Saturn Looking Glorious on 18th September 2022

Saturn in Infra Red – 18-09-2022

The second-best planet in the sky in terms of imaging is currently our old friend Saturn. Being a lot dimmer than Jupiter is, as well as lower in the sky from the UK (currently not rising above about 22 degrees), it’s still somewhat down in the murk (and will be for a few years yet). It’s only at magnitude +0.4 with a diameter (of the main planet) of 18.4 arc seconds.

I took the opportunity to try something new this time, to try and improve the final result despite the planet being low in the sky. When planets are low down, their light has to travel through more of the Earth’s atmosphere, meaning the image is far more distorted and detail is lost, as well as the red, green, and blue light being refracted to differing degrees, resulting in colour fringing on the capture.

There are several ways around this. The first is an Atmospheric Dispersion Corrector (ADC). This re-aligns the red/green/blue components of the image to help reduce or eliminate the fringing inherent in imaging low-down planets. The main disadvantage of these is that it’s another piece off kit in the imaging train that requires tweaking, and to be frank, they are a bit of a pain to use (especially in a non-permanent setup such as mine).

You can also wait until the planet in question is rising higher above the horizon. Unfortunately, it’ll be several years before that’s going to happen here in the UK for Saturn, and last time I tried, it paid no attention to me when I asked politely…

So the next option is to capture a wavelength of light that’s not as affected by the atmosphere, namely infrared. Using an Astronomik ProPlanet 742 IR-pass Filter removes everything except the IR. As the 224MC camera is sensitive to IR, it still captures an image, albeit in more-or-less monochrome.

This is fine, and the images you can get this way are very acceptable, even when the atmosphere seems to be bubbling away between you and the planet. They are, however, basically mono. This in itself isn’t a bad thing, as they are still very good. However, I do like colour in a planetary image. I know some aren’t bothered, but all things being equal, I prefer a colour planetary image. The first image above shows the result after processing of that IR capture. This is the best 30% of 8000 frames, stances in AutoStakkert with 1.5 Drizzle (to upscale it slightly), and sharpened with wavelets in Registax 6. Quite nice I think you’ll agree! I could stop there, but I decided to try and get that elusive colour data.

Slightly less detailed colour capture (though not bad!)

So, the next step is to grab the colour. This is done by doing a standard capture through a normal IR block filter (so basically the opposite of the other filter – lets through all the visible wavelengths, and blocks the IR. With the expected bad seeing down low, even with all else being equal, this will never be as sharp as the IR image. It lacks the visible Encke division near the edge of the rings, but all-in-all isn’t too bad to be honest. It does, however, give us the colour we need to combine with the above mono image. This is done in Photoshop CC 2022 by placing the colour image as a layer underneath the mono one (once oriented the same), and using the ‘Luminance’ blending mode to apply the mono picture as the luminance ‘detail’ on top of the colour image. To remove any noise and artefacts from the colour layer (as the colour is all that’s required), a gaussian blur was then applied to that layer only. Finally, the saturation of the colour layer was increased, as the blending washes the colours out slightly. The end result was the below image.

Saturn, in all it’s technicolour glory!

I’m really happy to have captured the Encke Gap near the outer edge of the rings, as well as the fainter ‘C’ ring extending form the inner edge of the rings towards the planet.

As an interesting comparison, here’s a slider that allows you to compare the straight colour image with all its artefacts etc to the combined colour/luminance image. Quite a difference!

Comparison of straight colour capture on the left with combined IR/colour image on the right.

Here are the details…

Capture and Processing Information

  • Object  – Saturn
  • Magnitude – +0.4
  • Annular Size – 18.4 Arcsecs
  • Camera – ZWO ASI224MC
  • Exposure – 22ms frames @ approx 45fps
  • Stacking – 30% of 8,000 frames (2,400 frames)
  • Filter – RGB image – Astronomik Luminance (IR/UV cut)
  • Filter – IR image – Astronomik ProPlanet 742 IR-pass
  • Telescope – Skywatcher Skyline 200p f/6 Newtonian Reflector @ f/15
  • Mount  – Skywatcher HEQ5 with Rowan Belt Mod
  • Capture Software – Firecapture
  • Stacking – Autostakkert3
  • Post-Processing – Wavelets in Registax and final scaling and Processing in Photoshop CC 2022

Clear skies all!

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