Cool down time for a SCT

I've read that it can take up to an hour for a SCT to obtain equalibrium with the surrounding air temperature and until then images may not be that good. Does anyone know where this assumption comes from? Why should a SCT take any longer than a refractor, for example, to cool down, as both have enclosed optics?

The lens in a refractor is at the end of the tube where it can radiate very effectively to the night sky. The SCT corrector (or Mak front element) can also cool quite effectively, but the mirror inside the tube can not radiate its heat in a similar fashion, nor is there air moving through the tube to hasten cooling as there is in most Newtonians.

First of all I would say an hour is about minimum a SCT needs and it could be alot more than that if you see a constant drop in temperature throughout the night. A refractor has the optics exposed and at the far end of the OTA. An SCT has a corrector plate in the same position but that is not the problem. The problem is with the mirror, it retains a good deal of heat that has nowhere to go because of the enclosed OTA. Another problem with the SCT is heat from the secondary, since it is positioned mostly on the inside of the corrector plate the heat plume will rise straight up, un-disturbed from any air movement because of the closed system. This heat plume can be seen easily in a out of focus star test as a dark snake like smoke trail of sorts comming from the secondary and going all the way thru the defocused star and pointing in whatever direction is up.



Robert W.

In addition to the points above, it's also important to remember than larger scopes in general require longer cooldown. For instance, an 8" SCT takes a while to cool down, but so does an 8" refractor! :^)

If I know ahead of time I will be observing at night, I just put the SCT out before dinner and it's all ready later. For last minute observing, I grab the small refractor (or lately the small Newt).