OK, it is 10 PM and you have focussed carefully and gone on to find your object, calibrate your tracking, and start your exposure. 30 minutes later you download the first image and something looks wrong. You are out of focus! What happened? The graph below tells the sad tale. This shows the "cleanest" of several graphs I made following focus drift thru the night.
The following two graphs are similar representations, one using OTA temperature and the other outside air temperature. I did not graph the response of my RC Cass, but have noted similar amounts of drift during routine imaging. The big point here is that unless your scope has been athermalized, you need to consider refocus quite often. One focus per night just won't do it! There has been some discussion in various forums recently suggesting the possibility of predicting focus position based on temperature sensor readings. This may be a solution, but I must admit it makes me very nervous since during my studies I have seen some very unexpected "jumps" in focus. As mentioned above, this is the "cleanest" graph I was able to obtain.