(TL;DR see below)
We have bad weather (yeah, I know, big news). So what to do with a scope, that I barely used but eager to test on some nice DSOs? Learn to use it properly and optimize the **** out of it. At least as far as a beginner like me can do. I only have made some pictures while the moon was up, because the only clear nights in last months had at least 60-70% moon. So I photographed the moon and star fields. But first comes first. The equipment I use is:
- EQ6-R (with the v5 handset, which is great because it has a USB port, so no "expensive" cables needed)
- 8" f/5 Newtonian (which is round about 1.000 mm focal length).
- GPU coma corrector (4-lens design, threaded to the focuser)
- M48/EOS adapter threaded to the corrector
- Canon EOS 6D Mark II (yeah, I know, not the best for astrophotography but I am a "daylight photography child")
Before buying the newtonian to get some serious astrophotography done , I only used a small refractor to learn the basics (Skywatcher EvoStar 72) and in between also used an SCT 9.25" (owned by my father in law). The EvoStar is now my guidescope.
So since I got the scope I tried to learn collimation and do it as best as I can, which I think i succeded. Because I had a lot of cloudy nights to train those steps. The attached images where made yesterday evening. Before this attempt I didn't improve the collimation, just the a quick check on a defocused star and saw a slight miscollimation but left it, because my main topic was to check the distance between camera sensor and coma corrector. And this is the point, where I am unsure about.
Some additional information: The GPU coma correcter is a 2" corrector and used in a 2" focuser. So the vignetting is expected and not the point, that is bothering me. Keep in mind: I use a full frame sensor (36 mm width).
The montage of the corner stars is from a image made yesterday, which has very little stars visible, because I made a very short exposure to elliminate effects of guiding or poor polar alignment. The background is bright because of the moon.
The stars are a bit distorted. Through my tests in the last weeks and month I also had way worth corner stars than this. One problem was a distance ring, that was not perfectly machined and introduced a slight tilt.
After the results got better and better, I downloaded the test version of CCDinspector, which opened the door to insanity for me. As you can see from the attached images, there is still tilt (which I think, I can ignore and maybe introduced by the M48/EOS adapter from Skywatcher).
The field seems to be okayish centered. But there is a lot of curvature (at least 34% sounds like a lot for me).
Is this amount of curvature to be expected with that setup or is this an issue of imperfect spacing betweend corrector and sensor? And if so, should I increase or lower the distance? I am not totally sure from the stars (not as I have been by star images of my refractor with bad spacing).
Thank you very much and clear skies for all of you!