shows the base ring leaning against the 5m, 16' ladder. I was trying to capture what little sunshine there was @ 48F to help the glue along. A near frost overnight is forecast with rain all day tomorrow. What to do with the ring? Lay it back down, with a tarp over it, I suppose. Luckily I can still stagger around while carrying the ring and cross braces. Though that may no longer be true when the second layer of 15mm is added.

It should obvious, even to me, that consuming a heavy sheet of plywood, by cutting it into arcs, does not make it any lighter. I am just happy I didn't choose to cut the arcs from 18mm [3/4"] ply!

I doubt I'd have the physical strength to move the completed dome carefully on the wheels even if I survived beyond the tipping point. There would be another movement of several feet before the sliding ladder fully bridged the octagon. Best to forget such heavy lifting!

I have bolted the boat winch to a 2x4 to have something solid to fix to the pier or the octagon. I simply don't have the strength to lift the ring bodily by a rope even with the help of the sloping ladders.

Thursday: Wet. Very wet.

Friday: A much more promising day but much cooler. It has now occurred to me that adding another layer of 15mm ply to the existing base ring will almost double its weight. It might be best to lift the original ring onto the supporting wheels on the octagon. Then add a similar completed ring on top. That leaves the problem of applying the hardboard ring underneath. It looks as if I shall have to build the second ring first, complete with the hardboard layer. Then take the original ring up last to plonk it on top.

Or, complete the second ring and attach the first on top of that. Then lift the whole lot using the winch as planned. This would make for a much stiffer but heavier ring to lift into place. I can lay 2x4s across the octagon to receive the ring without risking the wheels catching on the ring.

These timbers must be able to be removed easily with the weight of the full ring resting on top of them. Which suggests blocks between the wheels rather than full lengths of 2x4 right across the octagon. The height of the wheels is almost 8" so that needs 2 lengths of 2x4 set on edge and joined with perforated roofing plates. That is a potentially unstable set up so I may use blocks of 2x8s joined side by side as 4x8s. Only the gap between the pier and far side of the octagon needs continuous support for sliding the ring into place.

The tarpaulin is full of water where it rested on the ring overnight. I placed a couple of ladders across to keep the tarpaulin off the ring. That was a waste of time. It still looks like an ice-covered, round, paddling pool after yesterday's rain followed by frost!

The white tarpaulin drying in rare sunshine. I tried hanging it from the top like a curtain but it was mostly in shade.

I don't hold out much hope for the glue reaching full strength. Which leaves me wondering what to do about doubling the ring thickness. I can use screws but neither glue nor construction adhesive is likely to be useful at near freezing temperatures.

I had a trial lift using the winch to raise the original base ring, with its cross braces, up the ladder. The inclined lift went well enough, but It was quite a struggle getting the ring over the protruding wheels. 8" high blocks and well spaced, full octagon width, timber runners would definitely be required for anything heavier. I may leave the ring up there for the moment. No point in it lying on the wet lawn gathering ice and water.

I finished marking and cutting the slanting joints of the second ring and added it to the first up on top of the wheels. Lots of screws will have to do until it is warm enough for wood glue. Which could be another six months under normal weather conditions. Ideally, I need to invert the ring for a smoother track over the diagonal butt joints. Or insert the hardboard arcs underneath the doubled ring to smooth out the lap joints. Though this is not the best option.

The underside of the ring is accessible in the centers of the octagon sides should I wish to drive screws upwards to hold the hardboard. I moved one 'steering' wheel outwards slightly and that was enough to free the ring from tight spots. It rolls very easily now.

Saturday: A wet and windy morning makes any progress unlikely today. The sky to the north is black! I have no idea how to tarpaulin the ring if we are to have gales. Large and heavy puddles in a covering tarp on top of the ring would be very unhelpful. Any form of central support off the pier would have to be very well padded to avoid piercing the 'tent.' Lightweight tarpaulins are very fragile and barely waterproof from new.

Should I invest in a heavy tarp to place directly over the dome skeleton? The weight could be a serious problem when trying to work underneath it. I would like to continue building the dome using screws rather than glue. Then, when the temperature allows it, I can remove the screws to separate the components and squeeze some glue into the joints.

Perhaps I should build the complete dome skeleton down on the ground. Then cut and mark the trapezium panels to fit. The dome could then be properly rebuilt, with glue, as a proven 'kit of parts' up on the octagon in the warmth of spring.

A big, red, big bale loader keeps passing me on the local roads as if to emphasize the wisdom of a single lift of the completed dome. I presume it belongs to a contractor who fills vast farm buildings with big hay bales right up to the roof. Or it could be a local farmer who own one and hires himself out. They are increasingly popular for lifting roofing and building materials on construction sites.

I did some online research and these telescopic loaders can manage huge lifts [several 1000s of kg] to great heights. They certainly wouldn't break a sweat lifting a modest plywood dome just a bare few meters. The machines can be hired by the day for a manageable sum. This may the way forwards. Because I can't really see the point of taking risks, messing about up there in the wind, snow and rain if it all has to be taken apart again just to glue it back together.

Click on any image for an enlargement.

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Dome build: Lifting the base ring safely onto the wheels.

