But if you are like me and the ever increasing number of people who wish to start their telescopes, let them do their magic, then analyze results the next morning then read on. This little article is for you. How lovely the first 50 images look after you have focused. How aluminum they are. Therein lies the rub.
Aluminum loves to contract when the temperature drops.
So as the temperature drops, the distance between the primary and secondary in your SCT is smaller therefore the focal point moves farther out. So, given the evidence it is clear that the culprit is aluminum when the telescope focus changes due to temperature. Okay, that's the bad news. This means that for every degree C. the tube shrinks there is X mm. If you made a plot of the amount of change in the focal plane out compared to the temperature dropped, it would be a straight line.