A Cold Mirror

[Solanum]

We have all seen/know the problems with a mirror that is significantly warmer than ambient air, but in the summer I have the opposite extreme. The house (and presumably mirror!) is usually kept under 25C, but it can be well over 30 when I'm out observing at night (35C right now at 10 PM).

What effect will this have on my observing? I always put the scope out early with the fans on to warm the mirror up, but was wondering if I should leave it cold, would that create still air in the tube?

Any opinions?

[davidpretorius]

Yup you have a newtonian, so the best thing to do is get a Dick smith digital thermometer and put one end on the side of the primary mirror.

A few of us "actively" cool our primary mirrors and at times we can get the mirror below ambient. Either way, if there is a difference in temperature, then air will move.........causing you to lose detail in a big way

For high magnification imaging, then less than .5 degree difference is a must......and to be honest even observing jupiter or splitting double stars really works soooo well when less than .5 degree difference.

Get to know your local temperatures over the night.... you might find you consistantly only drop 10 degrees over night......down here in tassie, we can drop 15+ degrees and quite quickly.....hence the need for actively cooling the mirror

[Gargoyle_Steve (Steve)]

I remember last summer on a particularly hot and muggy night we had the air con running inside, keeping the house a pleasant 25°C. I took the scope outside, thinking it would be good to have the mirror well cooled before going out.

Well it took less than 3 minutes for the mirror to get that frosty wet look on it, the same that you get on a glass full of coke and ice. Another 3 minutes and it was wringing wet, water droplets running of it like there was a rainstorm inside the tube.

Yes, it's good to have a mirror cool to start - no, not noticably cooler than outside ambient temp especially when it's quite humid outside. I think ti took the mirror longer to stabilise it's temp than it does normally.

[Alchemy (Clive)]

thermal management of newtonian mirrors is an interesting subject

i refer to an article in jan 2002 sky and telescope (us) if you can get your hands on it its a good read, essentially it discusses the benefits of having a fan in FRONT of the mirror, a calculation program can be free downloaded from http://http://skytonight.com/resourc...tml?page=4&c=y

you can download the article but its not free.

it suggests that any temperature difference of about 1.5 deg f will produce heat waves that will degrade the image ... BUT with a front fan good images can be taken at up to 10 deg f difference.