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I have been pondering different ways to ensure the ring will be round rather than oval. If I use cross braces the completed ring must be able to be lifted onto the rollers without obstruction. The double thickness ring will not be light enough to handle easily unaided due to the sheer size making it very awkward. Perhaps I can build a lifting jib using the 5m [16'] ladder and the two builders stepladders? The problem is avoiding the ladders causing an obstruction in themselves.

Perhaps a simple inclined plane using the long ladder and the boat winch [or just a rope] is the answer? The inclined plane itself [the ladder] could slide up over the octagonal top ring under the control of the winch. Bringing the supported ring with it.  The ladder would easily slide between the rollers and could then be removed when the ring is safely in place. A rubbing strip fixed to the octagon ring will protect it.

My fear is that pushing the bare ring up the sloping ladder would break it when it reached the top and flopped downwards onto the pier. If I was at the top and pulling a rope, attached to the middle of the cross-braces, I would be in a position to 'field' the ring and lower it safely down onto the rollers from the stepladders. Though the top end would be high overhead even from the observatory floor.

It gets even easier if I allow another ladder to slide up the big ladder. This allows the bottom of the big ladder to be pulled out to provide a much more gentle slope. The winch cable can run over the top rung of the 5m ladder and connect to the bottom of the shorter, sliding ladder. The shorter ladder has the usual hooks for the rungs. So will be safely locked at regular intervals on the lift should I need to go down and adjust anything. Though the winch does have the usual locking tumbler pawl on a ratchet wheel which most boat/winch owners seem to rely on.

The sliding ladder will be easily stiff enough to support the ring throughout the lift and lower. It is also lighter and rides nicely between the rails of the wider and heavier, big ladder. A single X-brace to support the ring would be tied to the sliding ladder. A rope would be attached to the top of the sliding ladder for greater control of the load. I think I finally have a workable plan.

Tuesday: A miserably wet, cold and windy morning clearing after lunch. So there's a possible window open to cut the new arcs. And there was. I marked the first arc with the beam compass at 1.5 and 1.6m on the plywood. Then checked with an existing arc to see if both were accurate and matched each other. I then used the arc as a template to mark seven new 15mm arcs which were sawn out with the jigsaw. Each freshly sawn arc weighed 1.75kg. 15 arcs to complete the whole ring @ 30mm thickness = 26.25kg = 58lbs! Manageable but difficult to handle due to the sheer size.

I have to decide how to join the new ring onto the old. I cut the ends off square on the last one and made half lap joints. It might be stronger to use skewed butt joints on the second ring lamination and stagger the joints relative to the first. The wheels should roll more easily over a diagonal joint. Though I hope to use the longer arcs cut from the 4'x8' hardboard on the underside of the ring to achieve even smoother joints. The birch plywood is in 5x5' sheets so makes shorter arcs.

A contact warned me against having hills and hollows in the base ring's desired perfect flatness. Which would make the ring drag at intervals as the wheels had to climb 'uphill.' Hence the move to a double thickness ring and hardboard running surface.

Wednesday:  A promising, sunny autumnal day has now been muffled in thick mist and heavy overcast! At least it is dry so I can continue work on the dome's base ring.

I shall arrange and glue the original ring together on blocks laid out on what remains of the parking place lawn. The blocks can be carefully leveled with a long straight edge and 4' level.

The image shows the original base ring has been glued, clamped and screwed at the lap joints. The beam compass has been re-drilled and a tent peg used as a much firmer pivot than the original 4" screw. I use a loop of cord to hold up the business end of the compass to make quick work of a circuit. In fact I went round and round checking roundness and gently adjusting the tent pegs pushed through each joint.

The diagonals on one end of each arc of the second ring lamination will be used to scribe the next joint. I shall use the diagonal cuts as butt joints and rely on the lower ring, with staggered joints for even support.

The horizontal, bottom struts of the dome segments will heavily reinforce the completed 35mm thick base ring. With a second horizontal strut on the inside of each segment, to match the outer one, the ring will not dare to warp.

I had a trial with the sliding ladder and found the rung hooks did not want to lift clear when a rope was used for lifting. So I reversed the sliding ladder to put the hooks outwards. The ladder then rose almost effortlessly and gently tipped over as it reached its balance point. Where it tipped down gently onto the pier. After which it was very easy to carry the far end of the ladder over to the opposite side of the octagon. I will add some blocks to pack up the octagon ring and the pier to ensure the ring does not catch on the projecting wheels once it is safely lifted up top.

It looks as if this method will serve perfectly for lifting the finished base ring safely into place. I am quite tempted to lift the entire dome skeleton up the ladder after construction on the ground. Though I would have to beware of the dome tilting backwards and outwards away from the ladder. The base ring alone has no outward rotational moment like the dome would probably have.

Click on any image for an enlargement.

*

Dome build: Dome steering wheels part 2.

*

Wednesday: Completed drilling and fitting the shelf brackets to the main wheel pressings. No bolts to fix the wheels. Another shopping trip! Bought all stainless steel screws and nuts for the wheels and brackets. Should have finished the wheels by tomorrow lunchtime.

Thursday: A grey, windy day. Finished fitting the centering/steering wheels to the brackets. Then found the base ring is oval. So the ring goes tight on one diameter despite my allowing some clearance on all the centering/steering wheels. Rain expected after lunch so I have covered the wheels and taken down the base ring for closer examination and measurement. By the way: The beer crate is a handy workbench, seat and step-up stool and does not indicate excessive consumption. Beer is also an excellent slug trap.

