NGC 6979 in Cygnus

Pickering’s Triangle is the central portion of the Veil Nebula, the famous supernova remnant high in the northern summer sky. This portion of the Veil is also known as Fleming’s Triangular Wisp after astronomer Williamina Fleming, who discovered the object in the late nineteenth century while studying photographic plates at the Harvard College Observatory. The Veil Nebula, also known as the Cygnus Loop, spans nearly 3 degrees or about 6 times the diameter of the full Moon, which translates to over 70 light-years at its estimated distance of 1,500 light-years. The supernova itself occurred about 6500 years ago.

This is a bicolor image, with light from hydrogen mapped to red and light from oxygen mapped to green and blue. The resulting teal color for OIII has been shifted slightly to blue in processing. This a two-panel mosaic. North is to the right and west is down.

Exposure: Total exposure time 29 hours. The left panel (south) is 13.7 hours (17:24 x 20 minutes Ha:OIII) and the right is 15.3 hours (20:26 x 20 minutes Ha:OIII). All bin 1x1. Captured from July to September 2019.
Light pollution: Bortle 7-8 (white zone, NELM about 4.5)
Seeing: Average FWHM of subs about 2.2 arcsecs
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 1.2 arcsecs/pixel (50% reduction)

Equipment:
Scope: C11 (standard, not Edge) with Celestron 0.63 reducer
Mount: Paramount MX+, connected via ASCOM Telescope Driver 6.1 for TheSkyX, with MKS 5000 driver 6.0.0.0
Camera: SXVR-H694, connected via SX ASCOM driver 6.2.1.17140 (SX 1.2.2 also installed)
Filter wheel: Atik EFW2 with 7x1.25 carousel and Artemis 2.4.3.0 driver
Filters: Astrodon 5nm Ha/SII, 3nm OIII, Type IIe LRGB
Rotator: Optec Pyxis 2", connected via Andy Galasso's 0.4 driver (Optec Pyxis Rotator AG)
Focuser: Rigel Systems GCUSB nStep motor with driver version 6.0.7 on stock Celestron focuser
OAG: Orion Thin OAG
Guide cam: Lodestar (first generation). 4 second exposures
Automation SW: Sequence Generator Pro 3.0.0.8
Guide SW: PHD2.6.6, connected to guide cam via native SXV driver
ASCOM: ASCOM 6.3.0.2831
Platesolving: PlateSolve 2, failover to local Astrometry.net 0.19 server
Collimation: Metaguide 3, using ASI120MM connected via ZWO Direct Show driver 3.0.0.2
Processing Workflow by Workspace in PixInsight 1.85

1. Calibration and Alignment
BatchPreProcessing with flats, darks and bias, using Cosmetic Correction with master dark
Blink to preview and reject a few frames
Subframe Selector to confirm selections and weight the frames by SNR and FWHM
StarAlignment with mode set to Union-Mosaic to create a mosaicked image using the best OIII frame from each data set.
StarAlignment set to Register/Match Images, using the mosaicked image as the registration frame. This registered all the subframes to the mosaic.

2. Stack and Mure Denoise
Separate ImageIntegration runs for the left and right panels
Mure Denoise
Dynamic Crop (removes the black area from each integrated image, making the next steps easier)

3. Linear Processing and Mosaic Merge
Dynamic Background Extraction
Deconvolution with Dynamic PSF and star mask
StarAlignment set to Register/Match Image using the mosaicked image as the reference frame. This gives left and right panels of equal size, which appears to be required for the next step.
GradientMergeMosaic set to Average, with shrink radius 2 and feather radius 30. This produces a single mosaicked image for each color channel.

4. Stretching on each channel
Histo Transformation (2)
Masked Stretch
Curves Transformation (2)
TGV Denoise
Aggressive Multiscale Median Transform to remove lumpiness in background, using an inverted and blurred range mask to protect highlights

5. Color merge
Merged narrowband channels in ChannelCombination, mapping SII:Ha:OIII to R:G:B for SHO image and Ha:OIII:OIII to R:G:B for HOO bicolor image
Color shifting:
On SHO, use SCNR to shift green to neutral/yellow
On HOO, Color Mask script on cyan, then curves to shift cyan/teal to blue

6. Star Reduction
Star mask creation with AdvStarMask from Herbert Walter (skypixels.at)
Second star mask for large stars using StarMask
Merge star masks with PixelMath max command
Morphological Transform to shrink stars (3 times)
Multiscale Linear Transform to sharpen fuzzy star cores

7. Final
Mild Curves Transformation
Slight Multiscale Linear Transform to sharpen bright strands of nebulosity
sRGB
Downsize 50% and save as JPG