Re: ATM BVC Weirdness part II

[Aplanatic@aol.com]

Even Pyrex shows a significant change in figure during equilibration.  A case in point:  

We recently made a 14.5" convex secondary for our 42" corrected DK cassegrain.  We tested the figure of this mirror by interference against a matched concave test plate.  After a figuring session we would wash the mirror, dry it and lay it on top of the spherical test plate.  At first the mirror would show huge changes in figure (at least 1/2 wave wavefront) as the effect of washing (and thereby cooling the face of) the mirror would subside.  In addition, it was clearly evident where we placed our hands on the edge of the mirror to turn it over on to the test sphere.  This first set of changes in the mirror would last roughly 15 or 20 minutes as the mirror equilibrated through its thickness.  At this point the mirror was within, approximately, about a 1/4 wave wavefront of where it would eventually end up.

During the next hour, the areas heated by our hands (and other, non-uniform temperature regions) would slowly die out, mainly near the edge of the mirror.  So, after an hour and a half, the mirror would be within, say, a 1/10 wave wavefront of where it would finally end up.  It did not fully equilibrate for at least two hours, maybe three, depending on who was looking at the fringes.  All this with a Pyrex mirror in a very stable environment.

As you might recall, the thermal diffusion time constant goes like the length scale squared.  Thus, it thermalizes quickly through its thickness but very, very slowly across its diameter.  A two inch thick Pyrex mirror has a thermal time constant of about 15 minutes in the thickness direction, but a time constant of many hours across its diameter.

Even with Pyrex it's a good idea to let the mirror sit on the test stand for three hours (or best, overnight)before making that final determination of the quality of its figure.

So, given all this, how would the mirror perform in a telescope under various initial conditions and changes in the enviroment?

Dave Rowe