SH2-91, located in the constellation Cygnus, is a fascinating supernova remnant lying about 6,000 light-years from Earth. This celestial object is the aftermath of a massive stellar explosion, where the outer layers of a dying star were ejected into space, creating a cloud of ionized gas and debris. The remnant is part of the Sharpless catalog of H II regions, though its nature as a supernova remnant distinguishes it from the more commonly known emission nebulae in this catalog.
SH2-91 is sometimes referred to as “The Other Veil in Cygnus”, because a much brighter and popular supernova remnant called the Veil Nebula also resides in Cygnus.
This particular image is a 5-panel mosaic and only captures a small portion of the entire faint structure. The supernova that created SH2-91 is thought to have occurred several thousand years ago, and the remnant’s structure is shaped by the shockwaves from the explosion. These shockwaves interact with surrounding gas and dust, creating a complex, expanding shell of material. SH2-91 is relatively faint and not immediately visible to the naked eye, but it can be detected in deeper observations, where it appears as a faint, diffuse glow.
Imaging Details
- Workflow: Narrowband workflow and Mosaic Workflow
- Mosaic: 5 panels
- HA: 18*600 seconds (each panel)
- OIII: 18*600 seconds (each panel)
- Binning: 1×1
- Total Imaging Time: 30 hours
- Imaging Dates: (11 nights)
- 3/09/2024
- 3/10/2024
- 4/14/2024
- 5/06/2024
- 5/09/2024
- 5/10/2024
- 5/13/2024
- 6/01/2024
- 6/03/2024
- 6/16/2024
- 6/20/2024
Imaging Workflow
This image followed the Narrowband workflow and Mosaic Workflow
Integrated Image
The process begins with 10 images: Hydrogen-Alpha and Oxygen-III for 2 separate mosaic panels. The normal integration process was used except that drizzle integration was bypassed. Only showing the panels for the Hydrogren-Alpha channel.
Crop
The dynamic crop process was used to remove the bad edges.
Gradient Correction
The Gradient Correction process made it easy to remove the extreme darkness/brightness around the edges.
Mosaic Alignment
This image followed the Mosaic Workflow using a generated star field for the area. With a valid star catalog, each panel used Star Alignment to create place the panel in the correct location and orientation on the field.
Gradient Merge Mosaic
The DNA Linear Fit script balanced the brightness across the three panels. Once complete, Gradient Merge Mosaic blended the edges together into a single mosaic image for the red, green, and blue filters.
Dynamic Crop
The dynamic crop process was used again to clean up the edges.
Gradient Correction for Mosaics
With the single integrated mosaic image, there is often a gradient across the image. Gradient Correction process gets re-applied.
At this point, the mosaic integration process is complete and processing returns to the normal workflow.
Deconvolution
BlurXTerminator is applied two times. The first time is for Correction only. This takes care of the coma (blurring of stars along the edges).
The second application of BlurXTerminator handles deconvolution, which improves the sharpness and details of the nebula while removing the blur from the image.
Noise Reduction
With the blurring corrected, it is time to do noise reduction. This time I used the new NoiseXTerminator add on.
Channel Combination for RGB
The next step is to do a Linear Fit and then a Channel Combination to combine the channels into RGB.
In addition, once the color image was created, the luminance channel was extracted to be used for the luminance workflow.
Star Removal
To better protect the stars from bloating due to additional processing, they are removed with StarNet+
Histogram Stretch
I used histogram transformation to stretch the image. Although the image appears dark, the entire image will get brighter when the synthetic luminance is added.
SCNR
PixInsight SCNR was used to remove the green hue
Color Saturation
Luminance Workflow
Before doing anything more to the color image, it is time to go through the Luminance Workflow.
Deconvolution was skipped as it was done to the RGB image with the use of BlurXTerminator.
Luminance Integration (LRGB)
With the luminance image processed, it can get integrated with the color image with LRGB Combination process. This used 45% for lightness and 35% for saturation.
Stars
Pixel Math was used to re-add the star field back into the image. The luminance star field was used as a mask and the color star field was then applied.
Brightness
Curves transformation was used to brighten the overall image.
ACDNR for Chrominance
Although NoiseXTerminator did a great job in removing the noise from the image early on in the workflow, some of the processing might have added noise in the color. Applying ACDNR for only chrominance helps correct this without losing details.
Final Adjustments
To finish the image, I make a few minor tweaks to the brightness and colors of the stars using Curves Transformation. I also cropped the bottom and left side to remove the two bright stars as I wanted the focus to be on the reflection nebula.
