Crescent Nebula (NGC6888), HOO & Ha versions

[turfpit]

Captured over 3 nights (1 night of data lost due to a failed heater band) late September 2025.

Equipment: Celestron AVX mount, Vixen 81s, 0.67x reducer/flattener, Atik EFW2 9 slot filter wheel, Altair Ha & OIII 7nm filters, Atik 314L+ camera (set point cooling at -5°C), QHY mini guide scope with QHY5LII-M guide cam.
Capture: images were captured with 10x300s, 16x450s and 12x600s exposures (guided) giving a total integration time of 4h 50m.
Processing: PixInsight and Starnet++.

Crescent Nebula - NGC6888.JPG (65.98 KiB) Viewed 1648 times

The OIII never fails to disappoint!

Workflow for processing the colour image:

• Channel combination for HOO

• ScreenTransferFunction (STF) - autostretch, unlink, autostretch

• Gradient Correction

• Colour calibration - SPCC

• STF - link, autostretch

• STF - set sky background

• Delinearise with STF + Histogram Transformation

• Export file as a TIFF for use with Starnet++

• Run Starnet++ to create a starless image (starless)and load back into PixInsight

• PixelMath, original - starless = stars_only

workflow.JPG (85.42 KiB) Viewed 1648 times

• Process starless - Histogram Transformation (stretch & black point), create Range Mask, Local Histogram Equalisation, CurvesTransformation (satuaration, 'S' curve)

• Process stars_only - Morphological Transformation (Selection) to reduce stars

• PixelMath, starless + stars_only = final

• CurvesTransformation (saturation, 'S' curve)

• TGVDenoise & MultiScaleMedianTransform to remove layer 1

The Ha only image followed the same (applicable) steps.

I ran both images through StarFixer at https://www.starfixer.org/. Here are the results:

StarFixer_HOO.JPG (77.22 KiB) Viewed 1648 times

StarFixer_Ha.JPG (75.05 KiB) Viewed 1648 times

In conjunction with Brian (@oopfan) some analysis was done on the exposures (300s & 600s) and it was decided to compromise on 450s to mitigate against potential star bloat.

This workflow will need some study and I may post improved images here in the future.

Dave

[admin]

Hi Dave,

thanks for posting both the images and the workflow - this sort of detailed 'howto' and analysis of how the results turn out is so useful for those wishing to try this sort of processing themselves

cheers,

Robin

[turfpit]

Thanks Robin.

Writing up the steps helps me clarify the processes in my own mind plus I have a reference to go back to.

I have reprocessed the Ha & OIII data and produced this doc which only took me 3 years to get here.

Processing NGC6888 Using PixInsight and Starnet.pdf

(728.07 KiB) Downloaded 80 times

This is the final image with processing steps as described in the doc.

final.jpg (583.05 KiB) Viewed 1531 times

The Crescent is moving out of position for me now. The race is on to capture more Cygnus Wall and Pacman before the clocks change.

Pacman to date:

Pacman_HOO_Ha-15x600s_OIII-10x450s.jpg (1022.17 KiB) Viewed 1531 times

Thee Cygnus Wall - Ha 5x600s so far:

Cygnus_Wall_Ha-5x600s.jpg (549.26 KiB) Viewed 1531 times

Dave

[turfpit]

A comparison 2 days apart after some work evaluating how best to process the starless version. Version 2 (processed only with PixInsight & Starnet++) also has more aggressive star reduction.

comparison.JPG (74.02 KiB) Viewed 1492 times

The workflow for version 2 (PixInsight & Starnet++) is contained in the 13 page PDF above.

Final assessment:

• I intended to exceed 10h of data capture when I started imaging the Crescent. Bad weather (4 weeks of August and 2 weeks of September lost) plus the dew heater band failure (1 night of imaging scrapped) only allowed for 4h 50m of capture.

• For objects in rich star fields, the stars can detract from the subject. Working with starless and stars_only images I think has produced the better image (v2) - thanks to the power of PixelMath. The separation of the images has allowed for more appropriate noise reduction and stretching.

• The new (to me at £250) Atik 314L+ with set point cooling has performed well at -5°C. Being able to build a darks library on cloudy nights with the scope located in the greenhouse has been a real time saver (no more watching the sun rise while capturing 'temperature matched darks'). The new (to me at £400 including 7 Altair filters) Atik 9 slot filter wheel is really convenient as it helps simplify capture.

• The 3 year journey with PixInsight has been worth the effort.

Maybe next year ....?

Dave

[admin]

Hi Dave,

all good stuff, and it's certainly nice when you get equipment that just works day in, day out...

I have been thinking for a while that separate star and background processing would be a big benefit to SharpCap live stacking - reduced star sizes, better star colours and improvements to background processing with no stars to worry about would be among the benefits. However, automatically generating a star mask (or, equivalently, doing something like the starless image generation you are using) is a significant image processing challenge... If I ever get somthing like that written then everything else in the separate processing (at least for live stack type processing) is fairly routine.

cheers,

Robin

[turfpit]

A reprocess of the data. I think this is as much as I can wring out of the current dataset.

Crescent_HOO_Ha-12x600s_OIII-14x450s.JPG (46.9 KiB) Viewed 1384 times

Out of position for me when the clocks go back - see you next year Crescent Nebula.

Dave

[kaymann]

Wow lot's of work lot's of reward - great captures!

[turfpit]

Thanks Randall. These objects are a bit light on data - Crescent & Pacman ~4h each and Cygnus Wall 1h.

I purchased PixInsight in May 2022. Learning was uphill at first but the release of the official PixInsight Videos was a turning point https://www.youtube.com/@PixInsight for me. Professionally produced and scripted, being broken down into 10 minute chunks which didn't overwhelm the viewer. Books which have been helpful are:

Inside PixInsight https://link.springer.com/book/10.1007/ ... 19-97689-1 by Warren Keller
and
The Deep-Sky Imaging Primer https://digitalstars.wordpress.com/primer/ by Charles Bracken

The hours invested are now staring to pay off.

Dave

[kaymann]

I am just now transferring my processes from Siril to Seti Astro Suite. Have you looked at it? As much as it is a FOSS option to PixInsight, it's learning curve is just as steep if not not higher. It's author is throwing in new wrenches in this amazing toolbox all the time. I can hardly keep up. Check it out and tell me what you think... available for Win, Linux and Mac. The author is just as much a geek as we are.

Randall

[turfpit]

I have not looked at the suite. I know the author has written some scripts for use in PixInsight. I think I will stay focussed on PixInsight. So much to learn.

Dave