I've been wanting to return to the Andromeda Galaxy since upgrading from a 61 ZenithStar 61 APO doublet.

Now that the AstroTech AT115EDT (a fantastic APO triplet) seems to be dialed in, it was the perfect chance to not only return to this beautiful target, but also to try the scope out on a mosaic. At 1.2"/pixel, I can fit a LOT of M31 one in an ASPC frame, but not all of it. This framing has about 20% overlap between the two panels. 

It was also a great opportunity to try out combinations of RGB, Ha, and [OIII]. On night 1, I captured 4 hours (80 x 3 min) on each frame in broadband, no filter. On night 2, I captured 4.5 hours (54 x 5 min) in dual narrowband with an L-extreme filter. 

Ha and [OIII] were extracted and stacked for each panel in AstroPixelProcessor. As usual, I processed light pollution as much as possible in each individual RGB, Ha, and [OIII] panel, then mosaiced each channel separately. (The RGB-only mosaic is in C.) 

I then registered and combined the separate RGB, Ha and [OIII] mosaics, and this is where it gets fun: choices! I looked at HaRGB and RGBHOO. HaRGB (which is a traditional representation for M31) did indeed strengthen the details, but i found that some combinations ended up losing some of the lightness of the outer disk and the connection between M31 and M110. Brining in the [OIII] for RGBHOO struck a nice balance between bringing in more of the Ha nebulas in M31 while keeping the blues and purples of that broader disk. 

The other interesting trial and error was how to combine the channels: add, multiply, add-scale, or multiply-scale. The default setting in AstroPixelProcessor is multiply-scale, but this made the reds far too harsh. (See for example D with light processing.) Add-scale struck a nice balance between the original RGB and the crisper detail of the Ha mosaic. That's what you see here. Overall, this is the best luck I've ever had on M31, and I've learned a lot in the process! 

And truly, this AT115EDT punches far above its weight!