I arranged the dome base ring on blocks on the lawn and measured it. Then I brought out the beam compass I had used to draw the initial arcs for the ribs and base ring. The beam compass was far more useful than pushing the ring around. With a long screw as a pivot in the lawn it quickly showed where the radius changed rather than the diameter.

The only obvious way to progress is to glue the ring's half lap joints with the ring first made perfectly round. Then to lift the entire ring up onto the wheels using cross braces to increase the strength of the ring. At 3.2m [ten feet] in diameter the 15mm ring is far too flexible to risk the lift unsupported.

Should I add another layer of plywood to the ring for greater strength? I have no idea but don't like the extra cost and considerable added weight. Carrying all of the base ring arcs at the same time is hard work! I usually only carry two or perhaps three.

I had hoped the lowest, horizontal, dome braces would add their considerable stiffness to the base ring. With the added stiffness of the dome structure the ring becomes inflexible. I shall also add horizontal braces to the inside of the base ring but all that is somewhere down the road.

Friday: I drilled the lap joint areas to tent peg the ring to the lawn while the glue dried once perfectly round. Unfortunately it started raining and became steadily heavier. Attempt abandoned.

Sunday: Having decided to add another layer of birch plywood to the base ring I drove 10 miles to a stockist. To find they had closed despite the signs saying they were open on Sunday. Grrr?

Monday: Another outing in the car and I brought home another 5'x5' sheet of 15mm Birch plywood. Plus a sheet of oil-treated hardboard. [Masonite] The latter will be added to smooth the surface of the wheel track. The 15mm ply will be cut into arcs and glued to the original ring to produce a 30mm thick base ring. With the hardboard track providing the extra [odd] lamination.

I have been pondering different ways to ensure the ring will be round rather than oval. If I use cross braces the completed ring must be able to be lifted onto the rollers without obstruction. The double thickness ring will not be light enough to handle easily unaided due to the sheer size making it very awkward. Perhaps I can build a lifting jib using the 5m [16'] ladder and the two builders stepladders? The problem is avoiding the ladders causing an obstruction in themselves.

Perhaps a simple inclined plane using the long ladder and the boat winch [or just a rope] is the answer? The inclined plane [ladder] could slide up over the octagonal top ring under the control of the winch. Bringing the supported ring with it.  The ladder would easily slide between the rollers and could then be removed when the ring is safely in place. A rubbing strip fixed to the octagon ring will protect it.

My fear is that pushing the bare ring up the sloping ladder would break it when it reached the top and flopped downwards. If I was at the top and pulling a rope, attached to the middle of the cross-braces, I would be in a position to 'field' the ring and lower it safely down onto the rollers from the stepladders.

It gets even easier if I allow another ladder to slide up the big ladder. This allows the bottom of the big ladder to be pulled out for a much more gentle slope. The winch cable can run over the top rung of the 5m ladder and connect to the bottom of the shorter, sliding ladder. The shorter ladder has hooks for the rungs so will be safely locked at regular intervals on the lift should I need to go down and adjust anything. Though the winch does have the usual locking tumbler pawl on a ratchet wheel.

Tuesday: A miserably wet, cold and windy morning clearing after lunch. So there's a possible window to cut the new arcs. And there was. I marked the first arc with the beam compass at 1.5 and 1.6m on the plywood. Then checked with an existing arc to see if both were accurate. I then used the arc as a template to mark seven new 15mm arcs which were sawn out with the jigsaw. Each freshly sawn arc weighed 1.75kg. 15 arcs to complete the whole ring @ 30mm thickness = 26.25kg = 58lbs! Manageable but difficult to handle due to the sheer size.

I have to decide how to join the new ring onto the old. I cut the ends off square on the last one and made half lap joints. It might be stronger to use skewed butt joints on the second ring lamination and stagger the joints relative to the first. The wheels should roll more easily over a diagonal joint. Though I hope to use the longer arcs cut from the 4'x8' hardboard on the underside of the ring to achieve even smoother joints. The birch plywood is in 5x5' sheets so makes shorter arcs.

A contact warned me against having hills and hollows in the base ring's desired perfect flatness. Which would make the ring drag at intervals as the wheels had to climb 'uphill.' Hence the move to a double thickness ring and hardboard running surface.

Wednesday:  A promising, sunny autumnal day has now been muffled in thick mist! At least it is dry so I can continue work on the dome's base ring. I shall arrange and glue the original ring together on blocks laid out on the lawn. The blocks can be carefully leveled with a long straight edge and 4' level.

I can use my ratchet straps to pull the ring into perfect roundness using the beam compass to check. The diagonals on one end of each arc of the second ring lamination will be used to scribe the next joint. I shall use the diagonal cuts as butt joints and rely on the lower ring for even support.

The bottom struts of the dome segments will heavily reinforce the completed 35mm thick base ring. With a second horizontal strut on the inside of each segment, to match the outer one, the ring will not dare to warp. 

Click on any image for an enlargement.

*

Dome build: Dome steering wheels. Pt.1.

