The Holmberg 124 interacting galaxy group in Ursa Major group consists of the beautiful spiral galaxy NGC 2805 and its peculiar companions NGC 2814, NGC 2820, and NGC 2820A.  The cluster of tiny galaxies in the lower right center is Abell 764. 

According to TheSkyX, NGC 2805 measures about 6.3’ x 4.8’ and shines at a blue magnitude of 11.5, but it seemed much fainter while I was capturing the data.  It bears some similarity to M101 in that it is seen face-on and has open arms.  Like M101, its surface brightness is low, and it was a challenge to obtain good signal:noise ratio from my Bortle 4+ location.  Few images of this galaxy captured with small telescopes show any substantial H-alpha emissions, and its outer arms seem to be dominated by young, blue stars.  I captured a few 10-minute H-alpha subs, but the signal was so weak that I decided not to use an H-alpha component in this image. 

NGC 2820 is the elongated galaxy at upper left, which has an irregular protrusion from one end (NGC 2820A or IC 2458  ).  It measures 4.1’ x 0.4’ and is of magnitude 12.5, and it is classified as type Sb, meaning it is an edge-on spiral.  To its right is the smaller irregular galaxy NGC 2814, which measures 1.1’ x 0.3’ and is of magnitude 13.5. 

The cluster of tiny, ruddy-colored galaxies in this image is Abell 764.  I could not find much information about this cluster except that it is redshifted by 0.166, which would place it at a distance of roughly 1.5 to 2.0 million LY.  None of its members are large enough to show any substantial detail with a 100 mm refractor.

This image is a standard LRGB composition with luminance data obtained in Mode 3 (extended fullwell) (Gain 14, Offset 15) of the QHY268M camera.  RGB data were acquired in Mode 1 (high gain) (Gain 56, Offset 10) with the same camera.  Star shapes and colors came from the RGB data, whereas the rest of the image is based on the luminance captured in Mode 3 with Mode 1/RGB colors. 

I originally intended to use Mode 3 for luminance in order to prevent overexposure of the brighter stars.  However, it was extremely cold (under 10 degrees F) on most of the nights when I captured luminance data.  My Esprit 100ED shows signs of pinched optics in cold weather.  Therefore, the brighter stars had some ugly little spikes.  I ended up using Starnet++ to remove the stars from the luminance channel and replace them with stars that were captured this month in substantially warmer weather, mostly with Mode 1.  It does not make much sense to use Mode 3 to suppress overexposed stars, only to erase the stars and replace them during processing, but the misshapen stars were too distracting. 

Overall, I am pleased with the luminance I got from Mode 3, but I am still left wondering if I could have done better by shooting the whole project in Mode 1 / Gain 56, which offers lower read noise.  The image was stacked using 2x drizzle to bring the image scale down to a reasonable 0.7” per pixel.  The version shown here is cropped to frame the subjects well, and it is only about 15.6 % of the total sensor area.  There were not many interesting background galaxies outside of the cropped area.