NGC 891 is an edge-on spiral galaxy in the constellation Andromeda that is sometimes called the "Flying Saucer Galaxy.”  Measuring 11.7’ by 1.6’ at an overall magnitude of 10.1, NGC 891 is much larger and brighter than many of the galaxies that I imaged during 2022. 

This image combines data from two telescopes.  The RGB frames were shot with an Esprit 100ED refractor at f/3.6 with the Starizona Apex-L 0.65x reducer from a Bortle 2 dark site back in 2020, but the detail resolution was poor due to the large image scale.  The luminance channel was obtained in my backyard (Bortle 4+) this month using a C11 EdgeHD SCT at f/7 on a night with above-average seeing.  All subs were shot with the QSI 660ws CCD camera.

I’m still trying to figure out how I got this result with just over five hours of integration.  It all makes sense if you consider the acquisition details.  NGC 891 passes nearly overhead at my location, which makes a huge difference compared to something like M81 that never gets as high in the sky.  Shooting RGB from an excellent dark site with a fast f-ratio and large image scale (2.4” per pixel) allowed fast RGB capture on a night with excellent transparency.  The luminance data were shot with shorter subs (180 s) and good seeing on a night that was quite cold, which allowed me to drop the sensor temperature to -30 C.  The sensor in my old CCD camera really responds well to low temperatures, but I never tried running it this low.  It turns out that the 2.1 hours of luminance looks like 6 to 8 hours at my normal sensor temperature of -16 C.  I wish I realized that a few years ago! 

A few words on processing are merited.  For noise reduction, I used a blend of Topaz AI (with sharpening turned off) and standard noise reduction (Noiseware) on the starless image.  The goal here was mainly to reduce noise in the sky background.  I probably could have skipped noise reduction because the luminance stack was so clean, but I wanted to stretch the image a bit more aggressively.  For sharpening, I employed the multi-resolution sharpening feature in ImagesPlus by MLUnsold, which I believe is similar in concept to wavelet sharpening.  The sharpening was applied to the dust lane in the galaxy only, with the rest of the image masked.  The final LRGB image was constructed with layer-based processing in GIMP, as usual.

Acquisition details: 

Nov. 8 2020 (Esprit 100ED @ f/3.6, Astrodon E-series RGB filters - Bortle 2 dark site);  46 frames of 240s each = 3 h 4 min

Dec. 12, 2022 (C11 EdgeHD @ f/7, Astrodon E-series L filter, 43 frames of 180s each = 2 h 9 min)