Imaging with CGE1400: temperature factor

In the beginning of July 2010 I tried to shoot luminance part for M57 image. The evening was very hot, but later it started to cool down quite fast, I think the total temperature difference between the beginning and the end of the session was at least 20F. It caused a significant shrinkage of my C14 tube resulting in a loss of focus during the 2.5 hours of imaging. Interestingly the temperature change wasn't uniform (as the focus change) - initially it hold almost steady, and then started to drop dramatically. Instead of producing a beautiful luminance for M57 I produced a video of thermally induced defocusing of an SCT. Interesting study, but not what I had in mind. To add an insult to injury defocused stars showed some miscollimation . The video is below:

I had some indications of a slight focus change in the past (a bit "soft" focus in the last frames), but this one was the largest one I have ever seen. This problem can be alleviated by refocusing during the session (annoying!) or using a temperature compensating focuser (much better!). I like to have my visual observing sessions unperturbed (I observe with my Obsession when images are being acquired) so of course I opted for a new focuser ! I bought Optec TCF-S temperature compensating focuser to fix the problem.