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I
am presently considering how I might use the raw materials of the
nesting components of the 10" steamer to best effect. Adding internal
plywood rings would increase the weight and undo the entire purpose of
making the counter-cell as a thin and hollow but very stiff shell.
Besides my lathe is already at the limit of maximum turning diameter.
Fortunately the router with circle cutting jig does a good job of
achieving roundness and patience in setting will provide accuracy in
diameter. Carbon fiber would be ideal for a counter-cell but I don't
want to get involved in the mess or the expense. I have had quite enough
experience with building things in GRP to last me a lifetime! The 10" steamer base pretending to be an APO dewshield.
A search of the village recycling shops provided another, larger, but still lightweight, aluminium steamer. A 10" diameter one this time. It looked absolutely huge as I stuffed it into my cycle shopping bag. To add to the fun the charity shop was having a half price, sales day. Once the steamer was safely brought home it shrank dramatically when held up against the 8" diameter main tube. Despite its considerable depth the steamer base pot looked rather tragic as a potential dewshield. The sense of scale in a larger, classical refractor demands a dewshield length of at least twice the aperture.
Dewshields are odd things from a purely aesthetic point of view. I have very specific tastes in dewshields which simply cannot be ignored. It must be neither too long nor too short. Of the correct diameter, without protruding cell flanges. Above all dewshields must be larger in diameter than the main tubes or it looks completely wrong.
My fixation is probably the result of my childhood yearning for a 3" Swift refractor. This was prominently displayed in a library book, on basic astronomy, which I often borrowed. As a schoolboy I only had a 1" draw-tube refractor at the time in which the image of the Moon at 30x was a fuzzy mess. Seeing through a pair of ex-govt. binoculars was a revelation! It was years later before I finally had a pair of binoculars to call my own. Decades before I had a half decent pair.
The image above shows the steamer base opened out to fit the objective cell OD and denuded of handles and drainpipe. Note the closeness of the main tube and cell diameters. It occurs to me that I could reverse the "pot" to make a shallow, mechanical dewshield. This would support the plastic dewshield and provide a really stable base on which to to rest the OTA vertically. The problem is getting a strong fixing onto the main tube. Though I have some new ideas for that too.: A couple of plywood rings can clamp the counter-cell/dewshield over the main tube flange. The rings would have to be split and glued once they are sprung over the flange. I have cut a couple of circles but could make some more with extended ears for a bolt style clamp. I'd rather avoid using a mechanical clamp over the plywood rings for aesthetic reasons. This fixing is absolutely vital from both a stable collimation and security point of view. I can't have the heavy lens falling off the end of the telescope tube! Perhaps I should reconsider a counter-cell which is inserted into the main tube? Or a belt-and-braces design.
I split the two 12mm thick Birch plywood rings I made yesterday and sprang them over the main tube flange. Once i was happy with the fit I glued and clamped them together with every clamp and vice I could muster. Wood glue is suppose to be stronger than the material it joins so it should be up to the task.
Fuzzy image in a dark shed of the clamping operation. One day they will invent a light for photography in dark places. "Flash" might be a good name for it. I deliberately cut the rings at an angle to increase the joint area for extra strength. Once these rings are bolted through the dewshield/counter-cell pot the whole arrangement should be more than strong enough. A few radial screws will ensure that the plywood rings never come loose even if the disintegrate over time. Though there's no reason why they should as they are WBP birch multiply. The donated steamer base weighs almost nothing and the rings are narrow enough not to add much weight of their own. I would have preferred not to use plywood but nothing else suggested itself. Only the router could manage circles of the required diameter. They were at least an inch larger than my lathe can manage.
Of course we had a cloudburst right in the middle of the clamping operation! So everything had to be rushed back into the workshop. I had been working outside for good light and maximum freedom to move around the job. I wanted to ensure I had missed no gaps between the rings and the main tube so that the cut joints were tight. I put a slight chamfer on the foremost ring to ensure it bedded as closely as possible against the main tube flange. A notch was also required for the main tube seam. Now it's just a matter of patience until the glue sets. It's a shame the temperature has dropped 20F in the last few days.
I went out in search of some 4mm screwed rods [studding] and some thin alloy flashing to start making up the baffles. Anything thicker than 4mm quickly gets quite heavy when a minimum of six is required. The first store only had two rods and the second store was already closed. Denmark has a long history of shops closing early on Saturdays. Somewhat ironically the same shop will be open tomorrow. The important baffle hole to get right is the middle one. Since this is where the pairs of rods join leaving little room for manoeuvre. The other baffles can be moved up and down inside the tube simply by adjusting their fixing nuts. I think I shall have five baffles separated by two sets of three rods. Each set will be rotated relative to the other to avoid having to join them end to end. The middle baffle will become the joining plate for the two sets of rods.
I have been testing the balance point of my still unfinished OTA by hanging a 5kg weight to represent the objective. It will need considerable weight added to the focuser end. I can add a sliding weight and a finder but it will probably still need more ballast if it is not going to look like it has slipped down through the rings. No to mention the lack of ground clearance this causes when pointing at the zenith. My wife suggested I strip our old [small] trailer down to the chassis. So I could easily push the MkIV mounting around on its pier. That would get me some instant ground clearance! It would need some thought as to rapid stabilization and a brake once it was parked.The suspension had rusted up years ago [as purchased!] which made it an automatic failure on the MOT. So, at least, suspension sag isn't a worry. Stripped of the bodywork it would be much lighter and have better access to the telescope at high pointing altitudes. I haven't looked at the tyres in years since it was never actually used on the road.
The supposed "Vixen" tube rings I have just received in the post have a crack in the casting right across one of the hinge "ears." The quality of the casting, These rings will have to go back. It's lucky I didn't open out the fixing bolt hole to match the needs of the sturdy MkIV mounting saddle. It's a shame because the next ring size up is even larger than these. Nobody seems to do a budget 202mm ring. Except for Orion UK which offers "rolled" alloy rings in many sizes including a 202mm which I need. Perhaps I should make some tube rings out of birch plywood using the router? The problem then is fitting a hinge and a clamping screw. I shall have to do some online homework to see how clever people have coped with the clamping problem.
I now have 6 x 1m x M4 screwed rods to build the baffle assembly. The rods alone weigh 1lb! Fortunately the weight is evenly spread along the entire OTA.
Click on any image for an enlargement.
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