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M31 Andromeda Galaxy (and associates)

July - September 2013

This is a Narrowband for Stars (nb4stars) project.  For more on the theory of what I am trying please consult the nb4stars page.
The Andomeda Galaxy is the most distant object that can be seen with the naked eye. It rises as the summer Milky Way is setting.  Both Andromeda and the Milky Way belong to the  Local Group which is the collection of relatively close galaxies. The Milky Way, Andromeda Galaxy (Messier 31 or NGC 224), and M-33 (which is associated with M31) dominate the Local Group.  The rest of the members are small galaxies or even smaller dwarfs.

M31's has a number of satellite galaxies, These are spread over the fall sky.  As mentioned above the 3rd largest galaxy in the Local Group, M33,  appears to have interacted with M31 in the past. It is located in Triangulum some distance from M31.  Two other smaller galaxies  (NGC 147 and 185)  are located between Andromeda and Cassiopeia.  Two others orbit close to M31 (M32 and M110).  A telescope with a wide field of view will easily show M32 as a fuzzy ball next to the bright center of M31.  M110 is more of a challenge since its surface brightness is lower, but can be observed in a dark sky.

I started collecting data for this project right before sunrise in July.  By September I was able to collect for most of the night.  This project used my nb4stars filter set; Strömgren V and Y filters (which I designate as sV and sYel), a 20 nm Hydrogen filter, and a 3nm Hydrogen filter.  The sV and sYel data were only collected with the moon fully down.  The Hydrogen was collected with partial moon.

Half Size Image

This image is displayed at half resolution (~ 7"/pixel).

M31 with nb4stars enhanced with 3 nm Hydrogen

Click on the image above for a full size image.
The full size image shows the structure of the spiral in more detail.

Note also NGC 206.  For comparison here is a closeup using a larger scope.

The annotated image is also presented below.

The first thing I noticed when processing the image was how well the spirals show.  Visually it is tough to see the inner spirals.  Even with a 30" telescope you can only see the outer spiral and even then it requires a good night.  The 3 nm Hydrogen filter easily collects the star forming regions so adding it to the base RGB image makes for a much more dramatic image.

Composing the Images

I completed the processing of this project immediate following the M8 project.  This processing was simple compared to the struggles I had with that project.  All processing was done using Pixinsight 1.8.

Data Collection

Even with 15 minute exposures my narrowband filters collect a very weak signal.  Thus I purposefully collected data until the moon shut me down.
Number of Images
sV (Strömgren V)
sYel (Strömgren Y)
20nm Hydrogen (Ha20)
3nm Hydrogen

All filters were Astrodon

Combining the H data

The first step was to form a composite of the Ha20 and 3 nm Ha that will enhance the Hydrogen rich areas without blowing out the red portion of the stars. The best method I have found is the Vicent Peris technique described in this Harry's Astro Shed tutorial.

With my filters the final Red Image was formed in two steps using PixelMath

  1. H_VP = ((Hydrogen*20)-(Ha20*3))/(20-3).  You can see the result of this operation here.
  2. R_VP = Ha20+(H_VP-Med(H_VP))*4
An initial RGBpH image is then created by using ChannelCombination to combine R_VP, sYel, and sV to an RGB image.

As an example here is a before and after

RGB only sample of Vicent Peris method

This shows the rather flat appearance of the RGB before adding Ha.  At this point Color Calibration has not been done.  The Ha20 filter will collect more hydrogen relative to its bandwidth than a conventional R filter.  Thus the star forming regions are just starting to show.
Example of Vicent Peris method of adding Ha data

Adding the 3nm Ha makes the star forming regions pop.

At this point the image is a bit red.  That will be corrected in later steps.

G2V Color Calibration

To insure that stars are the correct colors one images a reference G2V (i.e. sun like) star.  You then balance the colors so this star appears white.  I shot M31 in a wide swath of sky so no single calibration was going to be perfect.  I tried letting Pixinsight calibrate via structure detection, but that did not give good results.  Thus I used a color calibration for a star that is high in the sky (since almost all images were shot about 50 degrees elevation).

For this project I used HIP 96037.  From this I determined that the correction should be


These were used for color correction in PixInsight.  The initial color correction worked well with the galaxy mostly a white color with a slight red tinge. However, after stretching the galaxy got a yellow tint (too little blue).  I removed this using a masked CurvesTransformation that increased the blue.

How accurate are the star colors? For comparison here is an version of the image with the BV Index of the stars annotated.  The reader can decide for himself if the colors are accurate.

Once color calibration was done I used TGVDenoise to reduce the noise in the background.  I was happy with the stars so I did not bother using deconvolution .


So far the image had been linear. I used a simple HT to stretch it.  I was not happy with the results when I tried a masked stretch.

Bringing Out the Structure

A lot of structure was apparent even at this point.  To make sure the final image did not look over processed I used the sharping tools very judiciously.The first is HDRMultiScaleTranform which spreads the brightness over a wider range of values. I then added a bit of DarkStructureEnhance and LocalHistogramEquilization.  

Final Steps

Finally I used MMT to further sharpen the structures the previous steps disclosed.  Then another pass of TGVDenoise to remove the last noise.


Overview of M31

Annotated image of M31 area

Copyrights for Photos

Creative Commons License
Except as noted, all work on this site by Robert J. Hawley is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. This permits the non commercial use of the material on this site, either in whole or in part, in other works provided that I am credited for the work.