The Ghost of Cassiopeia - Urban Astrophotography

The bright star in the top-left is called Gamma Cassiopeiae, and is around 550 light-years from Earth. It’s almost 20 times more massive and 65,000 times brighter than the Sun, and rotates so fast — 1.6 million kilometres per hour — that it blasts out intense ultraviolet radiation, eroding the nearby Ghost Nebula and causing hydrogen gas to glow red. Other parts of the nebula reflect Gamma Cassiopeiae’s strong blue light.

Like most of my targets this object is really faint, so I needed to collect a lot of data to get a decent image from my light-polluted city centre location. The final image contains 32 hours of light, collected over nearly seven weeks. For tips on achieving long integration times, check out my article here. Despite all that it’s still quite noisy, but I’m happy considering it’s a bit of a feat to image it to any degree under my skies!

Being a reflection and emission nebula meant that I decided to collect data using my Optolong L-Quad Enhance (for RGB) and Optolong L-Ultimate (for Ha and OIII). I used this approach recently for The Flaming Star Nebula and it worked well. For more info, check out my guide to choosing filters for your OSC camera. Use the slider below to compare quick edits of the RGB and Ha/OIII data. You can see how various elements from both make it into the final image. I think it’s interesting from a scientific viewpoint too, as when looking at the two images side-by-side you can compare the emission (red) and reflection (blue) elements really well:

RGB is on the left (Optolong L-Quad Enhance); Ha/OIII is on the right (Optolong L-Ultimate).

The halo around Gamma Cassiopeiae is big and nasty, and was a pain to remove during processing. I outline how I went about this in the processing video you’ll see when you scroll down. (Spoiler alert: I used Photoshop’s new Generative Fill function).