Astrophotography forum > Reflectors Telescopes Forum

Why do secondary cup holders exist (instead of stocks)?

(1/4) > >>

Mohamed Wiest:
A cup holder adds about 3mm the CO, and it is a larger surface for light to bounce off on the way to the primary. Why don't all secondaries use a stock instead?

I'm mostly bothered that my 130mm has a cup holder, wasting its light. Of course that is in addition to the primary being too close to the secondary to give focus to cameras.The Zhummel 130mm mini-dob is much better quality visually than the Celesteron Astromaster 130mm. Instead of buying this piece of junk for $140 on craigslist, I wish I had bought the Zhummel. Although, it might not have been around back then. Also, the back end does not allow air flow. Seems to be a trend at 130mm and below. You get a real telescope at 150mm.

Christopher Mendez:

--- Quote ---A cup holder adds about 3mm the CO, and it is a larger surface for light to bounce off on the way to the primary. Why don't all secondaries use a stock instead?

I'm mostly bothered that my 130mm has a cup holder, wasting its light. Of course that is in addition to the primary being too close to the secondary to give focus to cameras.


--- End quote ---

It only affects the off-axis illumination and the effect visually is quite small. These are two reasons I prefer a Astro-Systems type secondary holder rather than a stalk:

- The secondary is more secure, no chance of glue debonding and dropping the secondary. It is also easier to remove the secondary for cleaning or recoating.

- A bonded secondary can be stressed by the adhesive resulting in astigmatism. The holder holds it lightly but securely so that it is stress free.

Jon

Dan Square:
The secondary that came with my 8" F/9 was only 1.25" wide, a good size for a planet killer. But it was glued to the secondary holder, and I badly chipped the edge of the secondary while changing some screws, causing the image of every star to flare like an exploding bubble. So I bought a new secondary and secondary holder from Astrosystems. The secondary holder is 1.36" wide, but I will gladly accept an additional .11" of obstruction to have a secondary with a protected edge.

James Bradmon:
I use foam tape. Even if it thermally expands and contracts, it has such a low modulus of elasticity that I doubt it would stress the mirror much.

As for tape dropping the mirror, peal is the main factor there. There are ways to apply tape and other carefully placed glue to eliminate peal. I make the tape surface area a bit larger than the stock, so it stays stuck to the mirror. Then, on the top and bottom edges of the stock, where pealing might start, I put wood glue that connects the stock to the tape more securely. Note: the wood glue does not contact the mirror. It is stiff, so I don't want it applying force. The tape I got has a strength of 1 pound per square inch.

fefeldarsro:

--- Quote ---I use foam tape. Even if it thermally expands and contracts, it has such a low modulus of elasticity that I doubt it would stress the mirror much.

As for tape dropping the mirror, peal is the main factor there. There are ways to apply tape and other carefully placed glue to eliminate peal. I make the tape surface area a bit larger than the stock, so it stays stuck to the mirror. Then, on the top and bottom edges of the stock, where pealing might start, I put wood glue that connects the stock to the tape more securely. Note: the wood glue does not contact the mirror. It is stiff, so I don't want it applying force. The tape I got has a strength of 1 pound per square inch.


--- End quote ---
More thanone way to skin a cat. Your ideas make sense to me. By the way
I like your signature...a man with a plan.

Navigation

[0] Message Index

[#] Next page

Go to full version