Author Topic: Why do secondary cup holders exist (instead of stocks)?  (Read 352 times)

Mohamed Wiest

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Why do secondary cup holders exist (instead of stocks)?
« on: December 28, 2017, 02:56:33 AM »
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

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Re: Why do secondary cup holders exist (instead of stocks)?
« Reply #1 on: December 29, 2017, 06:04:10 AM »
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.


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

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Re: Why do secondary cup holders exist (instead of stocks)?
« Reply #2 on: January 03, 2018, 02:35:00 PM »
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

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Re: Why do secondary cup holders exist (instead of stocks)?
« Reply #3 on: January 08, 2018, 07:55:19 PM »
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

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Re: Why do secondary cup holders exist (instead of stocks)?
« Reply #4 on: January 09, 2018, 03:40:27 PM »
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.

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.

penliipamex

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Re: Why do secondary cup holders exist (instead of stocks)?
« Reply #5 on: January 10, 2018, 12:06:21 AM »
The rim of the secondary holder can prevent some edge issues with the ellipticalfrom being seen.I believe one of Mel Bartels' old tests of differentsized secondary obstructions talked aboutthe smallest secondary testing suffering from someedge problems. this should not apply to a high quality secondary mirror.

At 130mm the benefit of a rim is dubious.

Keith Dixon

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Re: Why do secondary cup holders exist (instead of stocks)?
« Reply #6 on: January 10, 2018, 05:02:40 AM »
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.

Without commenting on your queries, note that the correct terms are: "shrouded secondary holder" for the type with the surrounding lip that prevents the secondary from falling, and "stalk" (not stock) for the 45° cut surface to which a secondary is attached by some form of glue bond.

As for the WHY of the holders, see Jon Isaac's reply above.

schorerabhat

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Re: Why do secondary cup holders exist (instead of stocks)?
« Reply #7 on: January 11, 2018, 09:08:11 AM »
Oh go ahead and comment on the queries themselves. Somebody? Please.

Rodney Slater

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Re: Why do secondary cup holders exist (instead of stocks)?
« Reply #8 on: January 12, 2018, 04:26:52 AM »
Well, in addition to what Jon said, a secondary glued to a center stalk has shiny sides that reflect low angle of incidence light directly into the light path.
You could paint the side of the secondary, but removing this completely when the secondary is recoated might prove impossible.
Or the side of the mirror could be flocked, but doing a neat job of this is difficult.
In contrast, a shrouded secondary holder has flat black sides, to reflect considerably less into the light path.
Additionally, the shroud covers the outer lip of the secondary, which, on every secondary, is the least accurate part.
And, one can reach in and rotate the secondary during collimation with no fear that rotational force will separate the secondary from its stalk.

redsfilrerig

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Re: Why do secondary cup holders exist (instead of stocks)?
« Reply #9 on: January 12, 2018, 08:34:27 AM »
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.

Heat could make the tape drop the secondary. Don't use the scope for solar viewing at least without a full aperture filter.

redsfilrerig

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Re: Why do secondary cup holders exist (instead of stocks)?
« Reply #10 on: January 13, 2018, 01:57:28 AM »
I use sharply on my mirror sides, and grab it by the stalk.

Donnell Keown

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Re: Why do secondary cup holders exist (instead of stocks)?
« Reply #11 on: January 18, 2018, 10:37:11 AM »
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.


Foam tape hardens, it can get sticky in the heat. Normally people use silicone.

Jon

Tom Doyle

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Re: Why do secondary cup holders exist (instead of stocks)?
« Reply #12 on: January 20, 2018, 03:57:07 PM »
This thing has remained attached for 17 years. Zero added obstruction.

Logan Budd

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Re: Why do secondary cup holders exist (instead of stocks)?
« Reply #13 on: January 21, 2018, 01:52:40 PM »
Does silicone melt in the heat?

The secondary in my 8" was attached with foam tape, though it was oriented wrong. Would tape be too risky?

A problem with silicone is I need to get it flat. Tape is already flat.

reapriavoland

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Re: Why do secondary cup holders exist (instead of stocks)?
« Reply #14 on: January 23, 2018, 07:03:38 AM »
Talk about minimalist!