questions on making a mirror cell for a 13.1" thin mirror

[5ash]

I am going to make a better mirror cell for my 13" oddyssey or should i say proper mirror cell. Has anyone got any suggestions for a source of suitable springs for this project and silicone glue to attach the mirror(I dont like push pull mirror cells).
regards philip

[koputai]

Hi Philip,
Suprisingly, Bunnings has a very good variety of spring sizes and rates, you should be able to find them near the bolts/hinges/door fittings.
Otherwise, http://www.smallparts.com.au have a HUGE range of stuff.

Cheers,
Jason.

[Ian Robinson]

Check out American S&T archives , there have been some good articles in that magazine over the years that are well worth referring to.

If you can get hold of Berry's book http://www.company7.com/books/products/berrybuild.html- look there too , it's excellent.

Wilman Bell sell direct to the public.

[Satchmo]

Mount your mirror on a 6mm thick ring of acetic acid cure silatic at the 60% zone. Springs come from any good hardware.

[5ash]

Isn't that the type of silastic used in bathroom and gutter sealents? Also why a ring? I thought that 9 points arranged in the prescribed positions as in a floating cell would be the shot?
regards philip

[Ian Robinson]

I think you'll find 9 dots of the stuff wont be much good at hanging onto the mirror - not good if it falls off or out. If you want to do that how about structural grade Liquid Nails (least that'll grip the mirror).

You could make a 9 point flotation cell , nylon screws segments are used in my 10" mirror cell , but you need to have at least 3 fingers attached to side clamps to hold the mirror in place (stop it from drifting or rattling about or falling out) , but not distort it by gripping it with much if any force).
I'm in the process of modding my mirror cell to effect "safer" side clamping as my old side clamps were not properly sized and unadjustable.

ASTRO SYSTEMS in the USA make flotation mirror cells , and ship overseas and are more than happy to sell direct. http://www.astrosystems.biz/

Orion Optical in the UK http://www.orionoptics.co.uk/ made my mirror cell http://www.orionoptics.co.uk/OPTICS/opticsmirrorcell.html but when I ordered it I didn't notice the side clamps weren't floating angle sections , only thing I don't like about these is the non-adjustable side clamps - the cells are made for their own standardized mirrors ONLY , but they will happily make special parts for you to your exact specs and drawings , ( Parks USA also do mirror cells http://www.parksoptical.com/index2.php?cPath=26&cat=Telescope+Components&osCsid=84261642741c00c4608651d785a3 290d as
I bought my mirror as custom job from Newport Glass Works http://www.newportglass.com/index.htm see http://www.newportglass.com/angwcat.htm - good people to deal with , I've been a customer of their's for over 15 years and have bought several mirrors and kits (for other people mostly) from them over the years (my 10" is a PA Pyrex thin mirror, they didn't do enhanced coating when I bought it , else I'd asked for it).

[Satchmo]

Nothing sticks to glass better than Silastic. They use it to lift the 8 meter VLT mirrors :)

Sure if you have a 9 point cell ( 3 flotation triangles ) use them, but 9 dots placed on a board is not a floating cell as any warping of the back plate will be transmitted to the mirror . A single ring flotation is extremely forgiving and gives excellent results on 14" and under mirrors, in fact better than a 9 point cell. I have mounted 3 mirrors between 14" and 12" with a single ring of velcro and it works superbly. I pipe up about it every now and then when questions like this come up.

Use Search function and you'll find plenty of prior dicussion of it. Have fun.

[skies2clear]

Wouldn't it matter how "thin" the mirror is in deciding whether to use a ring of silastic at the 60% zone or a proper 9 point flotation cell? It's easy to try the "ring" treatment and verify the performance on a star test. Making a proper 9 point cell will be a lot more work, but I confess, this is the way I'd be going to not take any chances, especially if you have a really nice optic. If it's so so, then maybe keep it simple.

Philip, how thick is your 13.1" mirror and what's it made of?

Cheers

[Satchmo]

Some more info: I've used the 60% ring successfully on 11.7 " and 12" X 27mm thick and 14" X 37mm thick The 13.1" Coulter is likely to be 30mm or so.

Its not so hard to cut up 3 triangles of 1/2" ply for a flotation cell if you aren't happy with the results:)

[rmcpb]

Mark,

Sooooo, a 1" 13.1" Coulter would work on six dobs on a single ring at the 60% diameter. Boy, wish I had known that before I played with the cell I eventually made. Even so I think I may just give it a go and compare the result, lots of cloudy weather at present :)

Thanx

[5ash]

Hi ,
the mirror is about 1" thick , What i dont understand with the flotation points for a cell is how they are fixed, i get the impression that some movement is necessary and think that silastic support at the right points might provide same?? Sidgwicks book"amateur astronomers handbook provides details on position of the points for a 9 point flotation cell.
regards philip

[Ian Robinson]

If I remember correctly they were full thickness plate glass. (I think 1" or 1.5" thick was about as thick as it came then). I don't think they were annealed in anyway either prior to grinding and figuring and polishing. They were the poormans big scopes of the day... I think they also had a 16" dob too.
Too bad the owner of Coulter Optical (the maker of the Odessy Line of dobs) had a massive heart attack and died. He was a very nice chap. Similarly with Kenneth Novak who did very fine mirror cells.

Velcro's fine for dobs , where the mirror is never likely to be rotated to an orientation where it might fall out. I'd be very hesitant about trusting velcro to hold a mirror in telescope on a equatorial which might wind up pointing downwards or even just very near or just past horizontal in park position .... that mirror might pull free and fall off the cell .... Ouch !!!! :sad:

[5ash]

[QUOTE=Satchmo;389158]Nothing sticks to glass better than Silastic. They use it to lift the 8 meter VLT mirrors :)

. A single ring flotation is extremely forgiving and gives excellent results on 14" and under mirrors, in fact better than a 9 point cell. I have mounted 3 mirrors between 14" and 12" with a single ring of velcro and it works superbly. I pipe up about it every now and then when questions like this come up.

What exactly is the definition of the 60% zone? How exactly does the ring of velcro work?? The ring of silastic holds the mirror down at the 60% zone and the velcro??
philip

[GrahamL]

love the velcro :thumbsup:

please excuse my mirror cell I originally planned to make the whole thing frame and all bolt together but changed my mind

Those are heavy duty velcro brand adheasive squares cut from a metre length you buy it in
I have three edge supports which barely touch the mirror.. the ring you see is centred @ 60% of the mirror D
Pic 2 there is me trying to shake it out .. velcro don't let go .. so much so I clean my mirror in house :)
I routinely place my mirror box on the grass beside my rocker box(on edge) as i set up and sometimes leave it there for hours.
The mirror falling out just is not an issue .

[5ash]

What is the 60% zone? and when using a ring of silastic is it used to stick to the mirror or as a support when set .
philip

[Satchmo]

60% zone is a ring support of 60% the diameter of your mirror.

The silastic is used in a ring to glue the mirror down , but not compressed less than say 6mm.

I recommend Velcro instead for a 13" mirror. Steady pressure with a wooden wedge over a few minutes will release the Velcro. There is also enough slight give in Velcro to allow a floatation cell to work so you could also use it to attch to a 9 point cell. Ring supports however a very good at not transmitting any twisting of the back plate to the optics , so are very forgiving.

The last time I glued a mirror down I had to destroy the cell to free it for coating !

[skies2clear]

Ouch! In that case I think Velcro would be the easy method on the agenda to try. Silastic is pretty tenacious stuff.

[5ash]

Thanks for all the info i think ill try to mount the mirror at the 60% zone using velcro . Will post my results sometime in the future when i ve finished making the cell.
regards philip:thumbsup::hi:

[5ash]

Have now finished and tested the mirror cell using velcro at the the 60% zone to attach and float the mirror . The images look good at 150x , only had an opportunity to test it on M42 and the pleiades from my verandah. I am quite suprised by the images since the mirror is "only 17millimetres thick:eyepop:" and 330mm in diameter. Pictures of the mirror cell are attached .
regards philip