Re: (ATM) ATM planetary scope window
bruce mcmath (jbmcmath@intellinet.com)
Tue, 22 Aug 1995 20:35:52 -0500
>>Re: Quality of optical window.
>>
>What would worry me is the effect of wavefront error caused by temperature
>differences across the plate. Lets take alpha=7.1*10^-6/K, index n = 1.517
>(i.e. BK-7). Consider light passing through a thickness a of glass followed
>by thickness b of air. Light with wavelength l in air has wavelength l/n (I
>think) in glass so the path length is (an+b)/l wavelengths.
>
>Now make the glass thicker by d. Path length becomes ((a+d)n + b-d)/l
>wavelengths, i.e. (n-1)d/l wavelengths more. Say we want to limit this to
>1/8 wave, l = 22*10-6 inch then d<=5.3*10-6 inch. If I want an optical
>window 0.5" thick I must not have a peak to peak variation of temperature
>within it of more than 1.5 K.
Roger: how does the above compare to heat differances in the mirror itself.
Anyone who has done a focault test knows how sensitive the mirror is to temp
changes. Unless the above is much more sensitive than changes in the mirror
surface do to temp changes then it clearly would not be a factor. the
mirror would obviously be much more massive. Presumably if the mirror has
aclimated the window would as well, would it not?