Re: (ATM) ATM planetary scope window
Max ben-Aaron (xeno357@ix.netcom.com)
Tue, 22 Aug 1995 21:33:22 -0700
You wrote:
>
>>>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?
>
>
High their:
Has it occurred to you that your analysis may be proving that
refractors could not possibly work?
Mb-A