Sunday: I spent last night [repetitively] dreaming of ways of fixing the centering wheels to the main dome support/rotation wheels. Without horizontal centering wheels or rollers the dome would have no 'steering' and would just roll off the building sideways at random. The centering wheels keep the main support wheels firmly on track by running along the inside of the dome's base ring. The curvature of the main wheel's contact surface avoids tyre scrub on the endless corners.

I have some 2"x4"channel which could be cut into strips on the miter saw. Sadly the 2" webs aren't quite wide enough to allow the skate wheel axles to fit easily. I'd hate to have to add another plate on top because it adds to the complexity and would look rather clumsy.

I also have some 3x3 angle profile but that isn't deep enough to reach the main wheel axle. Ideally it needs to be a 3" wide by 5" tall, L-shape. Perhaps I'd better go back to looking at roofing steelwork for a centering wheel support?

Here is the first attempt for measurement:

A short length of base ring has 

been added for the image.

I cut a 40mm wide strip from the 2" x 4" channel section. Then sawed off one web by hand. The initial, main axle hole was too high so I filed it longer to allow the bracket to sink below the track's bottom edge.

The main wheel tracks rather near the inside of the base ring. So the skate wheel could be moved nearer the edge of the alloy top section by a few mm. [1/8"] I could also turn down the diameter of the wheel by as much as 20mm if necessary. That would move the track nearer the center of the base ring.

The angle profile isn't really tall enough. Nor the web wide enough. Perhaps I can find some simple shelf brackets as donor material to do the job properly. There are dozens of potential shelf bracket donors at a big DIY outlet in the city. I intended to visit the outlet to see what would best suit my needs. Since the brackets come in assorted lengths I imagine they must be stronger/wider in the larger sizes. Since there will be waste material cut from the top, shorter leg this can be used to reinforce the upright leg around and above the main wheel axle hole.

I have came up with umpteen ways of stopping the dome from lifting in a storm. My favourite, so far, is having alloy disks on top of the centering/steering wheels to catch the base ring if it lifts. The dome's vertical, plywood ribs might need a small slot to clear these retention disks to avoid blocking dome rotation.

Always one to listen to sensible advice, particularly from experts, I have ordered some normal skateboard wheels online. Luckily, plain white is right at the budget end of the market. The 45mm width and smaller diameter [60mm] are both in favour of greater success. I shan't need so much inward offset for the wheels to get the base ring back on track. Though the difference in radius is barely 8mm it all helps the cause.

The height of the wheel, when turned to horizontal, means reduced cantilevered loads beyond the main wheel's axle hole. The wider wheels will allow much greater variations in dome ring height due to wind, wobble or warping.

My 42 mile cycle ride resulted in eight, stainless steel, shelf brackets. With excellent depth and form for strength at the bend, and rust free into the bargain, I was delighted to find them at quite a modest price. They should be amply stiff enough in use with a 50kg, 110lb load capacity. The 40mm width is absolutely perfect for self-centering and alignment in the pressed steel fork of the main wheel housing.

The image [right] shows the shelf bracket in place with the skate wheel attached. I marked the center of the track to ensure it lay over the center of the 'crowned' main wheel.

It is hard to believe how well the bracket fits its role to such perfection. I added a small bolt to retain the lower leg when the main axle bolt wanted to alter the angle of the bracket as the large Nyloc nut was tightened.

The wheel's colour match is as close as could be desired and both will hopefully show up as dome perimeter landmarks in the dark. The shorter leg of the bracket will need to be removed to avoid sharp obstacles projecting in the dark. In fact the bracket can be cut off within the radius of the small wheel for safety.

Needless to say, I am delighted with how well the bracket and its wheel turned out. I may turn off the dished outer rim of the skateboard cruiser wheels in the lathe. It serves no useful purpose and there is ample tread width without it. I may use some 8mm stainless steel, threaded rod [studding] for fixing the centering/steering wheel. Spacers are needed to lift the wheel clear of the racket. This function can best be fulfilled by an adjustable Nyloc nut. Or even a pair of normal nuts locked together.

Whoops! I have just been reminded to leave some wheel clearance for non-circularity of the dome base ring. It had completely slipped my mind!

Wednesday: Completed drilling and fitting the shelf brackets to the main wheel pressings. No bolts to fix the wheels. Another shopping trip! Bought all stainless steel screws and nuts for the wheels and brackets. Should have finished the wheels by tomorrow lunchtime.

Click on any image for an enlargement.

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Dome build: Wheels and lifting octagon braces.

 *

Friday: Dull and grey but dry. I centered the dome base ring and then marked the 2x8s ready for fixing the wheels.

There will be a conflict in the wheel's coach screws overlapping the posts.

There was no obvious way to have the wheels situated over the posts. The logical answer is to fix all the wheel trucks a few inches anticlockwise of the posts. The wheels can then be bolted straight through the 2x8s without any problem. I have drawn arcs on the 2x8s as a guide to wheel placement using the base ring as a stencil. The lines can also be used to guide a jigsaw to curve the 2x8s.

By lunch time I had all eight wheels bolted to the 2x8s a couple of inches away from the nearest octagon posts. I inverted the 10mm coach screws to put the domed heads down, threads upwards.

A tap upwards with a hammer on the 2x6s and a Bessey F-clamp was enough to indent the bolt heads into the cross braces. So that the 2x6s fitted tightly underneath the 2x8s for extra support. It looks much neater too than in their original, lower position.

