10" f/8 It ain't heavy, it's my OTA.

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[17]

Now I'm wondering if I really need to pull in the focuser end of the beams so tightly.  Or even need to do so at all. The beams need no help from such 'triangulation' to remain stiff in any orientation.  I have added an inverted channel section to hold the front of the pot via one of the handle brackets. Attempts to bend one of the channels to match the pot's curvature was time wasted. The material was just too stiff to bend as I wanted it.

Next day: It started very cold working outside but steadily improved all day in full sunshine under a cloudless sky. In the end I added another channel profile and fixed the pot to that by the handle bracket. I then made an arc of 1" aluminium strip and riveted that to a piece of square tube resting on the channel. Finally I bored two holes in the strip and screwed the pot to the arc with large spreader washers. The idea was to be able to remove the pot later if it proved necessary. It seems solid enough but can be beefed up further if necessary.

Balance test with 10lb steel weight to simulate the mirror. I have just weighed the mirror for the first time at 8.5lbs. So I can safely move the balance point further up the tube. Which is all to the good. It means a lighter OTA as well.

Next day: I have now fitted a channel at the top of the tube to make the beams parallel. Adding a metal weight to simulate the mirror places the balance very close to the pot. I know the focuser weighs one pound so I added 2 lbs to the focuser end to allow for the focuser, secondary mirror and spider. This shifts the balance point just enough to get the Fullerscopes saddle between the beams. It just clears the pot mounting channel.

The 12"/30cm ID plywood ring is just to get a feel for the dimensions of the secondary ring/cage.
I have had no inspiration for anything in this area as yet. Other than a desire for a curved vane spider to avoid spike diffraction. 

The bare spar and pot weigh exactly 10lbs. There will be some added weight to the cell end to support the mirror. Though that pushes the mirror forward slightly to balance things out again. After I add a (50mm) finder I should have a perfect balance point. This will place the saddle quite near the pot (for a nice low pedestal) but far enough away to allow a little variation for balancing the OTA. If I clamp the finder to the beams somehow I shall be able to use that as a sliding balance weight. This will avoid adding unnecessary dead weight at the top just to be able to balance the OTA. The complete and working OTA should weigh about 25-27lbs. The beam construction is about half the weight of the cardboard tube.

There is a price to pay for this twin beam spar design. It places the optical axis a long way from the nearest declination axis bearing. Whether this is a disadvantage will depend on the weight involved. I could spread the beams further apart to allow the saddle to be brought to the top surface of the beams. The ends of the MkIV saddle is considerably wider than the alloy channel profile I have been using. This allows me to clamp the beams to the saddle without the need for fixing bolts. It would be difficult to reach the Declination clutch adjusting knob if the saddle was inserted between the beams. The mirror is the heaviest object and its centre of gravity placed 10-11" from the face of the saddle. The beams could be turned by 90 degrees and clamped to the saddle. Though this would not change the mirror overhang by very much.

By spreading the beams to either side of the pot the degree of cantilever could be reduced by clamping to one beam. This would reduce the moment arm around the polar axis by about 4". Compared to using the cardboard tube and conventional tube rings I don't think I need to worry too much about the lighter weight beam design. There is still unlikely to be any need for a massive counterweight.

I am still struggling to come up with a suitably clever design for focuser rotation around the optical axis. I'd really like to avoid plywood if possible. It adds unnecessary weight in the worst possible place on the OTA. Thin plywood rings could be workable with a thin aluminium (tubular) skin. I would ideally prefer aluminium tubular rings for their stiffness. The problem is finding suitable donor material from everyday objects. There are no scrapyard outlets to pick over. No obvious culinary items spring to mind. Nor is there anything in the builder's merchants which presents itself for adoption and adaptation.  I have no Tufnol in large enough sheets to make rings and would probably produce a stink if I did. I have scoured all the charity shops for inspiration. No pot is large enough. Nor any outdated magazine rack. I have a couple of push rings from an old wheelchair. They are wonderfully stiff and light but are far too large in diameter to to be useful for a 10" telescope.

I could use flat surfaces and clip the focuser into several fixed places to provide comfortable viewing. Though I worry about collimation issues if I follow this route. This very clever telescope builder shows how to make aluminium tubular rings with no more than a vice:

www.binodob.de/12_eng.html

I have tubing from old TV aerials which I can try with this method. Well I made a couple of wooden curved forms but had no luck with my modest vice in bending the square aluminium aerial tubing. I couldn't get remotely enough pressure. So I tried a car screw jack to press the forms onto the tubing. This had no effect either. Perhaps I need a much bigger and better vice?

Later I fitted the beam tube temporarily to the MkIV. Then tried covering the southern sky from East to West. A fixed focuser would work but this would rather depend on the angle it was set at. It would be easiest (construction wise) to have it pointing from behind the beams towards the optical axis. Though this would present a focuser requiring the observer looks upwards when viewing. Not the most comfortable of situations.

Flipping the OTA to the other side of the mounting would help. Except that the mounting is now in the way of the observer. To enjoy a comfortable eyepiece position (looking at least partially downwards much of the time) I would have to lean over the mounting to reach the eyepiece. A potentially dangerous situation if I am standing on a box to reach the eyepiece. Very difficult to achieve if I am facing and leaning against a stepladder. There would be no room for the stepladder with the telescope physically in the way.

Ideally, I need a focuser which looks towards the twin beams (and saddle) from the other side of the optical axis. This requires a ring or some other firm support for the focuser at least 12" from the beams. Alternatively, a focuser at right angles to the beams would work. Again this needs physical support for the focuser well away from the twin beams. Requiring a firm "dogleg" construction or partial ring to reach the focuser base plate. Since it is cantilevered it lacks the stiffness of a full ring unless seriously beefed up to avoid flexure.

I'm really going to have to find a better bench vice which can cope with the forces involved in bending an aluminium tube. The Dutch telescope builders have produced some wonderfully light and smooth rings using this method.

I have no idea why I have never owned a decent metalworking vice. It would have been so useful over the years. I have a small swivel vice with soft jaws which was very useful for clock making and model work. I have also had a Record woodworking vice for many decades. The faster Acme thread and soft jaws made it rather limited in usefulness for metalwork. Most of my G-cramps came from flea markets.

Click on any image for an enlargement.
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