Imaging M31, the Andromeda Galaxy, from an urban location – Bortle class 5.

Whilst waiting for Jupiter to reach the Meridian in mid October 2022, I thought I would attempt to image M31 using an Altair Astro 294mc, colour camera coupled to a Teleskop Service ‘Imaging Star 65mm f/6.5 Quadruplet Astrograph’.  This telescope uses a triplet objective employing one FPL-53 element and an ED glass field flattener element within the tube assembly.  With a focal length of 429 mm, it claims to have an imaging circle of 44 mm so when used with a Micro 4/3 sensor should provide pinpoint star images across the whole frame with no vignetting.  It does.   A helical rack and pinion focuser is employed which is far better for supporting cameras than a Crayford focuser.   [There are now many short focal length astrographs using this or similar optical layout.]

A total of 290, 18 second, frames were taken under the control of SharpCap Pro and saved as Tiff  files – so 87 minutes total exposure.   The camera sensor temperature was set at -15 C and no flats (definitely not needed) or darks were taken – though I could have added suitable darks if I felt that they were needed.   A single stretched, 20 second frame, is shown below and, as there was no evidence of any hot pixels or amp glow  which dark frames would have eliminated, I did not feel it necessary to use them.  Another point is this: when imaging under light polluted skies the dark current noise when the camera is used at low temperature is swamped by the sky glow noise and so becomes insignificant. 

The telescope was mounted on an Astro-Physics Mach 1 mount (along with a Celestron 9.25 Schmidt-Cassegrain for later imaging Jupiter).  It was not guided and the image moved very slowly across the frame – but just ~0.03 pixels per exposure so no star trailing would be apparent.  In fact, this is a good thing as it will avoid ‘colour mottling’ which can give rise to a coloured sky background.  This very slow movement of the image across the sensor also has the effect of ‘dithering’ the frames which sometimes allows a higher resolution to be obtained if the result is ‘drizzled’ when aligning and stacking the data – though I have rarely seen any real improvement.

The frames were aligned and stacked in Deep Sky Stacker.  As an experiment I also stacked the data using a 2x Drizzle.   Surprisingly, I found that the stars were tighter in the x2 drizzle result as seen below when the x1 image is compared with the x2 result downscaled by a factor of 2.  So, for once, dither and drizzle has seemed to improve the image.  [See article about dithering and drizzling in the digest.]

As usual, the image has a green cast as no compensation is made for the fact  that there are two green pixels for each red and blue.

Post processing

The output from Deep Sky Stacker was processed in Adobe Photoshop, but could be done in the low cost – but excellent – program Affinity Photo or even the new free program GLIMPS.

Removing the Sky Glow

Andromeda was at a reasonably high elevation, so a reasonable assumption is that the sky glow (light pollution) will be uniform across the frame.  If so, it is easy to remove in a 4 step process:

1) Duplicate the image layer.

2) Apply a ‘Dust and Scratches’ or ‘Median’ filter with a radius of ~40 pixels.  The stars will disappear but leave the glow of the galaxy.

3)Use the paintbrush to select the background colour away from the galaxy and paint the whole layer this colour.

4) Flatten the two layers using the ‘Difference’ or ‘Subtraction’ blending modes.

Stretching the image

The image was stretched using applications of the ‘Levels’ tool with the centre slider moved across to the left by  a small amount.  It is better to stretch with a few small applications rather than one large one.

Separating the Galaxy from the Stars

The stretched image was applied to the free ‘Starnett++’ program whose latest version has a graphical interface.  This produced an image of the galaxy with the stars removed which was saved as ‘Galaxy’.

The original image was copied and pasted over the Galaxy image and the two layers flattened using the difference or subtraction blending modes.   This left an image of the stars which was saved as ‘Stars’

Enhancing the Galaxy image

The  detail in the galaxy structure was enhanced by applying some ‘local contrast enhancement’  using the ‘Unsharp Mask’ filter with a large radius and small amount.  It was then sharpened using, in Photoshop, the ‘Smart sharpening’ tool or this could be done using high pass sharpening or other sharpening method in other programs.  The green cast was removed using the HLVG filter from Deep SkyColours and the saturation was increased to bring out some colour.

Enhancing the Stars

In the final image one does not want the stars to overwhelm the galaxy.  In this case the stars were pretty tight but their size can be reduced if desired by using the star size reduction tool in the free program ‘Images Plus’.  The saturation can be increased to bring out the star colours.

Combining the two images

The Stars image is copied and pasted over the Galaxy image and the stars were added by flattening the two layers using the ‘Screen’ blending mode.  If desired, the brightness of the stars can be reduced by adjusting the opacity downwards.  I used a value of 85%.

The Andromeda Galaxy

The final result was far better than I had expected given that the sky transparency was quite poor.  In particular I was surprised at the amount of structure shown in the galaxy.  This exercise did show that deep sky imaging of galaxies can be carried out from an urban location.  It must be said that the extreme ends of the galaxy were not apparent – hidden, I suspect, by the effects of sky glow.