After a very wet July and cloudy first week of August the night skies opened up for me between the 6th and 9th of August. A target that I have not tried before I think because using the APS-C sensor in the DSLR the simulated field of view, with the 533mc's tight crop with my Z61 it makes for a more appealing target.
In the centre of Cygnus the Crescent Nebula, NGC 6888 which in my eye looks more like a brain than a crescent is around 5000 light years away from us. It is an emission nebula, which glows by the excitation of atoms of specific elements that give light off at specific frequencies (colours). What makes this emission nebula unusual and what gives it a shell like structure is the star that lies at its centre. The star in question is catalogued as HD 192163 and what make it special is it is a type of star called a Wolf-Rayet, astronomers love creating lists so this is also known as WR 136. This type of star is huge when compared to the sun, 5 times larger, 21 times more massive and is 600,000 times brighter that our own star. One thing about stars this big is that in order to fight the immense gravitational forces that are attempting to crush it, the star must fuse its material much faster. In fact this star is only about 1000th the age of the sun. It is estimated that it will go Supernova soon (note soon in astronomical timescales is in the order of 100,000 years!).
Here is a close up of the nebula, you can see in the image two 'shells'. This is due to the Wolf-Rayet stars lifecycle as it rushes to its final end which will be a supernova, first, after about 4.5 million years it expanded to become a red giant star in the process ejecting its outer layers of material at a speed of 20,000 miles per hour, then after about 200,000 years (very short in terms of stellar evolution) the exposed inner layer of the star created a stellar wind (stream of radiation particles) that travelled outwards at a rate of 3,000,000 miles an hour. When this high speed stellar wind hit the previous red giant 'wind' a dense shell was formed.
Capture
Four nights spent on this 6th, 7th, 8th and 9th of August 2023.
All four nights using my usual equipment with the new ZWO ASI533MC Pro with gain of 100 offset 40 and running at -10ºC through the Optolong L-Enhance Filter and 300s exposures. A master dark for this exposure and temperature was used as part of the lights pre-processing.
2023-08-06
54 Images taken (300s) through l-Enhance Filter
30 flats (2 rejected as outliers (Median value)
30 dark flats used as bias
Seq # 000-053
2023-08-07
45 Images taken (300s) through l-Enhance Filter
30 flats
30 dark flats used as bias
Seq # 100-144
2023-08-08
64 Images taken (300s) through l-Enhance Filter
30 flats (removed 5 for bad medians (11,27,28,29,30))
30 dark flats used as bias
Seq # 200-263
2023-08-09
62 Images taken (300s) through l-Enhance Filter
30 flats (removed 1 for bad medians (1, 8))
30 dark flats used as bias
Seq # 300-361
Pre-Processing
Total of 225 Lights.
13 failures
212 registered files
Calibration File Processing
Failures:-
7 star trails above (see investigation for sessions 1-3) above
4 Clouds (303-306)
2 elongated stars - 316/317 - Initial investigation shows 6 minutes of phd trying to settle, I am putting this down to cloud issues. The night cleared after around midnight both of these occurred just on this Transition.
Registration and Stacking
I used the following cutoffs to remove some of the weaker images:-
Removing all frames where there were less than 30% of the maximum of 1900 (570) = 14 frames
Removing remaining frames there were less than 75% round=14 frames
Removing remaining frames that are 350% of the minimum Background value (>0.009425) = 7 frames
Removing remaining frames that have FWHM > 3.5 = 4 frames
Removing remaining frames that have wFWHM > 6 = 10 frames
163 images of 300s = 13h35m
Total Integration Time = 13h35m
I was using a new version of Siril which provided some opportunities to use various 'improvements' that had been added (current v1.2.0 rc-1). I will provide a different post of my findings of the different stacking methods, the selected Stacking parameters was to use Winsorised Sigma Weighted using Noise with RGB and Output Normalisation On.
Post Processing (Aug 10, 2023)
Siril
Cropped to remove stacking artefacts at the borders
Background Extraction using the RBF method
Photometric Colour Calibrated
Deconvolution
New Siril Deconvolution used taking Dynamic PSF function of stars. Parameters for iteration and gradient decrease step size were derived after iterations over a highly cropped version of the full image.
PSF on 0.3-0.7 Amplitude stars within 60% of the Centre of the image. Used the PSF generated at size 9…
Starless processing using Siril integration with Starnet to produce a mask and a slightly stretched starless image.
At this point this is the auto-stretched version of the starless image:-
Generalised Hyperbolic Stretch - another new feature of Siril
Green Noise Removal (SNR)
Denoise using NL-Bayes denoise (mod=0.139, CC enabled) - very subtle, if any visible changes.
Saturation - Cyan/Magenta Increased then global increase
New Process using Trial version of Noise X-Terminator
Converted to 16 bit Tiff - Opened in PS and used Filter->RC_Astro->Noise X-Terminator
Saved back opened in Siril and converted back to 32 bit Fits format.
Major improvement - this will be a purchase!
Star Mask Opened and Generalised Hyperbolic Stretch applied to bring up the levels but control the size and brightness of the stars.
Recombined with a minor Generalised Stretch to get the final result.
Created JPG version for my Social feeds and Astrobin.
Result
The long integration time really helped and the new processing options in Siril I think made this one of my best images to date, I am really happy with how it turned out. Special mentions go to the new Deconvolution and Generalised Hyperbolic Stretch in Siril, also to the Noise X-Terminator Photoshop plugin which as I mentioned in the walkthrough will be a definite buy now I have seen it in action.
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