This is a project I've been working on for a while. I decided shortly after I got my FLT-120 that I wanted to do a new image of M31. I knew it would be a little zoomed in and not capture the full target, but I didn't care. I figured it would be more close up and intimate.

So for a couple weeks, I would set up my telescope and keep aiming it in the same direction. I got a fair few nights of good data and was careful to take flats every morning after each session. I wanted to make sure everything would calibrate well. Ultimately I ended up with 7 nights worth of data and 34hrs of it was good, usable stuff. 

Stacking took a long time but the results were immediately impressive. I got a ton of Ha signal, as is evident in the  final image. The OIII, however, was very weak. In order to get it to pop out, I decided to remove the overlap between channels through Pixelmath. An example would look like G-(R-med(R))*.22 where .22 is a scaling factor that should be tuned to produce a result that isn't clipping. Doing this provided much greater color contrast and the OIII started to show through much stronger. 

I exported the extracted stars, the starless RGB image, a "narrowband" version of the RGB image produced with additional Pixelmath, the R channel, and a pseudoluminance channel made from 40% R, 30% G, and 30% B. These were layered in Photoshop. The starless image was left largely unmodified and used as the background layer. The "narrowband" image was blended in screen mode, run through camera raw filter, and had some hue/saturation adjustments applied. The R channel was  blended with overlay mode and had curves and levels applied to provide more contrast. The pseudoluminance was blended with luminance mode, reduced to 60% opacity, and sharpened with two layers of unsharp mask. The stars layer was blended in screen mode, noise reduced in CRF, boosted in vibrance/saturation, and masked with its own extracted luminance to act as a basic star reduction. I also had levels, brightness/contrast, and curves adjustment layers to further tweak the black point and contrast of the image.

While I'm very happy with the results of my efforts, I don't consider this project done. Because it was shot through an Antlia ALP-T filter, you can only see the Ha and OIII signal of the target. I think the image would look more complete with broadband RGB data integrated into it. I hope to get some dark sky time to accomplish this before it's no longer visible. Wish me luck!

I strongly encourage you to take the time to explore this image by zooming in. I'm including a link to a full resolution (98MP), full quality jpeg so you can easily download and pixel peep to your heart's desire. I've already had a lot of fun exploring this image and I hope you do, too!