The observant will notice I've left the octagon wall wide open above the [stowed] observatory ladder. I have plans for a pair of hinged doors to allow easy access to the veranda between the ladder handrails.  The ladder has been tilted almost upright to make more room for the dome components stored 'downstairs' out of the inclement weather.

The octagon cross braces were never intended to be visible. Being hidden [eventually] by the plywood cladding. Having the braces at the top of the posts will also provide a better fixing for the ply walls. A lower cross brace near floor level will provide further support for the plywood, observatory walls. I'll probably leave the middle braces in place to stop wall flexure in gales. Though I don't want to clad the building until I am satisfied I don't need easy access though the framework for the building work.


Saturday: I found a pair of cheap inline skate boots and bought them just for the wheels. I had to grovel on the ground outside with my cycle tool to remove the wheels because I had no room, nor use for the boots. The vendor had no need of the boots either so they were discarded.

I think I shall add alloy angle brackets to the main wheel axles to carry the skate wheels for centering the dome. This should make a neat and compact arrangement without the need for very tall "stalks" on the 2x8 base ring.

I may remove the bearings [temporarily] and turn the wheels down in the lathe to make them even more compact. Or I could use an angle grinder with the disk working across the tyre to remove rubber without the bearings over-revving. That might lead to eccentricity unless I resort to turning the last bit of tread down to size.

Later I applied poly. freezer bags to the naked support wheels to protect the needle bearings from rain and possible rust. The bearings are already greased but there is no point in taking risks before they enjoy the protection of the dome.

Sunday: I spent the night [repetitively] dreaming of ways of fixing the centering wheels to the main dome support/rotation wheels. I have some 2"x4"channel which could be cut into strips on the miter saw. Sadly the 2" webs aren't quite wide enough to allow the skate wheel axles to fit. I'd hate to have to add another plate on top because it adds to the complexity and would look rather crude. I have some 3x3 angle profile but that isn't deep enough to reach the main wheel axle. Ideally it needs to be a 3" wide by 5" tall, L-shape. Perhaps I'd better go back to looking at roofing steelwork for a centering wheel support?

I came up with umpteen ways of retaining the dome from lifting in a storm. Favourite are disks on top of the centering wheels to catch the base ring. The vertical ribs might need a small slot to clear these retention disks to avoid blocking dome rotation.

Click on any image for an enlargement.

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Dome build: Base ring adjustment.

*

Thursday: It soon clouded over but remained dry. Everything always takes far longer than I think it will. Lowering the mounting took ages to do it safely. Because I had to guy the ladders with ratchet straps.

Then I was able to center and measure the diameter of the base ring with it lying on the 2x8 octagon.

In the end I had to cut off quite a chunk of ring to reduce its size and then re-cut the half lap joint. The laser rangefinder was in use again confirming the diameter of the ring to within a mm or two. Dull weather, or even dusk, makes it far easier to see the red dot. Then the device has to be stabilized while a reading is taken with the spot centered on the narrow plywood ring 10' away.

This is the first time I have had the ring on a reasonably flat surface and easily accessible. Two opposing posts of the octagon are closer together than the others. Not that I can do anything about it at this stage. The whole structure is absolutely solid. I'll just center the ring as best I can and then mark the 2x8s ready for fixing the wheels. After that I can fix the timber brackets.

There will be a conflict in the coach screws overlapping the brackets because I couldn't get the exact length I needed. If I cut the screws off they will rust on the cut surface.

There was no obvious way to have the wheels situated over the posts. The logical answer is to move the wheel trucks a few inches anticlockwise. The wheels can then be bolted straight through the 2x8s without any problem. I have drawn arcs on the 2x8s as a guide to wheel placement using the base ring as a stencil. The lines can also be used to guide a jigsaw to curve the 2x8s. It started drizzling mid afternoon so I packed everything away by 4.00.

Friday: Dull and grey but dry. I now have two wheels bolted to the 2x8s a couple of inches away from the nearest octagon posts. I inverted the coach screws to put the domed heads down. A tap of the 2x6 and a Bessey F-clamp was enough to indent the heads into the cross braces. So that the 2x6s fitted tightly underneath the 2x8s for extra support. It looks much neater too. Though the cross braces were never intended to be visible. Being hidden eventually by the plywood cladding. Having the braces at the top of the posts will also provide a better fixing for the ply.

Click on any image for an enlargement.

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Dome build. Doing Swedish wheelies.

*

Wednesday 1st Nov:

The driver arrived with my wheels at lunchtime and I wasted no time opening the boxes and trying them out. They roll extremely easily and seem well made. Probably complete overkill but I'm not getting any younger. So I don't want to be struggling to turn the dome later on. You never know how much additional weight might be necessary for water-proofing.

The inevitable, freshly unpacked shot of the gorgeous wheels: Swede Wheel | 5PF 160-PA6 R

http://www.swede-wheel.se/Product/ShowPdf?Sku=5D16680222N-HD

Now, with the base ring balanced on top of the inverted wheels.

Photo taken from a ladder resting on the shed's gable end for an aerial view.

The cladding on the tall pier is cheap, flooring grade, crap plywood.

The Red oaks are still looking colourful.

