Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
· Get a Cloudy Nights T-Shirt · Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User

Equipment Discussions >> Reflectors

Pages: 1 | 2 | (show all)
tag1260
scholastic sledgehammer
*****

Reged: 10/07/12

Loc: Ohio, USA
12" GSO mirror removal and re-mounting new
      #5525625 - 11/18/12 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


Post Extras: Print Post   Remind Me!   Notify Moderator  
Old Rookie
sage


Reged: 09/05/08

Loc: North Central Ohio
Re: 12" GSO mirror removal and re-mounting new [Re: tag1260]
      #5525734 - 11/18/12 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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
yonkrz
scholastic sledgehammer


Reged: 11/16/06

Loc: SW Minnesota
Re: 12" GSO mirror removal and re-mounting new [Re: Old Rookie]
      #5526872 - 11/18/12 09:13 PM

The fishing line works great.

Post Extras: Print Post   Remind Me!   Notify Moderator  
tag1260
scholastic sledgehammer
*****

Reged: 10/07/12

Loc: Ohio, USA
Re: 12" GSO mirror removal and re-mounting new [Re: yonkrz]
      #5526928 - 11/18/12 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


Post Extras: Print Post   Remind Me!   Notify Moderator  
tag1260
scholastic sledgehammer
*****

Reged: 10/07/12

Loc: Ohio, USA
Re: 12" GSO mirror removal and re-mounting new [Re: tag1260]
      #5535600 - 11/23/12 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


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mirzam
Post Laureate
*****

Reged: 04/01/08

Loc: Lovettsville, VA
Re: 12" GSO mirror removal and re-mounting new [Re: tag1260]
      #5536040 - 11/23/12 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


Post Extras: Print Post   Remind Me!   Notify Moderator  
tag1260
scholastic sledgehammer
*****

Reged: 10/07/12

Loc: Ohio, USA
Re: 12" GSO mirror removal and re-mounting new [Re: Mirzam]
      #5536087 - 11/23/12 07:01 PM Attachment (52 downloads)

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.

Edited by tag1260 (11/23/12 07:04 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
tag1260
scholastic sledgehammer
*****

Reged: 10/07/12

Loc: Ohio, USA
Re: 12" GSO mirror removal and re-mounting new [Re: tag1260]
      #5536098 - 11/23/12 07:05 PM Attachment (40 downloads)

another

Post Extras: Print Post   Remind Me!   Notify Moderator  
Mirzam
Post Laureate
*****

Reged: 04/01/08

Loc: Lovettsville, VA
Re: 12" GSO mirror removal and re-mounting new [Re: tag1260]
      #5536136 - 11/23/12 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


Post Extras: Print Post   Remind Me!   Notify Moderator  
tag1260
scholastic sledgehammer
*****

Reged: 10/07/12

Loc: Ohio, USA
Re: 12" GSO mirror removal and re-mounting new [Re: Mirzam]
      #5536151 - 11/23/12 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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mirzam
Post Laureate
*****

Reged: 04/01/08

Loc: Lovettsville, VA
Re: 12" GSO mirror removal and re-mounting new [Re: tag1260]
      #5536190 - 11/23/12 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


Post Extras: Print Post   Remind Me!   Notify Moderator  
tag1260
scholastic sledgehammer
*****

Reged: 10/07/12

Loc: Ohio, USA
Re: 12" GSO mirror removal and re-mounting new [Re: tag1260]
      #5536221 - 11/23/12 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


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mirzam
Post Laureate
*****

Reged: 04/01/08

Loc: Lovettsville, VA
Re: 12" GSO mirror removal and re-mounting new [Re: tag1260]
      #5536781 - 11/24/12 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.com/scope/mirroredgecalculator.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


Post Extras: Print Post   Remind Me!   Notify Moderator  
tag1260
scholastic sledgehammer
*****

Reged: 10/07/12

Loc: Ohio, USA
Re: 12" GSO mirror removal and re-mounting new [Re: Mirzam]
      #5536936 - 11/24/12 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


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mirzam
Post Laureate
*****

Reged: 04/01/08

Loc: Lovettsville, VA
Re: 12" GSO mirror removal and re-mounting new [Re: tag1260]
      #5536946 - 11/24/12 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


Post Extras: Print Post   Remind Me!   Notify Moderator  
tag1260
scholastic sledgehammer
*****

Reged: 10/07/12

Loc: Ohio, USA
Re: 12" GSO mirror removal and re-mounting new [Re: Mirzam]
      #5537151 - 11/24/12 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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mirzam
Post Laureate
*****

Reged: 04/01/08

Loc: Lovettsville, VA
Re: 12" GSO mirror removal and re-mounting [Re: tag1260]
      #5537177 - 11/24/12 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


Post Extras: Print Post   Remind Me!   Notify Moderator  
tag1260
scholastic sledgehammer
*****

Reged: 10/07/12

Loc: Ohio, USA
Re: 12" GSO mirror removal and re-mounting new [Re: Mirzam]
      #5537289 - 11/24/12 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


Post Extras: Print Post   Remind Me!   Notify Moderator  
John rombi
scholastic sledgehammer


Reged: 08/14/05

Loc: Sydney, Australia
Re: 12" GSO mirror removal and re-mounting new [Re: tag1260]
      #5537389 - 11/24/12 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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
tag1260
scholastic sledgehammer
*****

Reged: 10/07/12

Loc: Ohio, USA
Re: 12" GSO mirror removal and re-mounting new [Re: John rombi]
      #5538750 - 11/25/12 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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | (show all)


Extra information
23 registered and 29 anonymous users are browsing this forum.

Moderator:  ausastronomer, Phillip Creed, JayinUT, okieav8r 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 2003

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics