ATM Big scopes

["Daniel Bakken" <dbakken@cet.com>]

> Hi Dan,
> 
> Thanks for good info.

You're welcome. 
> 
> I am impressed that you did it all by hand. I wonder if you did the initial
> grinding by hand too? It's funny cuz a lot of glass pushers  I talk to say
> its " nearly impossible " to do it by hand because of the force required to
> move over a surface that large. How did you handle this problem...more
> lubrication than normal for small mirrors, or is this just an old wives tale?

Curve generation was done at Glass Mountain. You can read more about 
the process on my web page:

http://www.cet.com/~dbakken/Fortyone.html

Grinding was done with a 30" tool made and used by GM and a 20" from the 
33" I did that had a very close R of C. Grinding was easy. Polishing 
was tough, but manageable with 2 people. Pushing the tool back and 
forth. Like Jeff Baldwin commented, after 5 minutes of working on it, 
the size wears off, and its just another mirror. 

> 
> I was told that the advantage of grinding out the initial curve by yourself
> vs: getting it done preform is that you now have a formed lap to use for
> later polishing. I need more info here..don't quite understand what they
> mean....

I see where they are coming from, but it isn't important. It is MUCH 
easier to get someone else to curve generate this size mirror. 
Generating with grit would take literally hundreds of hours. When we 
did the 33", we recieved a blank that was curve generated without a 
matching tool. We just cast a plaster tool on the mirror, epoxied 
ceramic tiles to the curved face, and ground for maybe 30 minutes, 
and we had a mated tool. No big deal.

> 
> I wonder if for a polishing machine if one were to get a used car front wheel
> assy...the bearing/axel assy, and mount that with a motor. What speed is
> suggested, or (radial turn rate)...is there some formula? 

I am ignorant of machine parameters ( I shouldn't be).

> 
> I wonder what will happen in the real world if there is a little wobble or
> vibration...will the arm holding the lap usually just follow this motion as a
> free moving assy??

See above.
> 
> I don't mind hand polishing at all if it is not so hard that it is much
> better to build a machine...I think doing it by hand would be much more fun
> and probably less apt to destroy the blank.

Unless you do many mirrors, I don't really mind working by hand. I 
wouldn't hesitate to do it by hand.

> 
> I wonder if anyone knows of a "slumping house" that has the forms already
> made from a previous project to do a 40" + mirror ???.....the F ratio could
> be anything from 3.5 to 5.
> The price is just too high to do new forms, though I like the idea of
> slumping.
 
I have heard bad things about Newport slumping. Glass Mountain will 
slump, but I don't know off hand what molds they have. Since my 
project worked out, don't feel like you have to get it slumped.

> I read that Swayze with his 40" mirror used some software that Mel Bartels
> wrote to produce the curvature lines produced in a Ronchi test. Mel, is that
> still available?? He thought of it as very important. How did you test yours?
 
I did use Mel's Ronchi software. The test that worked best for me 
when we got there was the wire test. It is a variation of the 
Foucault test, but much easier to use with large fast mirrors. Star 
testing in the telescope is a must.


> Dan, did you do the whole process with the blank in its mirror cell or a flat
> surface? If so, guess I will make up the cell first and make a pivot to tilt
> up for testing. Unless you can polish it on a flat surface and then use a
> sling for testing...since the sling is the only thing holding it in the real
> world when vertical anyway? 

I did the mirror on a flat surface with wet shag carpet. The mirror 
was tested in a seperate test stand with a sling and carpet behind 
it. A tilt up barrel top for testing was considered, but I already 
had the test stand from the 33" project. It would save you from 
physically moving the mirror from the barrel to the test stand.
> 
> What does the mirror weigh?... about 260 lbs as Newport says?
> Too bad about the tapered edges problem. If  not for that, a 48" mirror would
> be possible. I can't think of a way around that problem though.
> I wonder if there is a source of larger fine annealed Pyrex like this w/o
> this taper. I also thought of the possibility of fusing two sheets together
> or maybe fusing edges to make it larger before the circular cut. This is
> probably fantasy  considering the cost and other potential problems.

Yeah, kinda far fetched. My mirror weighs 185 pounds.
> 
> I was surprised to hear it cost $1800 to coat the mirror...man, guess it sure
> is an expotential thing. My 22" only cost around $350 for regular
> reflectivity, if I remember right.

Sticker shock for sure.

> 
> Do you have Mel's homepage? I want to give the Alt/az one more look. I still
> don't like the idea of a platform though, adding one more axis. I see that
> JMI with their NGC scopes and their 40" scope all use the split ring design
> and are pretty low to the ground. And evidently they track and point pretty
> well for a large amateur grade scope. I've got a lot of pictures of split
> rings from the various star parties and have a few thoughts in mind. I can
> see why you feel that way about the high C.G. of the GEM, but why about the
> split ring?

If you look at the height of the mirror in a split ring versus a alt 
az, you will see it is higher, unless you make the ring very large. 
This isn't a big problem in smaller scopes, but becomes very 
significant in these beasts. Remember if you want to look at the 
zenith, you'll have a big ring to get around. JMI does NOT use a 
split ring on their 25 or 40" designs, both use a tracking alt az 
mount.
> 
> You do need a suit and a big smile to sell the 40" er for $200,000 however.
> 
Amen brother.


> Enough for now.
> 
> CU 
> 
> Tom Cathey
> K1JJ@AOL.com
> 
> 
Take care.