GS 8" Dob mirror cell

[janoskiss (Steve H)]

While doing the mirror refit as per Geoff's Howto being discussed on the GS Dob optics thread, I've noted that the mirror is only supported on three cork pads at its edges. From the little I know about mirror cells I suspected that this may not be enough to stop an 8" mirror from flexing too much. I don't know what the mirror cells are like on the bigger Dobs, so this may only be relevant for us little dobbers.

After a bit of digging around, I found the relevant information in a review article on a bunch of 8" Dobs:

Alan M. MacRobert, Sky & Telescope, Jan 2000, volume 99, issue 1, page 60.

One of the Dobs considered was the Orion XT8, which has the same construction as the 8" GSO Dob. I am sure that the S&T people will forgive me for posting a few short relevant passages from the article:

I disassembled the Orion and Bushnell mirror cells. Both proved to be holding the mirrors only at the edges. A mirror should be supported more toward its center if it is not to deform under its own weight, especially if it is as thin as these, which had a width-to-thickness ratio of 9:1 (S&T: April 1996, page 75).

The Orion mirror cell offered the chance for an easy fix. I put three stacks of felt pads, made of cheap weatherstripping held together with double-sided tape, on the thick metal plate behind the mirror. The mirror now rested on pads 70 percent of the way from its center to its edge. I was careful to replace the mirror clips very lightly on the mirror's face so they would apply no pressure. Sure enough, once the telescope was reassembled and recollimated, the star test showed that the astigmatism was much reduced!

As a check, I took the pads out and restored the mirror cell to its original condition; the astigmatism came back.

So there's another mod to add to the list. Stuffing felt under the mirror does not sound all that appealing to me though. How much felt is enough how much is too much? The problem really is that any more than 3 fixed support points and you no longer have all of them in a plane... Has anyone got a better suggestion on how to improve mirror support?

[janoskiss (Steve H)]

Yelp!

I was going to fit the 180mm mirror mask tonight but waited for any response on the mirror cell question first, but I got none so far

I went to the hardware store and got the thinnest sheet of outdoor plywood for the mask, and a can of black chalk board spraypaint. But now that I know what I know, I'm going to try to experiment with making a better mirror support before anything else.

[RAJAH235]

Steve, in answer to the mirror support, provided you start with 2 flat surfaces, it shouldn't matter how many pieces of felt you place there. They will all take an equal amount of weight as they compress. Try it with 6 or 9 pieces spaced equally. You said you needed to re-collimate anyway. L.
99.

[elusiver]

i wouldda thought a little section of felt padding right behind where the centre spot is would do the trick, as that's out be the thinnest part of the mirror bearing the most weight. as to how much felt.. it'd be a trial an error thing.. kinda like milk bottle washers. Only thing to be wary of would be stacking the felt too high.. as i wooulda though this would cause astigmatism outwards.. and also.. to keep the felt pieces fairly small.. so to mot effect the eairflow behind the mirror too much.

el

[rmcpb (Rob)]

Originally Posted by elusiver

i wouldda thought a little section of felt padding right behind where the centre spot is would do the trick, as that's out be the thinnest part of the mirror bearing the most weight.

I think that is the last place to put a support because of the same reason. As its the thinnest part of the mirror it can be pushed "out" easiest. Its for this reason that the mirror is supported in a ring about 70% of the diameter becauses the narrow, thick outer circle balances the wide, thin inner circle and causes the least flexition of the mirror.

If you are using felt and not glue give it a try, it won't hurt anything except that you will have to recollimate the scope again.

[elusiver]

good point rob.. guess all we can do is try. There's been a couple of methods posted so far... so we'll get to the bottom of this soon

el

[Starkler (Geoff)]

If someone can post pics of the 8 inch mirror cell maybe those that havent seen it can make suggestions.

[janoskiss (Steve H)]

I've implemented a fix, see my post on the new ATM forum.

http://members.optusnet.com.au/~jano...ll-mod-012.jpg

[ausastronomer (John Bambury)]

Originally Posted by elusiver

Only thing to be wary of would be stacking the felt too high.. as i wooulda though this would cause astigmatism outwards.. and also.. to keep the felt pieces fairly small.. so to mot effect the eairflow behind the mirror too much.
el

El,

This would only occur if the mirror clips were overtightened but if the new pads are not of a very similar height to the existing pads they will be self defeating anyway.

Janos,

FWIW the primary mirror cell issues with these 8" scopes were rectified nearly 5 years ago. The mirror cell design has changed at least 3 times (probably more) in the period since Alan MacRoberts S & T test report.

If you have any astigmatism issues it should be evident in a simple star test as I described in the GS Optics thread. If its not broken don't fix it.

CS-John B

[janoskiss (Steve H)]

John B, I have already done the mod but have not had the chance to see if it made any difference. The GSO mirror cell I have looks pretty simple, and as described in the 2000 S&T report, with the mirror resting on three cork pads at the edge of the mirror.

Unfortunately the weather gods have seen what I've been up to so it may be some time before I can see the results of my efforts, if any.

[ausastronomer (John Bambury)]

Steve,

I just had a look at the photo of your original cell in the other thread, I didn't realise it had no central support at all, how old is your scope ?

What you have done should work fine anyways, just be a little wary of the plywood warping with moisture. I notice you have used "outdoor ply" and that should be fine.

CS-John B

[rmcpb (Rob)]

Ever thought of using silastic aquarium glue to hold the mirror onto the mirror cell? If the felt pads are replaced with a dob of glue and a spacer set to hold the mirror off the metal then all should work. I use a 3/16" drills for spacers then remove them when the glue has dried. This makes the issue of the side clamps distorting the mirror redundant as they are simply not used.

As for the moisture issue. Hopefully it is a non event as many ATM scopes have wooden frame parts that are not affected if they are sealed properly. For this a good couple of coats of polyurethane should do the trick.

[elusiver]

so you have your primary fixed to your mirror cell with that glue stuff and discard the side clamps? how strongs the glue??? that's the only thing that'd make me seem reluctant to use that method.

el

[rmcpb (Rob)]

Commonly done in the ATM world. It is the glue that hold aquariums together so holding a mirror is chicken feed for it. The main thing is to ensure an even contact of the glue to the mirror and mirror cell. As it never goes really hard (its rubbery) if the mirror cell expands or contracts relative to the mirror there is a bit of give in the setup.

[janoskiss (Steve H)]

John B, The scope should be the current or very recent model. I bought it from Andrews in January this year.

[ausastronomer (John Bambury)]

Steve,

I am somewhat baffled as to why they would go back to that design, especially when they had issues when GS made the original XT8's for Orion in 1999 and 2000 with a similar designed cell. That having been said, I didn't look at the design of the mirror cell on Gary's (Cahulian) 8" GS dob but it is only a recent purchase and it has no astigmatism at all, optically excellent.

CS-John B

[rmcpb (Rob)]

Mine has the same setup and is about a year and a hlaf old.