Casitas de Gila, Gila, New Mexico, October 18 - 20, 2022
Walton, Oregon, September 4, 23-26, 30, 2022

Ever since I started my pursuit of astrophotography, I wanted to produce a good photograph of the Andromeda galaxy. Having purchased a Stellarvue SVX102T refractor in April, I was ready for the autumn skies to put M31 in a good position for imaging.  This project included several first attempts: 1) the use of a two-panel mosaic in Sequence Generator Pro, 2) the use of the Photometric Mosaic tools in PixInsight to merge the individual panels, and 3) testing the new Weighted Batch Preprocessing script to calibrate and integrate subframes in PixInsight.  In early September, I took a few test shots with the Luminance filter to make sure that the mosaic orientation was correct, and that the 120 second exposure would not blow out the galaxy core.  I started the project at dark sky sites in September in Oregon and finished it in October in New Mexico.  I also added a camera rotation ruler on the telescope and tested the accuracy in New Mexico, +- 0.5 degrees or better.  The image acquisition process went smoothly and almost every subframe was of high quality.  Since I imaged during new moon cycles, there were no gradients or sky glow.  Only a few subframes were degraded by high, thin clouds.  For all nights, seeing varied from average to better than average.

While image acquisition was problem free, post processing was a real challenge, especially when attempting to add hydrogen-alpha filter data to the red and blue channels.  The luminance data were easy to denoise, sharpen and stretch.  Russ Croman’s StarXTerminator and NoiseXTerminator work so well on linear or stretched images that post processing was both easier and flexible.  It took me several days to process the Ha+RGB data.  While each channel was easy to process, and creating an RGB image from the Red, Green and Blue integrated images, adding the Ha caused many problems with color balance and introduced many artifacts and gradients.  I tried almost every approach described on-line and in books without success.  To solve this problem, I made a starless RGB image and then stretched it.  I then created a new Ha image that accounted for the Ha signal in the Red channel using filter bandwidths and exposure times in a PixelMath expression.  I then stretched a starless version of the new Ha image and clone stamped everything that was not an emission region in the arms of the galaxy.  The background was made dark, and the new Ha was combined with the RGB image with proportional factors in the Red and Blue channels.  By adding some of the Ha to the Blue channel, the emission regions had more of a magenta color.  The RGB stars image was denoised, stretched and saturated.  I used the GenerlizedHyperbolicStretch script to stretch all the images.   The HistogramTransform and CurvesTransform tools were used to fine tune the luminance and color images.  The MorphologicalTransform tool was used to reduce the RGB star sizes.  The starless luminance image was combined with the slightly blurred HaRGB color image to produce the LHaRGB starless galaxy.  The stars and LHaRGB images were combined with PixelMath’s combine function using opscreen() (StarXTerminator had the opscreen mode enabled).  Several masks were created using the RangeMask tool and GAME script to work on the cores of the galaxies or the arms.  The final image was touched up using Photoshop.

M31, the Andromeda galaxy is the nearest spiral to the Milky Way, type SA(s)b.  It is one of the very few galaxies that can be seen with the naked eye or binoculars at a dark sky site.  The galaxy spans over 3.9 degrees of the sky and its interflux nebula is much larger.  M31 is behind a very rich star field in the constellation Andromeda.  M32 and M110 are small elliptical satellite galaxies that have been stripped of much of their mass by M31.  M33, the Triangulum galaxy, has also been greatly affected by gravitational interactions with M31. While M31 is a spiral, its disk is distorted from past collisions with other galaxies.  Andromeda along with the Milky Way and M33 are a part of the Local Group of gravitationally bound galaxies.  M31 is moving towards us, and in about 4 billion years, Andromeda and the Milky Way will merge.  In 1923, Edwin Hubble proved that Andromeda was a separate “spiral universe” by identifying a Cepheid variable star in the galaxy to estimate its distance.  This was one of the most important discoveries in astronomy and changed cosmology forever.  Based on Hubble Space Telescope observations, M31 is about 2.5Mly from Earth with a diameter of about 220 kly (larger than the Milky Way).  It is estimated that M31 contains upwards of 1 Trillion stars.  Like the Milky Way galaxy, Andromeda is estimated to be about 10 billion years old.  There are about 460 globular clusters orbiting the galaxy center.  There are also a large number of star forming region as shown by the magenta emission nebulae in the spiral arms.  (Wikipedia, NASA and SkySafari Pro).

Imaging details:

Stellarvue SVX102T with SFR0.74 focal reducer (FL = 528mm, f/5.2)
ZWO off-axis guider (OAG-L) with a ZWO ASI 174MM mini guide camera
Losmandy G11 mount with Gemini 2
ZWO ASI 2600MM Pro cooled monochrome camera (-10oC)
Chroma 36mm Hydrogen-alpha, Luminance, Red, Green, and Blue filters
Equatorial camera rotation: 0o

Software:    Sequence Generator Pro, ASTAP plate solving, PHD2 guiding, 
    Losmandy Gemini ASCOM mount control and web client interface,
    SharpCap Pro for polar alignment with the Polemaster camera,
    PixInsight 1.8.9 with StarXTerminator (AI version 10) and NoiseXTerminator,
    Photoshop CC 2022

For each mosaic panel (about 12.5 hours):

Hydrogen-a    10 min x 12 subframes (120 min), Gain 100, Offset 68, 1x1 binning
Luminance    2 min x 135 subframes (270 min), Gain 100, Offset 68, 1x1 binning
Red    4 min x 30 subframes (120 min), Gain 100, Offset 68, 1x1 binning
Green    4 min x 23 subframes (92 min), Gain 100, Offset 68, 1x1 binning
Blue    4 min x 26 subframes (104 min), Gain 100, Offset 68, 1x1 binning

Total integration time for the completed mosaic was about 26 hours.