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12" GSO mirror removal and re-mounting

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#1 tag1260

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Posted 18 November 2012 - 07:33 AM

I have a GSO 12" Dob and I want to send my mirror out to be re-coated. I understand you have to cut the silicone that GSO uses to hold the mirrors in the mount. Is there any on the back or just the sides? Do you just run a razor knife edge around the outside to free it up and then just remove it or? Or will I have to gently pry it out?

After it's out, how do I re-mount it? In silicone again or is there a better way. Is there any sort of aligning it in the mount?

Thanks

#2 Old Rookie

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Posted 18 November 2012 - 09:09 AM

You can remove the mirror using either heavy fishing monofiliment, Spyderwire or simply dental floss to cut through.

We used cork on the mirror of the club's 16" Lightbridge when we re-installed it. On the bottom with some also on the sides for a snug - not tight fit. We allowed for a bit of expansion between the mirror and the clips.

#3 yonkrz

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Posted 18 November 2012 - 09:13 PM

The fishing line works great. :smirk:

#4 tag1260

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Posted 18 November 2012 - 09:53 PM

Thanks for the replies. The removal sounds pretty easy. How about re-mounting it? Do I use the silicone or do I try cork as John suggested? How tight do I want it or am I just spacing it evenly inside the mount? I'm going to try to get it out this week to have it re-coated.

Thanks again

#5 tag1260

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Posted 23 November 2012 - 02:14 PM

OK. Got it out and it's on it's way to Majestic for re-coating. Now when it gets back I'll need to re-mount it. My original set up has 9 small adhesive discs (3 on each mounting plate)mounted to the mount, but only 3 of them were actually stuck to the mirror, the other six still have the slippery backing attached to them. There was nothing around the sides to keep it centered in the mount. Do I do the same thing or is there a better way to mount this mirror to the mount? Will I benefit from something spacing the sides? Someone above mentioned using cork.

Thanks

#6 Mirzam

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Posted 23 November 2012 - 06:43 PM

I'm of the school of thought that thinks that gluing a mirror to a cell is a lazy man's approach to solving a problem (And I've done it myself so I should know). Sometimes the glue works well, sometimes it does not and causes mirror distortion. It always makes removal a bit more risky than it should be.

While your mirror is away, why not modify your cell to use edge clips and side supports that are guaranteed to avoid the problems mentioned above? Maybe your cell already has these and GSO just added glue for good measure? If you show us a picture we can better suggest how to proceed.

JimC

#7 tag1260

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Posted 23 November 2012 - 07:01 PM

Here's a couple pictures.The three white spots are the adhesive discs. Only one per mount were actually used. The mirror holding brackets are made of rubber.

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#8 tag1260

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Posted 23 November 2012 - 07:05 PM

another

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#9 Mirzam

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Posted 23 November 2012 - 07:36 PM

That looks like a pretty nice mirror cell. I would not glue the mirror back down on the triangles. I would connect the triangles with a ring of flexible plastic or perhaps string to make sure the triangles are held in the correct orientation (i.e points directly inward to the center). I would shim the sides to keep the mirror from shifting laterally, not too tight though. The best position for the shims is at +/- 45 degrees from the vertical plane. It's not clear to me whether the mirror clips are adjustable. But the ideal adjustment would be to position the clips so that they don't quite touch the mirror surface (use a business card to set the spacing). This allows the mirror to be safely held in the cell without any unwanted stress.

JimC

#10 tag1260

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Posted 23 November 2012 - 07:45 PM

The mirror clips are not really adjustable. Won't using, say a business card for spacing allow your mirror to move that much? What will actually hold the mirror from moving?

Please forgive all the questions as this is all new to me.

#11 Mirzam

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Posted 23 November 2012 - 08:12 PM

A dobsonian uses gravity to hold the mirror in position. The purpose of the 9-point floatation cell like you have is to allow the mirror to be supported without localized stress. Using shims at the sides of the mirror (+/- 45 degrees from the 6'oclock position) keeps it from shifting sideways and affecting collimation. Unless you point below the horizon the mirror will not fall forward 1/2 mm to hit the clips. Even if it does it will settle back when you point the scope skyward. The mirror may contact the clips while you are transporting the scope horizontally (i.e. in a car), although if you slightly elevate the eyepiece end during transport this will not happen.

If your clips squeeze down on the mirror this is not a good thing at all. It can cause warping and astigmatism. Maybe you could add spacers between the two parts of each clip to allow tightening without squeezing?

JimC

#12 tag1260

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Posted 23 November 2012 - 08:33 PM

Thanks. Makes sense. How snug do I want the spacers on the sides? You're saying put them in at say 1:30, 4:30, 7;30 and 10;30?

Thanks again

#13 Mirzam

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Posted 24 November 2012 - 08:21 AM

Don't make them tight because the floating support surface provided by the triangles might be compromised. Your positioning is correct. This page gives some understanding why:

http://www.cruxis.co...ecalculator.htm

Note that the page also mentions the ability to glue mirrors that are full thickness up to about 15". Full thickness in your case would be ~2" thick, and I'm pretty sure that the GSO mirrors are somewhat less than this. A floating support is really best.

