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Mirror cell Q

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#1 BCB

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Posted 24 January 2004 - 09:56 AM

After having used my new scope for about 3 days, I ran across something that disturbed me.. I was swinging the scope in the mount, from one side of the sky to the other, and I heard a "clunk". :o :(

I knew right off what it was, mirror shift.. SOOOOO, I remove the cell off the back of the scope, and find the 3 clips loose enough to allow the mirror to shift and turn under them..

The cell is a pretty simple design. supports the mirror at the bottom just around the edges, using cork pads for the mirror to sit on.
I then noticed there's quite a bit of space between the mirrors edge, and the ridge surrounding the cell, keeping it from slipping out.. Nothing around the edge at all, from what I can tell.

What I'm wondering is: what can be used around the cell edge, to keep the mirror from floating around (keeping it centered without pinching). And, how tight can I set the mirror cell clips holding the mirror in the cell ?? I snugged tham all down just enough to keep it from spinning/shifting, but loose enough to be able to slightly rock the clips on the screws.. I don't know how tight they can be before "pinching" starts.. :question:

Anyone else run into this, and what did you do to solve it ???

Thanks peeps. :)

#2 Tom L

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Posted 24 January 2004 - 11:37 AM

Some stiff foam comes to mind? The stuff you wrap pipes with...that type of foam.

#3 Don W

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Posted 24 January 2004 - 11:40 AM

First of all, it would help if you could tell us what telescope you have. There are a thousand variations out there. Generally speaking, you should have the approxmate thickness of a folded piece of paper between the clip and the front of the mirror. The same general rule goes for the edges. You have to allow room for the mirror to expand or contract or you'll get pinched optics.

#4 BCB

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Posted 24 January 2004 - 12:01 PM

Oh,

OOps.. Someone slap me.. Forgot to name the thing. LOL

It's an Orion 6" reflector 750mm FL.

One other thing I had forgot to make note of.. There's NO in-out adjustment on the clips, to keep the mirror centered in the cell.. It's just a round open backed aluminum casting. I'd say there was approx 1/8 to 1/16 gap when the mirrors off to to one side of the cell.

Think my having snugged down the clips to hold it in place was a mistake ?? I haven't noticed any difference in my star tests since I've done it..
One thing I did notice abut the clips, was that they're made out of stiff rubber, with a steel cap on the top for the screws to push on.

#5 Don W

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Posted 24 January 2004 - 12:02 PM

You do not want to put any pressure on the mirror. You MUST have clearance between the clips and the glass.

#6 Suk Lee

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Posted 24 January 2004 - 12:10 PM

Two interesting articles by RF Royce on the subject:

http://www.rfroyce.com/cell.htm

http://www.rfroyce.com/smallmir.htm

Cheers,
Suk

#7 BCB

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Posted 24 January 2004 - 12:57 PM

OK,

Now, if I have NO pressure from the clips, when I collimate in one direction, I will be fine but if I swing to opposite side of the sky, the mirror will shift, and collimation will go out of whack due to the mirror shifting in the cell no ? I've heard the thing clunk when I've had to shift to the opposite side of the sky, but before I spun the scope in the rings, to get the EP in a better position..

Sounds like I just need to find something to keep the mirror centered in the cell then, with slightly loose clips. Would thin strips of soft foam around the edges between the mirror and cell edge pinch it when the mirror expands when it warms up ??

I read both of those articles awhile ago. They both made perfect sense to me..

#8 Stacy

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Posted 24 January 2004 - 06:44 PM

When Al M. re-coated my 8” mirror, he put it back in the cell and used some kind of clear adhesive on the back of the mirror to hold the mirror into the cell. So no clips and no stress and no shifting. Whatever he used holds really well as I am storing it upside down right now. You might ask him about it in the “scope mod” forum.

