A Quantum Diaries Survivor

Building a telescope is one of those things you can do with a huge variety of levels of accuracy. As yesterday's post shows, one can build a telescope that would have made Galileo drool in five minutes, given enough spare parts. But the amount of ingenuity and precision that is required if the maker strives for perfection, is apparently bottomless.

As I delve in the documentation of simulations of thermoconvective motion of the air in a telescope tube, of axial stresses to the mirror surface due to edge supports, and of thermal hysteresis of large thick mirrors, in the hope of determining what is best for the 16" dobsonian scope I am having custom-built by Romano Zen (shown on the left at the optical bench with another 16" mirror he is testing - not mine), I feel kind of helpless…  

But I am not that kind of guy who throws his hands up and forgets about perfection. I am a perfectionist, in truth, and if stimulated enough (I am more lazy than perfectionist at times) I will do what is needed to ensure the final product is as good as can be.

One of the things I am studying right now is the thermal equilibrium of the mirror with the surroundings. Imagine the usual picture: the telescope is carried to a dark site - typically cold, since mountainous - from its storage (my home), then is assembled, and then used. The outside temperature is initially lower than that of the mirror, which weighs 30 pounds and takes a long time to acclimate. Fortunately, Zen's mirrors are made with 1.6" thick glass, which is not too thick for a 16" diameter, and so take less time to radiate their heat away. Also, Zen's dobsonian scopes have an open bottom, which eases heat exchange with the ambient. Detailed simulation of the heat transfer from mirror to air have been made, and the time constants for a given temperature gradient are known.

What do we care about the mirror temperature ? We do, because the parabolic mirror surface shrinks while the mirror cools down, and that causes variations in the surface which spoil the precise focusing and introduces astigmatism in the image. At high powers one does not see pinpoint stars but sizable balls of light if the mirror has not perfectly cooled down. 

So, heat transfer by conduction is important, but it has been solved. 

However, how about IR radiation ?

A mirror exposed to the outer sky exchanges IR radiation with it, according to Stephan-Boltzmann's law. The high reflectivity of the mirror implies that the emissivity is low, but not negligible. If one makes some assumptions (with which I will not bore the reader here), one finds a radiated power of 1.5 Watts from the mirror's surface. The bottom also exchanges IR radiation with the environment, and the -say- 20 degree difference will cause more power loss.

Here comes the interesting point: if the mirror's back is painted in black, one gets 3 more watts of cooling, but if the glass is kept transparent one gets way less than that. Also, the 1.5 W from the front are only achieved if the mirror is left "looking" at the whole sky - i.e., with no light shroud around the truss poles of the mount, and pointed at the zenith.

4.5 watts are not negligible, and they should help sizably in acquiring thermal equilibrium faster.

I think I will ask Romano to paint the mirror's back in black…

This is splitting hairs, but the aim is actually to split (double) stars! The highest magnification powers are only allowed if everything is perfect - thermically, optically, and atmospherically. And I know people with large dobsonians who have to wait three to five hours before using the scope on planets during cold nights. I hope I won't need to wait that long!