These are 3 images of the great Andromeda Galaxy (Messier 31) - our nearest galactic neighbor at just 2.5 million light years away and about 150k light years across. The 3 images represent different spectral bands in the filters used, and one combination image (I'll describe more below). Being able to image Andromeda is so amazing to me. It's visible to the naked eye on moonless nights outside of light pollution, and it's relatively large in the sky - this FoV being approximately 3 moons wide, and it takes up almost the entire diagonal in the image. There are two mini-me galaxies in the image - M110 and M32. There is an enormous amount of information known about Andromeda which I won't cover here, but Wikipedia's Andromeda page is a great resource. So, about the images above. These were taken in late summer of 2023 during a anomalous period in coastal LA County of several days of good seeing conditions (measure of light distortion due to turbulence and thermoclines in the atmosphere), clear skies, equipment that was working (as opposed to my last Andromeda attempt - see my prior blog for that adventure). Total imaging time is >30 hours.
The three images represent different post-processing of two filter bands. The total data set consists of 2 subsets of imaging - 1. ~15 1/2 hours of broadband filtered images (meaning most light is let through). I use "RGB" to label this (meaning Red, Green, Blue - the standard 3 visible colors) 2. ~11 hours of narrowband (tuned to H-alpha [red emission] and OIII [greenish-blue].
The "RGB Only" image is the pure broadband image - basically all light from Andromeda. Notable in this image are the very visible brown dust lanes in the spiral galaxy. The "Ha+O3" image is the pure narrowband image. You can notice that there is less overall light in the image, and you can see several very reddish spots in the galaxy that are heavy in H-alpha emission, often indicative of active or recent star formation regions, particular in the arms. The "Ha+RGB" image combines the Ha (red) data from the narrowband filter and adds it to the broadband image - highlighting a bit more of the Ha regions, although it doesn't come through nearly as clear as the Ha+O3 image.
It took a lot of persistence to find the right window to capture over 30 hours of this target. After some initial tries at processing the data and combining the data unsuccessfully, I entered a long period of procrastination, as it was a significant amount of time required (probably 40+ hours in post processing after all the trial and error over the last 1.5 years). After all of that work on combining the images....I think my favorite is either the pure broadband or pure narrowband! But, I am really happy with the final results overall, and as always, I learned a lot to apply to future images!
Now for the tech specs for those interested: Taken at 430mm focal length, 73mm aperture APO refractor, auto guiding on an EQR-6 Pro mount. Images were taken at 180s exposure (probably too long for the broadband in light pollution - amazing it turned out), meaning ~200-300 images for each filter type used. The images were stacked and processed, including use of neural network star removal to process the target details sans stars before recombining with the star field. Photoshop is used at various points in the workflow for touch up of detail, color balancing/saturation, etc.