Shooting with the modified ONTC has been a fun journey this year. We had lots of clear nights at the end of Spring, so plenty of opportunities to test my ESHR coma corrector and telescope modifications.
NGC 5371 and Hickson 68 galaxy group
Luminance from this image came from my GPU corrector, color was captured with the ESHR corrector. Both datasets were easy to align. The largest spiral galaxy NGC 5371 clearly shows some pink star-forming regions. This is a feature of the AstroDon E-series RGB filters, the red filter is a bit more narrow than usual and naturally enhances the contrast of H-alpha regions. (Click the image for a larger version)
Dates: 27 April, 02 Jun 2022
Exposure: 59 x 180 sec L, 20 x 180 sec per RGB (6 hours total)
Globular cluster Messier 13 in Hercules
Deconvolution is a fantastic tool for high resolution setups. I use it for sharpening of the stars and bright details in my images. Globular clusters are an excellent candidate for this process. The animation below shows subtle changes in clarity of the core and larger stars are reduced to a sharper, more focused point of light as the effect of atmospheric blurring is undone.
Below is an image comparison with a reprocessed dataset from May 2019, captured with my 80mm APO refractor (image on the right side). You will immediately notice the difference in the star colors between both images. Chromatic aberration is much better controlled in the image from the Newton (left side), and stars are also slightly sharper.
I intentionally chose to keep the brightness of the core on the low side to better show the star color all the way to the center, as high brightness washes out color. Full field of view below (click for larger size)
Date: 13 Jun 2022
Exposure: 30 x 10 sec L, 30 x 60 sec L, 12 x 180 sec per RGB (1.2 hours total)
Spiral galaxy NGC 4236 in Draco
A large galaxy with the size of almost the full moon (22′) that is surprisingly only rarely imaged. It has very low surface brightness and that immediately explains why it is not a popular target. The brightness of the barred core is not too bad but you need dark skies and lots of exposure time to separate the spiral arms from the background.
There are plenty of star-forming regions in NGC 4236 and a H-alpha filter was useful to enhance these pink patches.
Dates: 03 Jun 2022, 06 Jun 2022
Exposure: 93 x 60 sec L, 12 x 180 sec per RGB, 14 x 300 sec Ha (3.3 hours total)
At my location of 52 degrees North, there is almost no real astronomical darkness during this period of the year and the atmosphere will remain partially illuminated by sunlight. In the astronomy community these are known as ‘grey nights’, referring to the lack of real darkness. Views through the telescope will appear more washed out. There are still lots of possibilities to image during grey nights when using narrow band filters, especially with very narrow 3 nm filters as they block most of the unwanted sunlight.
LDN881 and the Gamma Cygni region
Many dark nebula are visible in this view, located close to the bright star Sadr (Gamma Cygni). The most prominent dark structure is called LDN881 and it looks a bit like a black soap bubble with curly edges. I could not find a specific name for the bright nebulous structures on top.
The color image was composed of only 2 channels: data from my SII filter was used for the red channel and H-alpha was used for the blue channel. The green channel is a combination of both SII and H-alpha data. The results look similar to a classic Hubble palette without using any OIII data. SII emissions are usually weak but I was really surprised by the amount of SII signal in this nebula.
I always find it very helpful when other photographers post their individual channels, so feel free to compare the starting stretched images (greyscale) with my final result.
Dates: 13, 14, 15 Jun 2022
Exposure: 71 x 300 sec Ha, 62 x 300 sec SII (11 hours total)
Sh2-115 and Sh2-116 in Cygnus
This is another collaboration image with French astrophotographer Axel. I was helping him with collimation and setup of his new Skywatcher 150PDS Newton. Axel lives in Paris – The city of light – so we choose an object that we could both image with an H-alpha filter as it suppresses most of the light pollution. Unfortunately, his Newtonian likely suffers from pinched optics and the final stack has triangular stars. I used StarNet2 to remove the stars from the combined dataset and I replaced them with nicer RGB stars from my ONTC telescope. H-alpha was used as luminance. The stack is pretty deep so I was able to apply some extra sharpening to key areas of the nebulosity.
Dates: 04, 05, 06 Jul 2022
Exposure: 8 x 60 sec for each RGB, 38 x 300 sec OIII, 39 x 300 sec SII, 27 x 300 sec Ha (ONTC), 113 x 300 sec Ha (150PDS) (20 hours total)
Again my individual stacks for comparison. The bottom right side of the OIII and RGB images shows a circular artefact: this is a reflection from the bright star Deneb (outside the field of view).
Sh2-117 in Cepheus
A very large nebula complex with lots of dark structures. Also known as NGC 7822. It’s a tricky one though, I see a lot of eye-popping images online but the signal in H-alpha was quite a bit weaker than the other nebula in Cygnus. Getting good color separation from the OIII and SII filters was not too hard though, despite the lower brightness. The image below is tone mapped with H-alpha as luminance.
Dates: 18 Jun, 10 Jul 2022
Exposure: 44 x 300 sec Ha, 21 x 300 sec OIII, 23 x 300 sec SII (7 hours total)
These are the individual stacks, the final image was cropped a lot because I didn’t want to rotate my camera after dialing in the tilt adapter.
Gear for all shots:
TS-Optics ONTC 20 cm f/4 with ESHR coma corrector
ZWO ASI1600MM-Cool and 3nm AstroDon filters
Mobile setup on Skywatcher EQ6-R mount
50mm guidescope with ASI120MM guide camera
Acquisition managed by Sequence Generator Pro
Staking and post-processing in PixInsight
Astro Virus 