Astro Archive - The Horsehead Nebula

10 minutes

Recently I had an idea to go back to my older astrophotography and try editing those photos again with new techniques that I learned along the way to see what happens and whether I can pull out some new stuff out of old data.

I asked on my Mastodon account if people were interested in seeing such content, and the majority said yes, so here we go. That’s how the idea of Astro Archive was born.

For the first image to retry I chose my old attempt to photograph one of the staples of the northern winter sky, the Horsehead Nebula in the belt of the Orion constellation.

I imaged Orion for three nights in March of 2022. Total acquisition time was 3 hours 36 minutes, shot only in narrowband using the Optolong L-enhance filter and an astromodded Canon 1200D DSLR.

Most processing was done in Pixinsight, with final touches in GIMP.

The telescope I used then was a Vixen ED114SS, which I sold this year because I was not happy with its large dimensions and low image quality.

The main point of this photo is the Horsehead Nebula, it’s striking shape of a, well, horse head, visible upside down in the centre. This black cloud of cosmic dust contrast with the glowing reds of the background emission nebula. There are not many stars visible above the horse, as the light is blocked by the larger nebula from which the horse stems. Only two stars shine strong enough to make their own small nebulae, IC435 and NGC2023.

On the right there is the magnificent Flame Nebula, NGC2024, a mix of glowing hydrogen and darker dust and smoke burning in the sky. Surrounding the Flame there are a few smaller nebulae, lighted by their stars.

Both the Flame and the nebula surrounding the horse are glowing because of the immense power of Alnitak, the leftmost star in the Orion Belt. Here visible as a shining ball of light just below the Flame. Alnitak is actually a trio of stars, the brightest of them being a blue supergiant, with a mass of 33 solar masses and 21 thousand brighter than our star.

In the lower part of the image, one can see the intricate lines of the hydrogen dancing in the solar winds, looking like a draped curtain. Sometimes the solar winds collide, and cause hotspots that shock warm the hydrogen ever more, making it glow brighter.

At the bottom of the image there are two interesting structures that seem not be named. It looks like hydrogen is lit by a nearby star and formed in its solar wind, once angled away from the star, once surrounding it. I do now know if this is the correct explanation, or are those two just visual companions, being in the same line of sight, but physically totally distant.

Finally, lurking somewhere in the background are galaxies other than our own, too faint to see, only their location marked on the annotated image below.

Image Processing

The image processing I chose was straightforward:

  1. Dynamic Background Extraction
  2. Photometric Colour Calibration
  3. SCNR to remove surplus green.
  4. Soft stretch (I am using EZ Scripts suite)
  5. Starnet2 to divide the image into a star mask and a starless image.

On the starless image:

  1. Unsharp Mask
  2. Final stretch

On the star mask

  1. Tiny bit of Curves transformation to slightly reduce the stars bloat
  2. More SCNR

Finally I moved to PixelMath to combine the star mark with the starless image using a formula I mentioned in the previous post: ~((~narrowband)*(~star_mask)).

I saved the combined image as .tif and opened it in GIMP for some final touches.

Final thoughts

This was at the time my best astrophoto. Looking at it now, I mostly see the downsides. There is not enough acquisition time to work with, less than 4 hours on an old DSLR will not give you wonders. And that Vixen telescope would only give round stars in the middle of the image, the corners being just a mesh of triangular points.

Nonetheless, it was fun to go back to my earlier works, and have a chance to share them with you.

Below you will find the annotated and the starless versions of the image.

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