12.01.20 Musings on new 6" f/10 H-a OTA

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Parts for the new 6" f/10 iStar, H-alpha telescope should be arriving very soon. I have yet to make a decision on skeleton or closed tube. From my experience with the folded 7" refractor, I worry about loss of contrast with a skeleton tube. If I then clad the skeleton tube, why bother to build one in the first place? The skeleton tube would have given me easier access to the internal D-ERF. Now I'm changing to a full aperture, external D-ERF I have no need of internal access.

I have had a 160mm bore, thin steel, galvanized, duct tube waiting to be used. This would certainly speed up progress to completion of the telescope. I just need to make an adapter for the new objective lens to fit this tube. Then make a tailpiece for 2" push fit, for the PST stack. So I can be imaging. While I am trying out the new OTA I can be considering alternatives builds and /or improvements.

For the 7" straight tubed refractor I made a glued, laminated plywood ring adapter for the objective. Long "pull" screws passed right through the cell into T-nuts on the back of the ring. The "push" screws pressed against the front of the plywood ring. The small flange on the 8" duct tubing prevented the ring falling off the  tube. This form of construction saved me finding a huge diameter of aluminium in the round to turn a solid adapter. My 9" lathe could not manage such a large diameter anyway.

The 6" has much more modest demands in adapter materials. I do have some 180mm Ø slices in round bar in aluminium. The iStar objective cell has a diameter of 168mm at the rear. Which means the lens cell must lie in front of the tube. Because it cannot enter.

However, the 6" cell collimation screws lie on a 180mm ring. While the greatest cell diameter is 192mm on the flange. This completely rules out a simple 180mm turned flange and tube in one piece. It would need an assembly of a tube and at least a 92mm Ø flange. Not the end of the world but slightly more complicated. The OTA mounted flange may need to support a dewshield.

Perhaps I should be looking at the thick, birch ring concept again. I can face the front of the flange with aluminium for the "push" collimation screws to press against. On the 7" I used Nyloc nuts on the push screws. The larger surface area of these locking nuts [compared to the bare screws] avoided serious denting. Or even drilling into the plywood during collimation. Not ideal because some denting still occurred even with the nuts. Which would alter the collimation over time depending on the resistance of the plywood surface. Hence the idea of an aluminium sheet flange to face the plywood ring on the 6".

The downside is that the Nyloc nuts have considerable axial depth. Which pushes the lens cell well off the front of the main tube. Which risks dust and dirt getting behind the lens. Unless sealed well. With something soft and flexible at the cell flange to adapter flange interface. Just plugging and unplugging accessories into the OTA focuser end will frequently suck and blow potentially damp and always dirty air. Which would pass right through the whole OTA via the lens cell/ adapter gap. Quite a large, open area when considering the circumference involved. Say, 21" x 1/4" = ~ 6 square inches. That's quite a large ventilation hole!

I could and should recess the front of the adapter flange to support the rear of the objective cell. This avoids the cell literally hanging in free space from the three, rather modest, 5mm, collimation "pull" screws. Which is what happened with the 7". Because I never thought about it at the time. Rotation of the OTA by any equatorial mounting might well have led to variations in sag of the [very] heavy objective. Though, fortunately, the "push" screws would have provided increased stiffness of the cantilevered assembly.

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