October 8, 2026

This page is the mage Processing Walkthrough page for the SH2-173 2026 Imaging Project

🔭 Project Summary

Target: Sh2-173 — The Phantom of the Opera Nebula / Sharpless 173 / Simeis 21 / LBN 593

Principal Ionizing Star: BD+60 39 (ALS 6151), an O9V star

Capture Dates: September 6, 11, and 14, 2026

Constellation: Cassiopeia • Distance: ≈ 8,200 light-years (± 1,600 light-years)

Type: H II emission nebula associated with molecular clouds and young stars in the Milky Way’s Perseus spiral arm

Imaging Period: September 6–14, 2026 • Total Integration: 15 h 46 m 00 s (SHO narrowband + RGB)

Filters: Ha · OIII · SII (Astronomik 36 mm 6 nm) + R · G · B (ZWO 36 mm LRGB Gen II)

Telescope: Astro-Physics 155 mm Starfire EDFS f/5.3

Camera: ZWO ASI2600MM-Pro (−15 °C; Gain 100 narrowband, Gain 0 RGB)

Pier / Support: iOptron Tri-Pier with column extension on custom steel pier

Processing: PixInsight (SHO narrowband + RGB) & Photoshop

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

Acquisition notes: Ha: 70 × 300 s; OIII: 39 × 300 s; SII: 69 × 300 s at Gain 100. R: 16 × 90 s; G: 16 × 90 s; B: 16 × 30 s at Gain 0. All exposures bin 1×1 at −15 °C; 226 light frames totaling 15 h 46 m 00 s after culling bad or questionable subs.

Image note: This SHO narrowband and RGB image captures Sh2-173, the Phantom of the Opera Nebula, with a bright orange-gold rim, a softly glowing interior, and intricate dark markings that suggest a ghostly mask. Faint surrounding emission and dark lanes extend across the Cassiopeia star field, placing the central nebula within a broader landscape of interstellar gas and dust.

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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

ACM Astro Color Mixer Script

ACE Astro Contrast Enhancer Script

BXT BlurXTerminator by RC-Astro

CC Cosmetic Correction

CT CurvesTransformation Process

DBE DynamicBackgroundExtraction Process

ET Exponential Transformation

HT HistogramTransformation Process

NXT NoiseXTerminator by RC-Astro

MLT MultiscaleLinearTransform Process

PI PixInsight

PS Photoshop

SCNR Subtractive Chromatic Noise Reduction Process

‍ SFS SubframeSelector

SPCC SpectroPhotometric Color Calibration

STF ScreenTransferFunction

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

Updated to match the [published Sh2-173 walkthrough](https://cosgrovescosmos.com/image-processing/sh2-173), while retaining your original styling and mobile layout. ```html

🔭 SH2-173 — SHO + RGB Stars Processing Flow

Summary of the actual SH2-173 image-processing walkthrough: calibration and integration, separate narrowband and RGB processing, development and comparison of HSS and SHO versions, final SHO refinement, RGB-star stretching and recombination with Astro Star Crafter, and Photoshop output.

1. 🔍 Blink Review and Frame Culling

1. Review Narrowband Lights Removed 5 Ha, 4 OIII, and 7 SII frames affected by clouds. OIII showed very little signal, and SII was weak.

2. Review RGB Lights Removed one frame from each RGB channel because of trees. Discovered that Blue exposures were 30 seconds instead of the 90 seconds used for Red and Green.

3. Inspect Calibration Frames Reviewed the darks, dark-flats, and flats; all appeared suitable for use.

2. ⚙️ WBPP 3.01 Calibration and Integration

4. Load and Configure WBPP Reset WBPP, loaded the calibration and light frames, and selected maximum quality, automatic reference selection, automatic pedestal, Cosmetic Correction, and autocrop.

5. Set Exposure Tolerances Set both dark- and light-frame exposure tolerances to 0.

6. Calibrate and Integrate WBPP completed in 41 minutes, 14 seconds with no errors.

3. 🎨 Construct the Color Masters

7. Load the Six Masters Loaded and renamed the integrated Ha, OIII, SII, R, G, and B master images.

8. Build SHO and HSS Versions Used ChannelCombination to create both narrowband color versions for later comparison.

9. Build the RGB Master Created the RGB image for the stars. Its initial color balance reflected the unequal RGB exposure times.

4. ✨ Process Both Linear Narrowband Images

10. Apply DBE Used DynamicBackgroundExtraction on both the SHO and HSS masters, with separate background sampling plans.

11. BXT Correct Only, Then Full BXT Applied BlurXTerminator correct-only mode, followed by full correction. With PFSImage unavailable after the PixInsight upgrade, settings were selected iteratively.

12. NXT V3 Applied NoiseXTerminator V3. The same BXT and NXT parameters were used for both narrowband versions.

13. Remove Narrowband Stars Used StarXTerminator on both images and discarded the narrowband stars, retaining the SHO and HSS starless images.

5. 🌟 Process the Linear RGB Image

14. RGB Background Extraction Applied DBE with subtraction and samples placed away from nebulosity. This also improved much of the initial color imbalance.

