Stephan’s Quintet - Stephan’s Quintet – HCG 92 / Arp 319 Compact Galaxy Group - Image Processing Walkthrough

Nov 22

Written By Patrick A. Cosgrove

November 23 2025

🔭 Project Summary

Target: Stephan’s Quintet – HCG 92 / Arp 319 Compact Galaxy Group

Capture Dates: October 16, 17, 26–28, 2025

Constellation: Pegasus • Distance: ≈ 270–300 million light-years (main group)

Type: Compact Galaxy Group / Interacting Galaxies (with foreground spiral NGC 7320)

Imaging Period: October 16–28, 2025 • Total Integration: 23 h 57 m (LRGB)

Filters: L · R · G · B (ZWO 36 mm LRGB Gen II)

Telescope: Sharpstar SCA260 V2 260 mm f/5.0 Special Cassegrain Astrograph (≈1300 mm)

Camera: ZWO ASI2600MM-Pro (−15 °C; Gain 0 LRGB)

Mount: iOptron CEM70 on custom steel pier

Processing: PixInsight (LRGB) & Photoshop

Location: Whispering Skies Observatory · Honeoye Falls, NY (USA)

Acquisition notes: L: 412 × 90 s; R: 181 × 90 s; G: 174 × 90 s; B: 191 × 90 s at −15 °C, Gain 0; total 23 h 57 m after culling bad subs.

🔗 Detailed Processing Walkthrough →

Table of Contents Show (Click on lines to navigate)

Special Note

Welcome to the New Image Processing Page for this project! You got here by following a link in the main Image Project Report, and you can easily return to that by using the back button on your browser.

Abbreviations Used

BXT BlurXTermminator by RC-Astro

CC Cosmetic Correction

CT Curves Transformation Process

DBE Dynamic Background Extraction Process

ET Exponential Transformation

HT Histogram Transformation

NXT NoiseXTerminator by RC-Astro

MLT Multiscale Linear Transform

PI PixInsight

PS Photoshop

SCNR Subtractive Chromatic Noise Reduction Process

SFS SubFrameSelector

SPCC SpectroPhotometric Color Calibration

STF Screen Transfer Function

STF->HT method – Drag the STF triangle to the base of HistogramTransformation, then apply it to the image to take it nonlinear.

SXT StarXTerminator by RC-Astro

WBPP Weighted Batch Preprocessing Script

Processing this Image

(All Processing is done in PixInsight, with some final touches done in Photoshop)

1. Blink

First, I screened the data for thin-cloud frames and obvious defects. It seemed that a lot more images were removed here than for my other targets shot during this cycle. Perhaps this had to do with where in the sky I was pointing - or perhaps LRGB shows the effects of clouds in a more pronounced way in LRGB than seen during Blink for narrowband - I really don’t know!

Ha
- 39 frames removed - mostly for thin clouds
Red
- 26 frames removed - for thin clouds.
Green
- 31 frames removed for thin clouds.
Blue
- 34 frames removed, for thin clouds
Darks
- All looks ok
Dark Flats
- All looks ok
Flats
- all good

A total of 130 frames were removed during Blink.

2. SubFrameSelector

Getting critical details was crucial for this image, so I wanted to remove the subs that showed statistically significant detail degradation. To determine which frames, I used SubFrameSelector to examine the FWHM size of stars and removed those outside the confidence bands.

Note the nightly pattern of star sizes increasing as the night progressed - this is almost certainly a dew effect, and it was seen in each channel.

This was the first project where I could clearly see dew buildup overnight in the SFS FWHM plots for every channel - that’s what drove the aggressive frame rejection.

Now the red channel, where 14 frames were removed. A few frames were removed because their star sizes were too small. This is almost certainly from clouds attenuating them - ones that I missed during the Blink operation.

Now the Green Channel, where 16 frames were removed.

And finally the Blue channel, where 19 frames were removed.

A total of 67 frames were removed during the SFS Operation.

3. WBPP 2.8.9

With the bad frames identified, I ran everything through WBPP with a quality-first configuration:

Reset everything
Load all lights
Load all flats
Load all darks
Select - maximum quality
Reference Image - auto - the default
Select the output directory for the WBPP folder
Enable CC for all light frames
Pedestal value - auto
Darks - set exposure tolerance to 0
Lights - set exposure tolerance to 0
Lights - all set except for a linear defect
set for Autocrop
I chose NOT to use Drizzle processing, even though I wanted the highest possible detail. In the past, I have had problems using the RC-Astro set of tools with images from the SCA260 V2 when running Drizzle.
WBPP run in 3:27:13 - no errors

Note that the WBPP process eliminated an additional nine frames.

4. Load Master Images and Create Color Images

Load all master images and rename them.
Using CombineChannels, create the Master RGB color image

The Individual L, R, G, and B master linear images.

5. Initial Process of Linear Lum data

Run DBE for the Lum linear image. Use subtraction for the correction method. Choose a sampling plan that avoids the galaxies and bright stars. (see below)
Run BXT - correct only. This cleans up the stars at the corners. Not much to do in this image as the scope is very crisp.
Run PFSImage script to measure star sizes. X = 5.70 Y= 4.97. This will influence the values used in BXT.
Run Full BXT - I am using an enhanced set of values to shrink stars more. These are about double the measured star sizes. See the BXT Panel Snapshot below.
Run NXT V3; refer to the parameters in the snapshot below.
Run SXT - no need to save the Lum stars, as we will not be using them.

