Sadr Region – A Bicolor L-Enhance test Shot

I decided to test my new L-Enhance filter around Sadr, a star in the Cygnus constellation in the middle of the Milky Way.

The Subject

The star Sadr (γ Cygni), in the Cygnus constellation, also known as the center of the Cygnus cross, is surrounded by a nebular complex rich in hα emissions. The emission nebula itself is designated IC 1318.

The Shooting

I took the shoot from my backyard at the edge of the urban area of Arezzo, under bortle 7 skies (that’s an estimation). I tested an old EOS 1100D: it’s an old camera but it has been recently astro-modified by me. I bought a l-enhance filter to use this camera for narrowband shots.

The field of view is quite large and allows me to frame big objects without the need of multiple shots. This is a simulation of the shot made with Stellarium (the final image will be slightly cropped due to dithering):

Also, i tested on this occasion my recently acquired Losmandy G11, with its Gemini 1 controller. The mount needed a bit of tinkering, despite this rig being very light compared to the mount carrying capacity. But after all, it perfomed its duty well and i had no tracking or guiding problems.

These are the specifications of the narrowband/widefield rig i’m currently using:

I tried to exploit my new mount and narrowband filter, choosing to shot 5 minutes subs at ISO 1600. Based on later tests i could have used even longer subs, but i was afraid this old camera might have problems with thermal noise and star bloating. After all i chose to shoot with the 1100D instead of my 6D solely because the latter isn’t astro-modified. I should test in the future the capability of the stock 6D with the l-enhance filter.

In order to have an integration time of at least 4 hours i needed two different shooting sessions one day apart.

0. Pre-Processing

For preprocessing I used PixInsight‘s new WeightedBatchPreprocessing script, leaving most of the options as default. I reused a master bias and darks from my last project, and a new set of flats.

The darks are useful to remove an annoying amp glow affecting this camera, otherwise they would have been practically useless thanks to dithering.

The image below compares a 50% crop of a single 180 seconds shot (to which only Debayer and AutomaticBackgroundExtractor were applied) with an integration of 55 calibrated 180 seconds shots stacked using 2x DrizzleIntegration (again,  AutomaticBackgroundExtractor was applied and the image was resampled to achieve the same scale despite the drizzling)

This is the result of the 53 x 300s integration using 2x drizzling.

1. Star halo reduction

Since i have no experience processing narrowband shots and i’m still experimenting a lot, i won’t illustrate the processing in detail.

I’ll show just a few important steps, the first being an attempt to reduce a nasty halo present around the bright stars (exspecially Sadr), in the Oiii channel. At the same time i tried to reduce the star size in order to have a better blend with the Hα channel in the bicolor image.

I extracted the Oiii channel by adding together the G and B channels with PixelMath, and did several steps with the  MorphologicalTransformation and CurvesTranformation processes, always using a StarMask. The result is satisfactory, despite i found very difficult to give the halo around Sadr a natural look. You can see it in the following 100% crop.

2. Bicolor composition

Once i pre-processed the Hα and Oiii channels, i tried several ways to compose them  in order to create a color image.

Following the indications in this tutorial:
https://www.lightvortexastronomy.com/tutorial-narrowband-bicolour-palette-combinations.html
i found the better results were from the classic LRGB-style composition and the blended channels composition. In the end i chose the first one, perhaps for the most natural colors.

3. Denoise

As usual, i used the two-steps denoise procedure as described by Jon Rista:
https://jonrista.com/the-astrophotographers-guide/pixinsights/effective-noise-reduction-part-1/
with good results. I have to say this old 1100D camera is significantly noisier than my full-frame 6D.

The following 100% crop show the effectiveness of this method.

4. Stretch

I used MaskedStretch in order to exploit the signal and keep the star size small at the same time. The difference between this and a simple STF autostretch is clearly visible:

5. LocalHistogramEqualization

I always find this step extremely useful, it really gives the image a punch in contrast and it enhances the beautiful and complex structures of the nebulosity, as this crop clearly shows:

6. Final Color correction

I made the final corrections with CurvesTransformation and HistogramEqualization, in order to enhance the colors and to precisely set the black point, and a final contrast boost with the DarkStructuresEnhancement script.

I really don’t like the star colors, i tried to obtain a more natural color using the Ballesteros method, but i didn’t have much success with it: i’ll try it again with more effort in my next l-enhance shot.

And here is the final image, resampled at a resolution of 2460 x 1690 pixels.

Conclusions

For a test shot, the result is not bad at all… the 1100D is quite noisy compared to my 6D and i clearly need better processing skills.

Anyway, the summer Milky Way is finally starting to rise high up in the sky, and in my next project i’ll shot the North America nebula using this same setup. See you soon 🙂

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