15. RGB BXT and NXT Ran BXT correct-only, then full BXT with iteratively selected settings, followed by NXT V3.

16. Calibrate Star Color Ran SPCC after BXT and NXT in this project, using a background preview, the Ideal curve, and ZWO R, G, and B filter curves.

17. Extract and Retain RGB Stars Used StarXTerminator, retained the RGB stars, and discarded the RGB starless result. Deferred star stretching until combined previews were available.

6. 📈 Take Both Starless Versions Nonlinear

18. Stretch the SHO Image Transferred the STF-derived stretch to HistogramTransformation to create the initial nonlinear SHO image.

19. Stretch the HSS Image Used the same STF-to-HistogramTransformation method to take the HSS starless image nonlinear.

7. 🎨 Develop the HSS Comparison Image

20. Establish Tone and Contrast Used CurvesTransformation for tone and saturation, then Astro Contrast Enhancer for global Texture, Clarity, and Dehaze plus a local lasso adjustment on the left.

21. Recover Detail and Polish Used Astro Tone Recovery to recover texture in the saturated red region, followed by NXT and localized Photoshop work.

22. Add RGB Stars for Comparison Used Astro Star Crafter with its default adaptive stretch and boosted star color to produce an HSS-plus-stars comparison image.

8. 🎨 Develop the SHO Comparison Image

23. Reduce Green and Magenta Applied SCNR Green at 0.85, inverted the image, repeated SCNR Green at 0.85, and inverted back.

24. Refine Tone and Residual Green Applied CurvesTransformation, another SCNR pass to reduce remaining green, and a further curves adjustment.

25. Finalize the Initial Palette Used Astro Color Mixer to refine the SHO color balance before comparing it with HSS.

26. Add Comparison Stars Used Astro Star Crafter with its default stretch and an 85% color boost to create the preliminary SHO-plus-stars image.

9. 🔎 Compare HSS and SHO

27. Review Both Interpretations Compared the preliminary HSS and SHO images with RGB stars and shared the comparison with astrophotography colleagues.

28. Select SHO The greater color variety of the SHO version was preferred. Continued refining its starless image for the final result.

10. 🖌️ Finish the SHO Starless Image

29. Recover Bright Orange Detail Used the Astro Tone Recovery prototype to reduce the bright orange regions and restore texture.

30. Refine Tone and Nebular Structure Fine-tuned color and tone with CurvesTransformation, then used an ACE lasso selection on the main nebula to add texture and a slight Clarity boost.

31. Final Noise Cleanup Applied another NoiseXTerminator pass to the refined starless image.

32. Local Photoshop Color Polish Exported the starless image to Photoshop and used lasso selections with the Camera Raw Color Mixer for localized adjustments.

11. 🌟 Reunite the Nebula and RGB Stars

33. Preview the Combined Image Loaded the finished starless image into Astro Star Crafter and previewed the RGB-star stretch against the nebula.

34. Set Star Presence and Color Experimented with the Star Presence and Color sliders, then chose the default stretch and increased color boost used for the earlier comparisons.

35. Output the Combined Result Used Astro Star Crafter to output the finished SHO nebula with its RGB stars.

12. 🖼️ Final Photoshop Output

36. Export a 16-Bit TIFF Exported the combined image from PixInsight as a 16-bit unsigned TIFF. Little additional Photoshop polishing was needed.

37. Refine the Frame Cropped the bottom and left edges slightly, removing an awkward star split by the bottom edge.

38. Prepare Publication Versions Added the watermark and exported clear, watermarked, and web-sized JPEG versions.

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Processing this Image

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

1. Blink

Ha
- 5 frames removed for clouds
OIII
- 4 frames removed for clouds. Almost no signal
SII
- 7 frames removed for clouds. Weak signal.
Red
- 1 frame removed for trees
Green
- 1 frame removed for trees
Blue
- 1 frame removed for trees. Oops! I discovered the blue frame was 30 seconds, and the R and G filters were 90 seconds.
Darks
- All looks OK
Dark Flats
- All looks OK
Flats
- All good

2. WBPP 3.01

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
WBPP ran for 41:14. No errors.

3. Load Master Images and Create Color Images

Load all master images and rename them.
Use ChannelCombination to create the Master Color images. Since I don’t yet know whether I am going the SHO or HSS route, create one of each.
Note the weird color of the RGB image. This is caused by the imbalance in the RGB filter times - a mistake made in setting up the sequence in NINA.

4. Initial Process of Linear SHO and HSS data

\Note how it cleared up much of the balance problem o
Run BXT for both images- correct only. This cleans up the stars at the corners. Not much to do in this image as the scope is very crisp.
I just upgraded to Pixinsight 1.9.5, and for some reason my PFSImage script is no longer showing. So I usually use it at this point to extract star sizes for my BXT runs. This time I will go without it and try to determine what I need iteratively. Note the same BXT and NXT params were used on each image.
Run Full BXT on both images- I experimented and got what you see below. See the BXT Panel Snapshot below.
Run NXT V3 on both images - see params from snapshot below.
Run SXT and remove stars for both images - we are not going to use the narrowband stars, so no need to save them.

