Re: [ATM] Updated web site: mirror testing and Draper machine

["Bill Kelley" <flex@commspeed.net>]

List,
Some reasonable argument can be made here for flex mirrors. We make them of 
plate glass and field tweak them for the conditions. Exquisitely smooth 
spheres are easy, and a Foucault null beats parabolizing and zonal testing. 
Before I get flamed, yes, I also parabolize mirrors occasionally- usually a 
replacement mirror for someone.
Bill Kelley

----- Original Message ----- 
From: "Mike Spooner" <spoon@PageAmerica.net>
To: "Ric Rokosz" <radicell2@yahoo.com>; "scott" <srychnov@chem.ps.uci.edu>; 
<atm@atmlist.net>
Sent: Tuesday, November 30, 2004 10:00 AM
Subject: Re: [ATM] Updated web site: mirror testing and Draper machine


> Hi Ric,
>
> For smaller mirrors (<10") it probably doesn't matter a lot but for larger 
> mirrors it makes a difference. Even in the small sizes, overcorrection of 
> any amount is a problem so my preference is also to have slight 
> undercorrection even with these. I often test (autocollimation) with my 
> shop temps dropping 2 to 5 deg F per hour and recall a high end 20" mirror 
> I refigured that was grossly overcorrected when I first received it. 
> Testing in the morning while warming the shop showed a fairly smooth null 
> but when cooling it grossly overshot (my guess was 3/4 wave). It was not a 
> smooth figure at that point either. This performance had been verified in 
> the field and the mirror had been declared "within spec" when sent back to 
> the original optician. I saved .jpgs of the before and after. Some few 
> small pyrex mirrors are almost as bad as plate glass but for the most part 
> the majority do well. If the mirror can be kept close to ambient then 
> close to full correction is desired. Guessing how to correct for good 
> correction in actual use is problematic but ATMs have the star test 
> available as a final judge. Most of my personal scopes have low expansion 
> substrates and I find the testing and figuring a joy using them (but not 
> so joyful for the pocketbook). A well annealed piece of pyrex is also 
> pretty good.
>
> Best,
> --Mike Spooner
>
> .
>>
>> There is one point he made,that really has me
>> perplexed,the statement,just below the image of mirror
>> B were he says one needs to keep pyrex mirrors
>> slightly undercorrected -since they cool off all
>> night.
>>
>> I wonder if this is the professional opinon,or just
>> Roger's view?This topic has been covered a lot on this
>> list and I for one got the feeling that the group view
>> was to correct the surface to the best figure one
>> could.
>>
>> Ric
>>
>
>
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
>
> 


_______________________________________________
ATM mailing list http://www.atmlist.net/