Last night the conditions were favorable to try my Canon 500mm f/4L IS lens for astrophotography. I was interested in the maximum exposure length with only a simple polar alignment (with no time-consuming drift alignment). Also wanted to check out any other issues that might arise from using such a heavy lens.

Well, it turned out that 2 minute exposures were good, so I made a series of 2 minute, 1 minute, 30 second and 15 second exposures. 35 in total – the shorter ones to be able tame the bright core of the nebula. When I loaded the images onto my computer half an hour ago, I realized that focus drifted horribly after the first three frames… I suspect the rapidly falling temperature being the cause. Temperature change induces a focus shift. That’s why you can focus lenses “beyond infinity” – to compensate for this shift.

So it turned out to be an exercise in recovering what’s possible in such a badly screwed up situation. Below is what I could recover from these three frames. I was surprised that the overall result is pretty good, just disappointed about the overly bright core.

M42

Because refocusing between frames is not a viable option (think image position changes and lack of bright enough stars), I’ll look into telescope heaters… I’ll let you know when I will have a working solution.