#9 BCB

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Posted 24 January 2004 - 08:56 PM

Hmmmmmmm

I was womdering just what you had done Stacy. Out of curiosity, I was wondering why you had your mirror done.. I thought your newt was rather new ???

I'll fire Al a PM, and see what he says. :)

Thanks Stacy

Only thing I've done up to this point, was to reloosen all the clips so there's a bit of play between the clip and mirror.. I haven't done anything with foam yet.. The gap is pretty small when the mirrors centered.. 1/16- 1/32 per side.

#10 Stacy

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Posted 25 January 2004 - 02:57 AM

Hmmmmmmm
I was womdering just what you had done Stacy. Out of curiosity, I was wondering why you had your mirror done.. I thought your newt was rather new ???

OK, so it really didn't need a re-coating very badly, I’ve only had it a year and a half. It was “slightly” oxidized and Al used silver for the re-coat instead of aluminum for a tad bit more performance. I’m very happy with the results and hopefully I’ll be able to actually use it soon. :crazy:

#11 Stacy

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Posted 25 January 2004 - 03:02 AM

I really like the idea of using the adhesive instead of clips. It’s a fine line between holding the mirror snugly and stressing it, plus the clips did overhang the mirror. This way I’ll have one less thing to worry about. It will be interesting to see if I notice a difference. :question:

#12 Mike Hosea

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Posted 25 January 2004 - 04:03 PM

It's important to use a flexible adhesive (I use clear GE Silicone II from a FRESH tube) in widely spaced dollops with a spacer during application to ensure that there is some thickness to the dollops so that they can flex laterally. Otherwise, mirror stresses will be created by the different expansion/contraction of the glass and mirror cell.

Here is another idea, wrap your clips with 3M Temflex rubber splicing tape to cut down on the gap.



#13 BCB

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Posted 25 January 2004 - 04:26 PM

I'm seriously considering using silicone/rtv to glue the mirror in place.. There's a set of 3 cork pads that the mirror sits on, right where the clips attach, and I'm leaving them in for auto spacing.. I had also noticed that there IS 3 thin pads around the sides, but they're not thick enough to hold the mirror in the center of the cell..

3 drops between the pads, and 4 small ones around the sides for centering, and I should be good to go..
I'm a bit leery of gluing the mirror in the cell, but from what I've read regarding it, it's pretty safe to do..

Thanks for all the tips people.. :waytogo:

#14 BCB

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Posted 26 January 2004 - 09:42 PM

Well,
With cloud cover tonight, and snow for the next 2 days it got done tonight. Plenty of cure time.

Next up is to replace the secondary collimation screws with longer cap head ones that have the ends rounded off.. Should make for a finer "feel" when tuning the secondary.
I also have to take a look at the focuser secondary left-right adjustment.. It's still a bit off, even with having the focuser shifted. One of those "it'll do till i hit a cloudy patch" jobs..:)

#15 BCB

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Posted 28 January 2004 - 05:23 PM

It's ALIVVVVEEEEEEEEE !!!!!

Turned out quite good too.. While I was at it, I went and checked the spot centering, and found it to be QUITE a bit off center.. Almost .040 (1mm).. So I went about finding true center, marked it, and lifted and moved the spot..

Fixed the left right secondary problem also, and changed the collimation setscrews to longer cap head screws that I can turn with my fingers. I also rounded off the ends if the screws to make setting the secondary easier....

It seems between the Center spot being off that little bit, and getting my secondary right fixed the errors I was having.. I'd do laser for secondary to primary, then collimation cap to do primary centering.. then I'd double check with the laser again, and EVERY time it would show my secondary to primary center being off...????

No need to use the cap now. I can nail it dead on with the laser first shot.. I double check with the cap, and it's exactly Where I placed the return laser dot. No need to mess with any more extra adjustments with the cap.. :)

Made my day, that's for sure.. :grin:

#16 Stacy

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Posted 28 January 2004 - 10:57 PM

That's great BC! Nothing like getting all the pieces to fall into place.


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