The low level view. The total wheel height is 190mm or about 7.5". 

The white nylon wheels are marked Swede-Wheel and are 48mm wide x 160mm in diameter with needle roller bearings and a claimed load capacity of 350kg @ 4kmh. [2" x 6.5" x 770lbs.] The base plate on the sturdy, pressed fork is 110 x 135mm.

As it was so still today I risked balancing the bare, dome base ring on top of the wheels. The woodwork is still dark from the earlier rain. The ring turned effortlessly but still needs centering rollers before I dare to trust it to stay on track. I might use skateboard wheels mounted on timber blocks for centering the dome. Unless I can find some hefty, galvanized steel, shelf brackets. Something with a hooked 'nose' would allow the skateboard wheel's axle to be firmly fixed. I'm keeping an open mind because there are lots of other wheels and potential supports out there.

I've been measuring up the 3"x4" for timber brackets to help to support the wheels and decided on 46cm lengths. I'll need to buy some 8" coach bolts to fix them properly. A quick trip to the timber yard produced a large box of various nuts, bolts and screws. Hopefully enough to finish the project. The miter saw makes quick work of chopping off lengths of the 3"x4" producing perfectly clean and square cuts.

The forecast is wet overnight but much brighter tomorrow. I have decided to take down the mounting to remove the obstacle to base ring accuracy. Two stepladders will have to be lifted up there, straightened, then tied together with the chain hoist.

I'll leave the mounting on the obs. floor well wrapped up. Then I can measure assorted diameters of the ring for the wheels. Once the wheel track is confirmed I can decide whether to replace the mounting.

Fixing the eight, timber brackets for the 2x8 octagon, wheel support ring will take another couple of hours. I'd better get cracking while it's still dry weather.

Click on any image for an enlargement.

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Dome build: Dome base ring trials.

 *

Monday pm:  I pottered on with the dome base ring this afternoon. Which I laid on top of the 2x8s to check overhang etc. The problem was trying to ensure the ring is perfectly round when the large mounting is in the middle of the dome. I can't run a straight edge across and the tape needs to bend around the asymmetric mounting.

So I progressed by adjusting the position of the ring relative to the 2x8s. There is about an inch protruding at the post miters beyond the ring which can be trimmed with the jigsaw. Though I'll wait for the wheels to arrive before making any final decisions. The wheels would seem to fit the center of the track without falling off the 2x8s.

Note that the ring will  be raised 195mm or 8" by the inverted wheels. The dome base ring will not be lying on the 2x8s but on the wheels. The 2x8 octagonal 'ring' is only there to sturdily support the inverted wheels at the correct radius. The radius of the octagon posts is too small for mounting the wheels directly.

I'll need to add timber brackets to the octagon posts on the undersides of the 2x8 miters for wheel support. Just as I did to support the main beams/joists for the observatory floor and veranda.

Perhaps I should add some suitable decoration to the bottoms of the brackets on my band saw this time? I simply mitered the lower brackets last time but they didn't look very exciting. Better, though, than a straight 90° cut-off. 2"x4" is undersized while 4"x4" is too heavy looking. Most of my sawn 4"x4" has split badly since purchase. I may look for some planed 3"x4" at the timber yard. Even 0.5m/ 18" adds up to 4 whole meters or 12' with eight lengths.

I wonder whether I should clad the observatory walls before fitting the brackets? If I fit them first I shall have to miter cut the plywood sheathing carefully around all eight brackets. This would demand considerable neatness and accuracy in cutting and might easily lead to leaks.

If I fit the timber brackets afterwards they will need solid packing to fill the empty void where the plywood meets the 4x4 octagon posts t an angle.

I will have to clad the outsides of the posts with shallow isosceles triangles anyway to give the cladding something solid to be fixed against. The back side of the brackets may need to be sawn into a shallow V to match the cladding at the posts. I notched and bolted the posts for the beam brackets but the loads are much lighter for the dome wheels. So bolting the wheel brackets may be enough.

Tuesday: Bought a length of 3x4" planed timber for the brackets. I may need to countersink the coach bolt heads or even for the nuts. I bought a load of 7" climate coach bolts and nuts and still have lots left. My lifetime collection of coach bolts may still come to the rescue if I can find enough of the right length.

Wednesday: Drizzle. Waiting for the wheel courier to arive.

Click on any image for an enlargement.

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Dome build: New clamps and wheels.

 *

Monday am. I had ordered 18 new Rawlink 15cm/ 6" spring/glue clamps despite having badmouthed my original Rawlink clamps. Now I am delighted to report that the new Rawlink clamps are a vast improvement! They have the same shaped handles but now have nicely grippy, rubbery pads. Making them very easy to use. They also seem just as powerful but with slightly shorter jaws for better leverage when opening them.

I think the spring wire is slightly smaller diameter so it may be improved steel for greater lightness. The previously loose jaws are now properly pinned so they can't fall out like the originals. Making them excellent value compared to the pricey and much weaker Bessey clamps.

Having opened my parcel of clamps I painted the new, 2x8, wheel support ring with the Safeway, mineral/water wood treatment. I have been delighted with the very even, silvery-grey appearance of the treated timber. The bare larch floor boarding has weathered to a very similar appearance. It may not be to everybody's taste but I like the "abandoned barn" finish.

