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I recently tried imaging M7 with my 6" f/4 Newtonian. I had earlier commimated it with a Cheshire and Howie Glatter and was sure of the collimation. However, when I imaged using my DSLR with the coma corrector installed, I get focused stars off centre and not on the optical axis. Anyone experience anything similar before? What could this be? Tilt in the optical train? The focuser was drawn out only about 5mm to reach focus along with a 50mm extension tube. Any suggestion is welcome.
By Anthony RS
I bought a GSO 2 inch Coma Corrector for my 8inch Newtonian. I'm still having terrible coma on one of the corners which is actually worse than without the coma. The rest of the corners look fine with proper guiding.
Here are some details and what I've done to try and solve the issue:
1- Spacing is 75 mm just as recommended.
2- Used a Cheshire, laser collimator, and a webcam to check collimation.
3- Squared the focuser so that it's orthogonal to the optical axis.
4- Tried another DSLR
Note that once I collimate and double check that I'm collimated with all the tools, I always try to do a star test and collimation appears off (the dark spot isn't in the middle). Last time I tried to collimate the primary using a star but it's really tough to fine tune since due to the focuser's sag, I can't be sure that the star is in the middle of the fov.
My take on the problem is that when I'm using the DSLR with the coma corrector, the weight is moving the focuser away from the center of the optical axis but I'm not sure this if this is the case. I've tried everything and I'm out of ideas.
Below is a test image I took for Lagoon Nebula. Notice the coma on the left top side mostly, while the right side usually appear to have no coma (not the case in the attached image though for some reason, but usually the right side is fine). One more thing I've noticed, the stars on the left corner appear to be out of focus while the rest of the image appears well in focus (used a Bahtinov mask with APT bahitnov focus tool)
There's also another image I took for a distant light to check collimation (since clouds covered the sky as soon as I decided to do a star test). It appears to be fine and I was able to center it better but didn't capture the image.
Please let me know if you have any idea what's causing this. My number one suspect is the focuser's sag preventing me from doing accurate collimation although everything appears to be normal while collimating. I'm giving this another 2 weeks of my time, if it doesn't work I might quit astrophotography till I'm able to afford an APO.
One more thing that needs to be added, when using the GSO CC I had to move the primary mirror up the scope by around 2 cm to be able to achieve focus.
Sometimes the weather gods look on you kindly. A nice new Samyang 135mm turned up from that well known jungle supplier, I managed to assemble it to my Atik 428ex mono along with an Ha filter and got the spacing correct, and low and behold the skies stayed clear - all on the same day!
I am sure there is room for improvement with the processing but this is IC1396 processed in PI; STF applied with HT, and a little HDRM + LHE to try to tease out some detail.
Any help on improving the processing would be gratefully received - I really don't feel I get the best out of processing mono images.
By the way this is 12x300s + 10x600s combined with HDR process in PI; flats, no darks and a master bias.
Thank you for looking.