Gads, turn my back for a couple of hours and look what happens.. a whole thread appears out of nowhere...
First, there's a link to the temperature graph for that night on my website along with three avi's showing how the seeing went from poo to perfect as the night went on and the temps all lined up. The link is
http://www.acquerra.com.au/astro/cooling/ballarat/
umm, and an article that describes the temperature sensor thingie that I built... it's really just a prototype, hence all the wires n stuff haning out where everyone can point and giggle, but it works:
http://www.acquerra.com.au/astro/equipment/temp-logger/
You can see where the temp was stable for a while and then suddenly started to plummet at around 11pm. After a few minutes of this I went for a walk around the various groups and heard a few grumbles about poor seeing, including the incident DP related in his post.
I'd agree with asimov and others here that getting the mirror to within about 1 degree of ambient should be considered the bare minimum, if you can do better then you should because the results will be worth it. If you look at the 2nd avi on that link then you'll see how a difference of about 1 effects the image, compare it to the 3rd avi when the difference is down to less than 0.25 and the improvement is remarkable. 1 degree doesn't sound like much but it's enough to set up a boundary layer that distorts the effective shape of the mirror by *a lot* (technical term).
If the mirror is more than 2 degrees warmer than ambient then you'll just see a fuzzy blob and you're not likely to get anything to focus.
It sort of puts things in perspective when you can have a wonderful scope, pefect collimation, beautiful mirror, expensive camera, and have it all ruined by a warm layer of air coming off the primary. That pretty much describes how I spent the mars opposition of 2003...
Okay, you refractor guys can stop sniggering now
regards, Bird