Master L Image DBE Sampling Plan (click to enlarge)

Background Subtracted by DBE (click to enlarge)

Measuring Star Sizes with PFSImage Script (click to enlarge)

Before BXT

BXT Correct Only

After Full BXT

After NXT V3

Master Lum Before BXT Correct Only, After BXT Correct Only, After BXT Full, After NXT

Master Lum Starless Image (click to enlarge)

6. Initial Process of Linear RGB data

Run DBE for the RGB linear image. Use subtraction for the correction method. Start with the same sampling plan from the Lum run. Choose a sampling plan that avoids the galaxies and bright stars. (see below)
Select a background preview, then set up and run SPCC. See the SPCC Panel shot below for the params used.
Run BXT - Correct only. This cleans up the stars at the corners. Not much to do in this image as the scope is very crisp.
Run the PFSImage script to measure star sizes. X=5.36 Y = 4.36. This will influence the values used in BXT.
Run Full BXT - I am using an enhanced set of values to shrink stars more. These are about double the measured star sizes. See the BXT Panel Snapshot below.
Run NXT V3; refer to the parameters in the snapshot below.
Run SXT - this time we will save the RGB stars.

Master RGB Sampling Plan (click to enlarge)

Master RGB before DBE (click to enlarge)

Master RGB Background removed (click to enarge)

Master RGB before SPCC(click to enlarge)

Before BXT

BXT Correct Only

Full BXT

NXT V3

Master RGB Before BXT Correct Only, After BXT Correct Only, After BXT Full, After NXT

Master RGB Starless Image. (click to enlarge)

7. Go Nonlinear

Using the STF->HT Method, take the linear Lum and RGB images nonlinear.
Using Seti Astro Star Stretch, take the RGB Star image nonlinear.

Nonlinear Starting Lum Image (zoomed) (click to enlarge)

Nonlinear RGB image (full frame) (click to enlarge)

The Star Stretch Script and parameters used.

8. Process the Nonlinear Lum Starless Image

Use HT to clip the left side of the histogram and darken the frame a bit
Apply NXT V3 now (see parameters in the screenshot below). I want to do this now, before I start sharpening and bringing out details
Apply LHE with a scale of 26, contrast limit of 2.0, amount of 0.14, and an 8-bit histogram. This helps bring out details.
Apply CT to darken things a bit.
Apply MLT Sharpening using the built-in linear mask (see params used in screenshot below)

NXT Parameters we are about to use (click to enlarge)

MLT Sharpening Set up (click to enlarge)

9. Now Process the RGB Starless Image

Use HT to adjust: the left side of the histogram is pretty empty, so bring it in and darken the image a bit.
Adjust tone scale with CT
Now apply NXT in preparation for bringing in the Lum data - you want the color to be as smooth as possible, as the detail will come from the lum image.
Use ChannelCombination tool to bring in the Lum image.
Apply a final CT

Initial RGB nonlinear image (click to enlarge)

NXT Params we will be using in the next step.

ChannelCombination Panel setup for bring in Lum data.

Add Lum data into the RGB data (click to enlarge)

10. Add the Stars Back In

Using the ScreenStars Script, add stars back into our RGB starless image.

The script used to add the images back in,

11. Export the Image to Photoshop for Polishing

Save the image as a TIFF 16-bit unsigned and move to Photoshop
The big issue here is cropping. I chose an initial crop and did a little polishing of the image.
I shared the image with others, and the feedback was to crop it a bit more, but unless I could get some effective sharpening done, I would have to limit this a lot. So I exported the cropped image back to PixInsight to experiment a bit more with sharpening.
I cropped the image by another 25%
Then I saved the TIFF Image, and returned to PixInsight

10. PixInsight Sharpening

I reloaded the image to PI
I experimented with various sharpening methods to take things further. I tried MLT, UnsharpMasking, and more LHE. None really helped and often harmed the image by creating artifacts.
I then experimented with BXT, used not for deconvolution but for sharpening. ( I felt a little guilty and ashamed even doing this!)
I found that when used with small values, I was getting some very pleasing sharpness enhancements. But even though I had set the stellar correction to 0.0 - my stars were starting to get some hard edges to them.
So I ran SXT and created a new version of the stars and starless images
I ran BXT for sharpening on the starless image (see the screen snap below for the parameters used)
Then used ScreenStars to put things back together.
I saved the image to a 16-bit unsigned TIFF and went back to Photoshop.

Starless version of the Final Crop. (click to enlarge)

The Stars Only Version (click to enlarge)

Final Cropped and sharpened image - ready for the final polish!

11. Final Work with Photoshop

Make final global adjustments with Clarify, Curves, and the Color Mixer - slight tweaks really
Added Watermarks

Export Clear, Watermarked, and Web-sized JPEGs.

The Final Image!

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Thanks,

Pat

Patrick A. Cosgrove

A retired technology geek leveraging his background and skills in Imaging Systems and Computers to pursue the challenging realm of Astrophotography. This has been a fascinating journey where Art and Technology confront the beauty and scale of a universe that boggles the mind…. It’s all about capturing ancient light - those whispering photons that have traveled long and far….

https://cosgrovescosmos.com/