Re: (ATM) ATM planetary scope window

Roger Moss (roger.moss@eng.ox.ac.uk)
Wed, 23 Aug 1995 10:17:06 +0100

>>>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) ino 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.
.............................................................. >Comment from bruce mcmath:
>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?

In theory, changes in mirror shape will have a much greater effect as a shift d in the surface makes a 2 * d wavefront error rather than a (n-1) * d error - so with a plate glass mirror, temperature effects in the window will be unimportant. A low expansion mirror will have an expansion coefficient half that of the window (pyrex) or almost zero (zerodur), and in this case one may have to worry about the window temperature. The problem (if there is one, I am only theorising!) is essentially that one cannot get low expansion optical glass. Mirrors also suffer more because any thermal stressing will distort the whole blank.

What one needs to is make a dummy telescope with an aluminised mirror blank and plate glass window, stick thermocouples over them, leave outside at night and see how large the temperature differences can be!

Obviously if one can maintain the scope at near night time temperatures throughout the day there will be no problem. .............................................................

Regarding the general problem of tube currents, the problem is not that the air is moving (air is invisible, after all) but that the convection currents move because of temperature differences, and the refractive index of air changes with temperature. What you have is what would be known by fluid mechanicists as a "thermal boundary layer". Now boundary layers are commonly removed by having a porous surface (or one peppered with tiny holes) and sucking air through it. I suspect that one would get seeing as good as with an optical window by making a double skinned tube with holes in the inner wall and sucking this warm away; for good measure one would have a central hole in the mirror and suck here also, in the shadow of the secondary. It would be much easier to make some holes in a baffle plate than to make an optical window.

I believe some observatories do this on a larger scale, sucking air downwards through the dome slit and then passing it through a large pre-cooled "cold room" so that when it emerges again, some distance away, it is at ambient temperature and does not make a plume of hot air.

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