Re: [ATM] ATM Digest, Vol 53, Issue 33

[chris forder <camkit@mweb.co.za>]

One solution would be to test the mirror on its cell whilst lying 
horizontal. Of course this is not easily done unless you have a staiwell 
and can hijack it for ATM purposes. However, such appropriation could 
star the third world war!

Chris


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> Today's Topics:
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>    1. FW: Large Thin Mirror (Ted Cohen)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Sun, 25 May 2008 19:43:00 -0700
> From: "Ted Cohen" <tcohen@blakeglobal.com>
> Subject: [ATM] FW: Large Thin Mirror
> To: <atm@atmlist.net>
> Message-ID: <000601c8beda$3709b470$4001a8c0@BLAKESF.LOCAL>
> Content-Type: text/plain;	charset="iso-8859-1"
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>
> Follow-up on earlier discussion related to mounting large, thin glass.
> ?
> Perhaps I was not entirely wrong about the need to consider support
> characteristics during manufacturing. Although, granted, the support
> characteristics must be different than those required during testing and
> mounting (especially if mirror's axis does not remain vertical). My point
> was primarily that it should be considered. For example, I found the
> following references to this issue in the book ?Opto-Mechanical Systems
> Design? Third Edition by P.R. Yoder in 2006: 
> ?
> Chapter 11, Page 518, paragraph 2 ? ?Even if the mirror is always to remain
> on Earth, conditions such as the direction of the gravity vector during use
> may be different from those occurring during grinding and polishing. The
> forces normal and tangential to the surface imposed during optical finishing
> must also be taken into consideration since they might be quite significant
> ? especially when large-diameter or very thin mirrors are polished by
> conventional techniques?.
> ?
> Ibid, paragraph 3 ? ?If the same mount is to be used to support a mirror
> during polishing as well as during testing, the additional loading due to
> polishing tools and auxiliary weights must be considered when computing the
> number of support points.?
> ?
> I think this supports my earlier assertion that proper support during
> manufacture of the mirror is important. However, I was wrong in implying
> that the various mountings under the different conditions of manufacture,
> test, and operation should be the same. 
> ?
>   
>>> Ted Cohen wrote:
>>> Rotation of the glass during grinding is not in question. The question 
>>> you are raising addresses whether one can grind, polish, and figure on 
>>> one type of surface (perhaps plywood, rubber, foam backing, carpeting, 
>>> or pitch) and then take the finished mirror, mount it in a floatation 
>>> mirror cell, with its different dynamic qualities, and expect the 
>>> figure to be the same in both cases. It would seem that when grinding 
>>> and figuring you would want the glass to have the same support 
>>> characteristics as when the glass is finally mounted.
>>> Am I wrong about this?
>>>       
> ?
>   
>> Mike Lockwood wrote: 
>> Yes, you are wrong.
>>     
> ?
>   
>> The mirror needs to have the same figure when it is being tested and when
>>     
> it is in its cell.
> ?
>   
>> The figure it has while sitting on the polishing machine really doesn't
>>     
> matter, and it is continually due to ?polishing and heating of the glass.?
> The glass will bend while it is being polished, so we require that a) the
> blank has the same physical characteristics (stiffness, thermal expansion,
> etc.) throughout the disk and b) that it is evenly supported so that it will
> deform by similar amounts during strokes across different diameters.? This
> maintains a figure of revolution.
> ?
> ?
> ?
> ?
> ?
>
>
>
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> End of ATM Digest, Vol 53, Issue 33
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>   

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