This is yet another image of M31, the Andromeda Galaxy. At the risk of violating any laws that one is not allowed to make more than one image of M31 per season, I took this data recently as a secondary target in order to study and practice combining short and long exposures using PixInsight's (PI) High Dynamic Range Composition (HDRC) process. For a more thorough description of the galaxy itself, please refer to my M31 post from earlier this season. Here I restrict comments just to this processing study.

I took both 30s and 300s duration frames of L 1x1, and R, G, B 2x2 to see how well I could combine the short and long duration exposures to capture both finer detail and control bright star saturation. In the end I made two final LRGB images, one using just the 300s exposure frames, the other with both sets of 30s and 300s frames with HDRC. Here I have uploaded the results and also created a couple of mosaics to examine the differences more closely.

Rev A : 300s LRGB

Rev B : 300s L

Rev C : 300s Inverted L

Rev D : 300s Annotated L

Rev E : 30s + 300s LRGB

Rev F : 30s + 300s L

Rev G : 30s + 300s Inverted L

Rev H : Mosaic of Two Cropped Regions of LRGB Images

Rev I : Mosaic of Full Field-of-View (FOV) LRGB Images

Referring to Rev I, you can see the results of processing just the longer 300s frames on the left, HDRC combination of 30s and 300s frames on the right. Although each incorporates the same long exposure data, they represent slightly different processing paths, so the colors don't quite match. I went for a bluer tint compared with my earlier image, but I think I went a little bit too blue. Of these two sets, I actually prefer the coloring of the 300s only image a little better. This can be easily revisited at a later time for another pass at the coloring.

On the full FOV scale it may be a little harder to tell differences between the two sets unless you look carefully. Certainly if you zoom in you can start to see the differences. I prepared a mosaic of two cropped regions in Rev H. The top row is the center of M31, and you can see that the saturated region of the bright core is larger when using just the 300s frames on the left (this is to be expected) compared with the HDRC combined version on the right. The bottom row shows the region around the bright blue star to the left of M31. This is nu And (35 And, HIP 3881, SAO 36699, HD 4724, HR 226), which, along with several other bright foreground stars in this cropped region, have tinier circles of saturation on the left compared with the right, just as expected.

The HDRC image is technically better in that I was able to utilize combined data that did not begin saturated. That was, as expected, not the case with the 300s data alone. However, what surprised me was that, when just viewing the wide FOV images, there really isn't that much difference, certainly not from a more casual glance. It is only upon closer inspection that one can see the benefit of using the short exposures.

It is likely that the differences would be more apparent from data taken with a longer FL and narrower FOV with larger star sizes. I may incorporate this acquisition scheme with future imaging. The short exposures, as you would surmise, don't take that long to acquire, and their data only gets incorporated around brighter stars and, in the case of this target, the central core of the bright galaxy. This constitutes a very small proportion of the final image, which is dominated by the long exposure data.