I finally ordered 8 off 160x50mm nylon wheels today in pressed steel forks to support and rotate the dome. Each, individual wheel has a capacity of 350kg.  That's 770lbs! Which would easily carry the full weight of the dome all by itself. Each wheel has ball needle roller bearings for easy movement. The hard wheels should avoid flats forming if the dome isn't turned regularly.

I liked the slightly increased height of the fork to provide a marginally taller dome. At nearly 6.5" diameter I imagined these wheels would roll more easily over inconsistencies in the track than my earlier choice of 5". My eye level, when standing on the observatory floor, would then be just above the intended base ring height.  So a horizontal view of the distant woods is still possible without needing a ladder to reach the eyepiece. 

Shopping around online provided considerable savings. The most heavily advertised Danish dealers on Google are wholesale only. However, they do not make it remotely OBVIOUS that they will not deal with private customers! I wasted considerable time on their websites picking and choosing! As it was, I found better wheels with higher load capacity for about the same money from a much more friendly dealer. They should be with me on Wednesday.

Click on any image for an enlargement.

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Dome build: Dome weight and wheel load.

*

It has been sunny but very windy all day after the storm. So I have been getting on with capping the octagon walls with mitered 2x8s. Only one post was out of level. My stock of 2x8s is of two different sizes! 

A bare segment/gore weighs just over 6 kilos or 13lbs.  So, 16 gores = 13 x16 = 208lbs.

45x45mm horizontal struts weigh 1.1kg per meter [0.7lbs per foot]

Vertical struts: Pi x 1.6 x 8 = 25 meters for the 8 vertical runs [half circumferences] over the dome. If the vertical struts weigh about the same per meter as the horizontal struts then [say] 27.5 kg = 60lbs total.

4mm birch ply weighs about 15lbs per 5'x5' sheet. [2.25m²]
Area of a hemisphere = 2.Pi.r² = 6.284x1.6² = 16m² / 2.25 x 15 = 106lbs for the ply cladding.

Plus any reinforcement around the obs. slit and shutters + hardware + paint. Say 500lbs total for the entire dome?

Support and rotation wheel loading:  500/8 = 62.5 lbs static load per wheel.

Hard, white, 5" Polypropylene industrial wheels with ball or roller bearings and fixed forks have a load capacity of 150kg = 330 lbs.

6" PP wheels capacity is  250kg = 550lbs. The price difference between 6" & 5" wheels is 2:1.

I had ordered 18 new Rawlink 15cm/ 6" spring/glue clamps despite having badmouthed my original Rawlink clamps. Now I am delighted to report that the new Rawlink clamps are a vast improvement! They have the same shaped handles but now have nicely grippy, rubbery pads. Making them very easy to use. They also seem just as powerful but with slightly shorter jaws for better leverage when opening them.

I think the spring wire is slightly smaller diameter so it may be improved steel for greater lightness. The previously loose jaws are now properly pinned so they can't fall out like the originals. Making them excellent value compared to the pricey and much weaker Bessey clamps.

Having opened my parcel of clamps I painted the new, 2x8, wheel support ring with the Safeway, mineral/water wood treatment. I have been delighted with the very even, silvery-grey appearance of the treated timber. The bare larch floor boarding has weathered to a very similar appearance. It may not be to everybody's taste but I like the "abandoned barn" finish.

I also ordered 8 off 160x50mm nylon wheels today in pressed steel forks to support and rotate the dome. Each, individual wheel has a capacity of 350kg.  That's 770lbs! Which would easily carry the full weight of the dome all by itself. Each wheel has ball bearings for easy movement and the hard wheels should avoid flats forming if the dome isn't turned regularly.

I liked the slightly increased height of the fork to provide a marginally taller dome. At nearly 6.5" diameter I imagined these wheels would roll more easily over inconsistencies in the track than my earlier choice of 5". My eye level, when standing on the observatory floor, is now just above the intended base ring height.  So a horizontal view of the distant woods is still possible.

Shopping around online provided considerable savings. The most heavily advertised Danish dealers on Google are wholesale only. However, they do not make it remotely OBVIOUS that they will not deal with private customers! I wasted considerable time on their websites!

Click on any image for an enlargement.

*

Dome build: Observation slit mock-up.

*

The observation slit provides the view of the sky from within the enclosed confines of the dome. Its minimum width is important for a number of reasons. Not least the difficulty of keeping an object in view as the sky passes across the slit over time. Though in reality it is the rotation of the Earth which gives this illusion of sky movement from East to West.

If the slit is too narrow then the dome must be moved so often it becomes a bore. Many amateurs prefer to drive the dome to avoid having to monitor and manually rotate the dome during observations. Those who image from a remote location will demand a dome rotator which closely follows the telescope's field of view. This will change depending on the altitude of the object being imaged or studied.

For simplicity I have chosen a slit width of 60cm or 2' which is the same as the normal gores. The mock-up was to see how such a slit, or rather its framework, would affect the rest of the ribs.

The gores on either side of the slit have no need of an adjoining rib so these nearest slit can simply be left out. Though the opposite side of the adjoining gores will still have a normal rib which stops at the slit framework. The two gore's horizontal struts can be carried right over the intervening inch to stop at the slit framework. To be fixed there as if they were normal ribs. Though the miters on the slit end will be at different angles to normal ribs. 

