Terrestrial 90mm binoviewing telescope.

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With the foul weather preventing outside work I'm having another look at the binoviewing telescope. I received a 1m length of 100mm aluminium tube which proved to be 100mm OD. Not very helpful when I had foolishly expected 100mm bore! Beggars can't be choosers and one must use what one can get. I found no other sensible Danish outlet for aluminium tube. Most are wholesale dealers only and sell in only 6m [nearly 20'] lengths!

The bare glass of the 90mm f/11 Vixen objective lens is actually 93mm in diameter. This doesn't leave much room inside my 96mm bore tubing. Any tube would have to be under 1mm wall thickness. There followed a search for potential "cell" material. None of my aluminium tubing was of the slightest use. Then I spotted a bleach bottle. Which turned out to be only slightly undersized. Further searching produced some flashing but nothing I tried would make a strip perfectly round. I tried rolling it around numerous diameters but it always made ridges.

Finally I spotted an old aluminium case from a mains clock now long forgotten. It proved to be only slightly tapered once I had very carefully cut the battered base off. It was now a perfect sliding fit in the 100mm tube and quite snug on the objective lens itself.

A lens does not want to be compressed or it will spoil the optical accuracy and thereby the image. If the aluminium shrinks in a frost it could actually damage the lens! It might be worth slitting the cell tube to avoid the risk of thermal damage.

So I set about sanding the old paint away to give just a little more headroom. Then I drilled pairs of holes at 1/6th intervals around the circumference and sawed a line between them. The idea is to allow the glass to survive a hard frost as the cell contracts. The cell tube obviously cannot become larger than the inside diameter of the main tube but the sleeve is only a snug, but not tight, fit. It's ugly, but beggars can't be choosers. I had nothing else which would fit in the main tube that was also thin enough for the lens to fit inside.

With the biconvex crown element resting on a foam cup I carefully lowered the tubular "cell" around the glass. Still snug but just manageable. Rings cut from the bleach bottle will make useful lens stops so the glass will not fall out. It would have to slide out because it can't possibly tip in such tight surroundings.

Careful measurement of the near focal point requires a 84cm length to the base of the Baader 45° erecting diagonal from the back of the objective glass. This is with the binoviewer in place in the diagonal and carrying a pair of Japanese 26mm Meade 4000 Plossls for about 42-45x. The actual power will depend on the effective glass path length of the diagonal. Assuming 100mm to be added to the original focal length plus a similar GP of 100mm for the binoviewer. Say 1200/26 = ~45-46x.

Some allowance for infinity focusing will be further inwards. I had no chance to check this distance in the rain but will as soon as the weather allows. I need this figure for infinity to ensure I have enough focus range. A suitable focuser has yet to suggest itself. A helical focuser would be nice for terrestrial use and avoid the usual focusing knobs/wheels sticking out.

Later I used the garden gate to check the likely nearest focus at about 20 yards. Anything nearer and one might as well just use ordinary binoculars. My earlier measurement of focus was strangely awry. I added a batten to rest on and internally blackened tubes to help reduce glare. The focus was now 78-84 cm from the back of the object glass to the front face of the Baader 45° diagonal's body. A range well beyond any helical focuser I have found so far. I had better start looking for a secondhand focuser with a range of at least 60mm. No obvious candidates within the Newtonian and SCT ranges. Which means a refractor focuser is required. It looks as if the original Vixen focuser will have to do.

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