Another way to make the edge supports is to drill and tap holes for nylon bolts. This approach provides some tightness adjustment, and is useful for mirrors that are mounted equatorially. However, in your case the edge supports doing all the work will be at 7:30 and 4:30. You can simply glue a couple shims at those positions--just be sure you use something the right thickness to maintain proper mirror centering. There will not be any pressure on the other two supports while using the cell in a dobsonian mount.

One last point--Make sure that the pads used on the triangles are not sticky or uneven. You might want to replace them with new felt furniture pads if they are.

JimC

#14 tag1260

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Posted 24 November 2012 - 09:57 AM

Thank you for all the information. What do you recommend for the shims? Will wood do the trick or should I use something more forgiving or maybe something like aluminum.

Sorry for all the question but this is all new to me.

Thanks again

#15 Mirzam

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Posted 24 November 2012 - 10:03 AM

Don't use metal against glass without something soft in-between. Wood or hard plastic would be fine. Avoid anything that tends to compress a lot (i.e. rubber) because it will compress to varying degrees with changes in telescope pointing angle. This could affect collimation.

JimC

#16 tag1260

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Posted 24 November 2012 - 12:08 PM

Thanks again< Jim.

If I want to go with the tapping and screws, would a 1/4" screw be enough? Just one NYLON screw per position? Is exact positioning important or is close enough good.

I do appreciate all this info. I feel I'm now in the position to do any mods needed while my mirror is out to Majestic.

#17 Mirzam

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Posted 24 November 2012 - 12:26 PM

The larger bolt the better--your mirror is pretty hefty. 1/4" is probably okay but 5/16" or 3/8" wouldn't hurt.

The bolts (one at each position) should be placed at the mirror edge center of gravity, which is very close the midpoint of the edge--just a tiny bit shifted towards the rear. The edge calculator gives you the numbers. For a mirror as thick as yours the tolerances are not very tight.

Looking again at your cell, it does not appear that there is a lot of clearance between the mirror and the inside rim. Your bolts will only protrude a short distance. You still need to be sure that the mirror clips will prevent the mirror from falling out.

JimC

#18 tag1260

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Posted 24 November 2012 - 02:01 PM

OK thanks. Here is my game plan as per your input. I'm going to replace the discs with felt. Then I plan to use "Spider Wire" (fishing line) to holder the mounting plates centered. Around the sides, I'm going to shim with some wooden shims about 1/2" wide at the specified spaces. I'll glue those in place with just a tiny bit of silicone to keep them in place.

Thanks again

#19 John rombi

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Posted 24 November 2012 - 03:27 PM

When I removed my 12" Lightbridge mirror (GSO) from its cell for recoating, the mirror maker recommended that I use adhesive velcro circles to remount the mirror.
I have been using this method for three years now with great success. It holds the mirror gently, and still allows expansion & contraction when necessary.

#20 tag1260

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Posted 25 November 2012 - 01:43 PM

Anyone have some good ideas how to go about stringing up something to keep my mounting triangles for moving around? I am think of using some spider wire (fishing line) but am unsure how to string it.

#21 Mirzam

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Posted 25 November 2012 - 02:10 PM

You can also use a flexible plastic ring cut from a file folder or other flexible plastic sheet. Just attach with double sided tape. There is a picture at the beginning of this thread in the ATM forum:

http://www.cloudynig...5538444/page...

JimC

#22 acochran

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Posted 25 November 2012 - 03:35 PM

I am a beginner. so take my advice with a grain of salt: I used tape made from a slippery plastic similar to Teflon. I think the product is called slick tape or slick strips. I bought it online.

#23 tag1260

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Posted 29 November 2012 - 09:55 PM

OK. Here's where I am. On the mirror mount I cleaned the sticky pads off and replaced them with felt sliders. Then I tied the three mounting plates together, with 30 lb Spiderwire fishing line to keep the sort of aligned. They still can move but are restricted considerably. Also installed some cork pads around the inside of the mirror mount to keep from having mirror to mount contact. I'm going to install two nylon screw "bearings" in the side when my mirrors get back and I can measure the thickness to find their locating.


Speaking of my Mirrors, I talked to Jeff at "Majestic Coatings" today. I can't believe my mirrors arrived Monday and were shipped back out on Wednesday!!!! I"ve heard nothing but good about him and now I can attest to it myself!!!

When it gets here, I have the center mark to install (Catseye). Again, Kudos to another vendor, Jim Fly at Catseye talked to me and explained his opinion on what I needed and didn't treat me like the idiot I felt like. Even after I ordered and changed my mind, he went out of his way to accommodate me.

Anyways, I'll post up a couple of pics as I finish up my project and hope I'm not being a pain but I've never really worked on anything like this. I know I couldn't be doing this without everyone here's input.

Thanks
Tag

#24 tag1260

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Posted 30 November 2012 - 08:54 AM

here's a picture of what I've done so far. All I need to do is drill and tap the holes for the side "bearings and I'm good to go.

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#25 David Castillo

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Posted 01 December 2012 - 04:26 AM

Before you break out the tap and dye set, try the mirror out in the cell with a star test. I don't think you need need to go that far- the mirror isn't that thin, and I'll bet a star test will confirm it. It's a big hunk of glass, dude!!I don't think you need to follow the lemmings.
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