As mentioned previously, the slit framework must be strong enough to compensate for the missing struts which would otherwise cross the slit. More than that, the framework carries the tops of all the ribs where they are cut away to allow viewing at the zenith. [i.e. The slit must be open overhead.]  So none of the ribs actually reach the dome's pole. In the image above I have pulled the base ring forwards by a 30cm [one foot] to simulate the open zenith. The arched sides of the slit's framework have been extended to compensate. Perhaps a foot [30cm] extra clearance, for zenith viewing,  is too much?

It would be safer to match the slit framework to the complete dome skeleton to ensure geometric accuracy. The top of the slit can be left square. Or a rounded collar could be formed from laminated plywood glued in enough layers for adequate strength. Weatherproofing at the zenith is obviously important as any leaks will fall directly onto the mounting or telescope, or both.

Click on any image for an enlargement.

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Dome build: Progress at last?

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A morning of light showers didn't put me off working on the dome. I had to replace all the 57cm C2s before I could add all the short gores to the three, full length ones. There was a minor panic when I realised I couldn't pull the resulting skeleton into a circle of the correct diameter. 

It seems the architect had decided to make only 15 gores to save on materials but forgot to tell the foreman or the labourer. Problem solved by adding a couple of extra struts for decoration where the observation slit will go.

By the time I had finished lunch the dome was enjoying heavy drizzle. I rapidly unclamped it all and put it in the octagon with a tarpaulin over the top. I have ordered more Rawlink clamps online. I know I keep saying it but they offer remarkable clamping pressure for small change. Yes, the jaws fall out all the time and the widely splayed, slippery plastic handles are very hard to grip.

Nevertheless, for my purposes in holding gores together, they are ideal. The Bessey clamps, for comparison in the same size, are very weak and need at least two or three, to hold as well as a single Rawlink. I never had any of these spring clamps before starting the observatory. They made dome construction so quick and easy I should call it Insta-dome. Or perhaps not. The entire dome can be erected and dismantled again in under five minutes.

What's next? I need to mark the observation slit on the gores. Then add side reinforcing ribs to replace the missing strength from, quite literally, cutting the dome wide open on one side. I was advised to stiffen the dome with cladding panels before attacking the ribs.

The opening must be left without any obstructions even up over the zenith. That means a considerable loss of strength without really stiff, additional materials built right around the opening. The dome is very likely to sag around the slit unless it is well support with vertical edge ribs. Conversely, I can't add these parallel reinforcements without the dome being supported evenly.

The gore on the left shows the latest dimensions and angles.

It also means cutting 4mm birch plywood panels on the table saw. Which means working outside to have room to swing a 1.5x1.5m [5'x5'] sheet freely. The largest [lowest] panels are 60x50cm but they'd have to be cut from 50cm wide strips. Three strips from each sheet. I'll lose a little bit of that width to the two saw kerfs as I divide up the sheet into three. Working outside needs dry weather. Or a temporary roof to work under.

Perhaps I'd better get some white, lightweight tarpaulins to hang a sloping canopy off the octagon and shed. Otherwise I shan't get much done in steadily deteriorating weather. It would be easy to run a 2x4 or 2x6 off the veranda to support the side furthest from the shed. A sturdy post will hold up the corner using truss plates. The shed will be providing support on the other side.

It needs to be windproof as well as waterproof and absolutely must shed rainwater easily. Rather than collecting it in heavy puddles ready to destroy the entire structure. One of my earliest memories was of a neighbour trying to push an enormous puddle off the roof of his tarpaulin or canvas "garage." Which was hung between two buildings. Unfortunately the length of timber he was using to prod the puddle pierced the canvas and he and his motorcycle were drenched! I remember being fascinated by the bright, lens-like puddle in the roof. History does not record whether I was also soaked.

The latest iteration of the dome has the upper C4 [35cm] struts fitted near the tops of the [still] shorter gores. Some to the ribs have taken on a slight set from being damp and twisted by earlier struts of the wrong length. They will soon respond to being straight again. Next step is to extend more of the ribs towards the pole.

By the strangest coincidence I saw a telescopic, big bale loader on my morning walk today. I was just discussing these machines as a means to lifting my completed dome into place. Though I still think I will lift each gore/segment effortlessly up the ladder using my hand-cranked, boat winch. Perhaps pairs of gores will be quicker to build with and more stable on the ladder. The important thing is to have rehearsed the entire build down on the ground before lifting anything. 

I can build a derrick off the pier with a pulley on top. Then the gores will remain upright throughout the lift and just slot neatly into place on the base ring while still safely restrained. Their forward balance point will ensure they don't tip backwards. Access, via a stepladder, through the open observation slit should be fine for working on the top of the dome.

I have received the white, 4x6m lightweight tarpaulin I ordered to form a work shelter. The 18 extra Rawlink spring clamps should be here tomorrow. I am using storage tubs for carrying the clamps about now. Bags were hopelessly awkward and bringing them in an out by hand meant lots of trips, due to the bulk. I can't believe how many clamps it takes to hold the dome together when I had none at all before starting. I need lots more to be sure the dome is safe as I build it up on top of the building. The gores should be mostly clad by then so I don't want 'bits' blowing away across the fields!

Click on any image for an enlargement.

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