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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.
The Fighting Dragons of Ara ( NGC 6188 )
( please click / tap on image to see larger )
The Fighting Dragons of Ara ( NGC 6188 )
Bright Nebula NGC 6188 and open cluster NGC 6193 are embedded 4,300 light years away in the Sagittarius arm of our Milky Way galaxy and can be seen with the naked eye south of Scorpius in the constellation of Ara.
With powerful stellar winds and energetic ultra-violet radiation, massive stars sculpt the interstellar gas and dust of the nebula into wonderful shapes and cause the interstellar gas to brightly fluoresce.
Closer to the hot young stars of the cluster, bright blue “sunlight” reflects off the clouds of gas and dust to produce the blue reflection nebulae seenin the image.
Magnitude +5.19, RA 16h 41m 42s, Dec -48deg 48' 46".
Approx. 3800 light years away.
This is an HDR image constructed from exposures ranging from 2 seconds to 240 seconds in length. The aim was to capture the faint stars and details in the nebula whilst at the same time maintaining colour in the bright stars without clipping the highlights.
Resolution ........ 1.336 arcsec/px
Rotation .......... 90.002 deg ( North is to the right )
Focal ............. 1475.57 mm
Pixel size ........ 9.56 um
Field of view ..... 58' 28.5" x 39' 0.8"
Image center ...... RA: 16 40 09.903 Dec: -48 41 27.00
Orion Optics CT12 Newtonian ( mirror 300mm, fl 1200mm, f4 ).
Corrector: ASA 2" Coma Corrector Quattro 1.175x.
Effective Focal Length / Aperture : 1410mm f4.7.
Mount: Skywatcher AZ Eq6 GT.
Guiding: TSOAG9 Off-Axis-Guider, Starlight Xpress Lodestar X2, PHD2 .
Nikon D5300 (unmodified) (sensor 23.5 x 15.6mm, 6016x4016 3.9um pixels).
Blue Mountains, Australia
Moderate light pollution ( pale green zone on darksitefinder.com map ).
Capture ( 24 June 2017 ).
8 sets of sub-images with exposure duration for each set doubling ( 2s to 240s ) all at ISO800.
34 x 240s + 10 each @ 2s to 120s.
Processing ( Pixinsight - 19 Aug 2017, 13 Jan 2018 ).
Calibration: master bias, master flat and no darks.
Integration in 8 sets. HDR combination.
This is a reprocessed version using the data I captured earlier in the year...