Lying at a distance of 13,300 light years, NGC 6791 is a rich open cluster in the constellation Lyra that could be mistaken for a globular cluster at a glance. It is unusual in that it contains a population of iron-rich stars that are about 8 billion years old, a population of white dwarfs that are younger (6 billion years old), and a third population of stars that are only 4 billion years old. There is some faint dust in the background which I confirmed to be present in others' images. The 16th magnitude background galaxy next to the bright white star at the upper right is LEDA 63067.
This image is an LRGB composite comprised of data taken exclusively in Mode 3 (extended fullwell 2CMS) of the QHY268M camera. Although I have been shooting luminance in Mode 3 for quite some time, I had not tried shooting RGB in this mode until recently. Instead of shooting 4-minute RGB subs in Mode 1 (high gain, gain=56, offset=10), I tried 10-minute RGB subs in Mode 3 (gain=14, offset=10) instead. The experiment was a success, because I found the star colors are better and the chrominance noise in the background was surprisingly low for just 1 hour in each RGB channel. It is too bad that I am forced to use a guide scope with my refractor rig, which makes 10-minute subs challenging. There is no back-focus left to add an OAG now that I opted to insert a Gerd Neumann CTU instead. The CTU was an instant success, as it allowed me to nearly rid my system of tilt after about two years of trying everything else. I think I will manage with the guide scope, considering the stars turned out as good as one could expect with this project. Perhaps I could try 5-minute RGB subs with twice the gain?
All stacks were drizzled 2x to achieve a final image scale of 0.7" per pixel. I did not apply any noise reduction or smoothing to this image because I did not feel it was necessary to alter the data any further. Seeing was between poor to terrible on the night when I shot the RGB subs. Seeing was average to good on the night when I shot the luminance subs. Like most of my recent images, this one is cropped down to size to frame the object well. Lucy-Richardson deconvolution was used to enhance the apparent resolution of my pathetic, undersized 100 mm refractor
while my 279 mm SCT remains inside the house collecting dust. Since astronomical darkness does not even arrive until 10:30 PM these days, and the SCT requires manual focusing and a lot of babysitting, I can't bear to use it until early evening imaging becomes a reality again. Until then, leaving NINA to do all the dirty work while I sleep, then waking up to perfectly focused subs, is a great joy.
References
1)
https://images.mantrapskies.com/catalog/NGC/NGC6791-BERKELEY046/index.htm2)
https://www.science20.com/news_releases/mystery_star_cluster_ngc_6791_has_three_different_ages
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