Turbulence and SCT Collimation

You will have, no doubt, read my tutorial about collimation. How, will you ask me, how are you supposed to collimate your telescope when an out of focus star looks like this. And that is out of focus, forget in focus a star as it looks like its break dancing. There is no way you are going to be able to see Airy disks in those conditions. Unfortunately, if your observing site is like mine, it looks like this 99 nights out of a 100…

This image is an avi made of Pollux in gemini with a webcam at prime focus. It was recorded because I just finished replacing the original collimation screws with “bob’s knobs” and I had to recollimate. To collimate, I used a 5mm eye piece, trying to center the unfocus star in the Fresnel rings. So, by looking at the video, you can tell that the collimation is not far off, but you have no way to tell if it’s spot on as it just moves too much…  Fortunately, there is a way to check collimation: you just need to stack all the AVI frames just as you would do with planetary photography, to average out turbulence under IRIS. I describe the procedure in “planetary imaging technique”. below is the result. While collimation is not perfect, it shows that it is not far out. And I managed to do it with turbulence. Just to show there is hope!

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The image matching the AVI is the one on the right. The one on the left is the same star, more defocused shot using the same technique.

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