Eta Carina Mosaic

One feature of Kstars that I’ve been curious to try out is the ‘Mosaic’ feature, buried in the ‘Sequencer’ page. I watched a few YouTube videos so that I got an idea of how it worked and then decided to give it a try on the next clear night.

The selected Target? The Eta Carina nebula, of course. It’s big, bright, and easy to find in the night sky.

I elected to use the ‘stock’ 420mm focal length of my telescope because I already had it configured within Ekos. The downside of this is the reduced field of view and the slightly elongated stars in the image corners. For the next mosaic I will prepare a bit earlier and use the flattener / reducer in the setup, and maybe drop the UV-IR cut filter in there as well. We’ll see.

Also, the default 10% overlap is not enough - the stacking artefacts eat into this overlap and may make if difficult to align the panels. So I’ll bump it up to 15%, or maybe even 20%, depending on what sort of FoV I have.

Because this was an experiment, I only took 90 frames for each panel - so about 22.5 minutes of integration time each. (The clouds were forecast to roll in at about the time I was going to finish. They didn’t, but that’s pretty usual for my luck when it comes to astro and the weather.)

I elected to do the stacking with SiriL, because it’s the fastest and has very handy (and powerful) pre-processing tools. Each panel was stacked in SiriL, after which came a Background Extraction, Green Noise removal and a Histogram Stretch. Oddly, SiriL 1.0.6 has started crashing when I right-click the Background Extraction window to remove a sampling point. Not good, but given that SiriL 1.2.0 has entered ‘beta’ stage, I’m not overly worried.

The four stacked and pre-processed panels were loaded into Affinity Photo’s Panorama tool and automatically stitched together. After cropping, I began the work of converting the very “dusty brown” looking output (see image V.1 below) into something that looked like an OSC image.g

Image V.2 is a “mid-way” image, whilst V.3 is the final output. The colours are a bit over-saturated, I think, but are much closer to what my other images of Eta Carina look like.

For what it’s worth, my working images were in the order of 70 megapixels (roughly 10,000 x 7000 pixels before cropping) and just under 900 megabytes in size.

Image details:

• Gain: 50
• Offset: 10
• Temperature: 0 degrees C
• Exposure: 15 seconds
• Lights: 90 (22.5 minutes of data)
• Darks: 50
• Flats: 50
• Dark Flats (Bias): 50
• Stacked and pre-proc’d in SiriL, then processed in Affinity Photo

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• Astro