Nebula Gum15

My method of selecting a DSO for imaging is rather relaxed and haphazard - I see what everyone else is doing and look elsewhere because I’m like that. I usually end up wandering through Stellarium or Telescopius looking for things that “look interesting”.

Gum15 is one of them - I saw it in Stellarium, but had no idea what it’s catalogue identifier was. A quick post on IceInSpace and I had my answer from the resident experts there. Thanks guys! Telescopius had a couple of images of it and some useful information - except for one important value: the surface magnitude (aka brightness).

Gum15 is a pretty dim object. Almost too dim to easily capture with a small scope and an OSC camera. People who get good images of Gum15 will be using large reflector telescopes, monochrome cameras and narrowband filters.

Undeterred (or perhaps overly naive and optimistic!) I setup the rig and started imaging as soon as I had a clear night. 3 hours of imaging later, I had a faint and noise-filled image. Not to worry - the new moon is coming up and I was going to the ASV dark sky site, so I could capture more data there. I captured another 5 hours of dual-band (LEnhance) data to enhance the Hydrogen-alpha signal and 2 hours of UV-IR Cut with the 3D-printed diffraction mask to give the stars that ‘spiky’ look that people seem to like.

This gave me (at this stage in my astro imaging career) an unprecedented amount of data - 10 hours of photon capturing and just under 60 GBytes of data once I’d added the calibration frames.

I initially tried using Sirilic and SiriL to process all 4 sets of data (2 sets of LEnhance and UV-IR-Cut from home and 2 further sets from the dark sky site) into one stacked image. The resulting image was OK, but difficult to process in Affinity Photo. I felt there was too much noise in the nebulosity, where I was having trouble getting anything sharp and with reasonable contrast. On top of that, the larger stars were all blown-out and massively over-exposed.

I ended up throwing out the 3 hours of light polluted data from home and just going with the dark site data, giving me “only” 7 hours worth of data.

I next attempted to use Sirilic’s “split the RGB into Ha and O3” tools, but had no success in combining the resulting Ha / O3 / RGB layers into anything that looked even remotely good. The LEnhance data gives me a good amount of Ha data, so I could always try to use that for the Luminosity layer, if I was to go back and try processing these stacked images again.

The main problem is the lack of RGB data - Gum15 isquite faint, and dominated by the red Ha signal that any other colours are pushed way down.

So to create some image with the data I had, I fell back to what’s worked before - keep the dark sky data and process the two sets separately before manually combining them into one image. It came out reasonably well but I really, really do need more practise in the processing side.

The image below is the final result - it’s not perfect (far from it) but I’ve got an image I can be (mostly) happy with and can now move on to another target. Preferably something a bit brighter!

L-Enhance Image details:

  • Gain: 100
  • Offset: 10
  • Temperature: 0 degrees C
  • Exposure: 90 seconds
  • Lights: 200 (5 hours of data)
  • Darks: 25
  • Flats: 25
  • Dark Flats (Bias): 25
  • Stacked and proc’d in SiriL

UV/IR-Cut Image details:

  • Gain: 100
  • Offset: 10
  • Temperature: 0 degrees C
  • Exposure: 45 seconds
  • Lights: 160 (2 hours of data)
  • Darks: 25
  • Flats: 25
  • Dark Flats (Bias): 25
  • Stacked and proc’d in SiriL

Final image processed with Affinity Photo

Update:

I worked out how to do multi-session stacking in Affinity Photo, so decided to try stacking and processing all the LMDSS data again. The resultant photo is below. I chose to saturate the colours a bit, as the images seem to get dimmer when I scale them down from their original size.

Nebula Gum15

Reprocessed Nebula Gum15