AHO - Before BXT

SHO - BXT Fix Only

SHO - BXT Full

SHO - NXT

Master SHO comparison: before BXT, after correct-only BXT, after full BXT and after NXT

HSS - Before BXT

HSS - BXT Fix Only

HSS - BXT Full

HSS - NXT

Master HSS comparison: before BXT, after correct-only BXT, after full BXT and NXT

After BXT/NXT Operations (click to enlarge)

5. Process the Linear RGB Data

Run DBE for the RGB linear image. Use subtraction for the correction method. Choose a sampling plan that avoids any nebulae (see below). You will notice that DBE removes a great deal of the strange color balance due to the RGB subs accidentally having different exposure times.
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.
I just upgraded to Pixinsight 1.9.5, and for some reason my PFSImage script is no longer showing. So I usually use it at this point to extract star sizes for my BXT runs. This time I will go without and just try to iteratively determine what I need.
Run Full BXT -Final params determined by iterative testing. See the BXT Panel Snapshot below.
Run NXT V3 - see params from snapshot below.
Normally I run SPCC before the BXT operation, but for some reason I did it after this time. Select a preview rectangle that samples the background sky, and then set up and run SPCC.
- Use the Ideal curve
- Use ZWO R, G, & B filter curves,
Run SXT and don’t save the RGB starless image - we are not going to use that one.

Before BXT

BXT Fix Only

BXT Full

Master RGB before BXT Correct Only, After BXT Correct Only, After BXT Full, After NXT V3

RGB Image before SPCC (click ot enlarge)

6. Take RGB Stars Nonlinear - But Not Yet!

In my last project, I introduced a custom star-stretching utility, and I used it to take the RGB stars nonlinear. This tool has evolved. I added the ability to stretch the stars in a way that preserves their structure and color, and now I can add my starless image to the tool and see how a particular star rendering looks combined with it.
In the past, I often made three versions of the stars: a targeted stretch, one that made the stars smaller, and one that made them bigger. Later, I would create three versions of the final image and pick the one that looked best.
With the new preview capability, I no longer have to do this, and I can now do this kind of exploration within the tool itself. So now I will leave the stars in their non-linear form and, at the end of the process, integrate them with their starless masters.
Since I can preview what this combination looks like, it was a simple task to also write out the final stars+starless image.
I will show a few screenshots of the prototype later in the process so you can see what this looks like!

7. Take the Starless SHO and HSS Images Nonlinear

Apply the STF-derived stretch to create the initial nonlinear SHO image using the STF→HT method
Do the same with the HSS image

8. Do an Initial Process of the Nonlinear HSS Starless Image

Apply CT to set the basic tone scale and color saturation
Use ACE to adjust global Texture, Clarity, and Dehaze (see screenshot below)
Use ACE to adjust a lasso-selected portion of the image on the left side (see screenshot below)
I then used another new tool I am developing called the Astro Tone Recovery tool. This is designed to recover detail from shadow and highlight regions. I used this to recover detail from the saturated red region. This is more sophisticated than just a curve adjustment; it also shaped detail of various scales to recover textures as well.

Apply NXT (see params).
Export Image to Photoshop - I wanted to work on specific areas with the lasso tool

Use my new Astro Star Crafter to do a default adaptive star stretch and then add the starless version for a preview. I opted to boost the star color as shown in the screenshot slider below and then create the final Starless+stars image.
This image will be used for a head-to-head conparision with the SHO image when it is partially processed as well.

9. Process the SHO image

Apply SCNR to green at 0.85. This removes most of the dominant green color.
Invert the image so magenta tones are now green
Apply SCNR to green at 0.85 again.
Invert the image again. Now green and magenta excesses are reduced.
Apply CT to get the basic tone and color where I want it.
At this point, I still think there is too much green in the image. So I applied SCNR again.
This was followed by another CT adjustment.
This looks better! I then used Astro Color Mixer to finalize the color position I want.
I thne added th stars back in using the Astro Star Crafter - using the default stretch and 85% color boost.

10. Compare SHO and HSS and determine the path forward

I shared the image comparison with several of my astrophotography colleagues, and everyone thought that the SHO version, with its greater color variety, was more interesting. While it doesn't use the full SHO palette you typically see, it was still more interesting to look at.

11. Finish Processing the SHO Starless Image

I used my prototype Astro Tone Recovery tool to bring down the bright orange areas a bit and restore some lost texture there. I took a screenshot of the tool before and after the change, but for some reason the after shot didn't save. That should have shown that I had the recovery slider up near the red zone and the Local Texture Slider up about as high.
Do a CT to fine-tune color and tone a bit
Use ACE with a lasso on the main nebula, add a bit of texture, and slightly boost Clarity.
Do a final NXT run - see params in the screenshot below.
Export the Starless image to Photoshop and polish - the main thing I used here was the lasso with the Raw Camera Color Mixer since I don’t have a lasso in my own ACM tool.

12. Add the Stars

Use the Astro Star Crafter Prototype to establish a baseline stretch for the stars. Then I added the Starless image in and experimented with a few different positions of the Star Presence and Color Sliders. I ended up with the same mix I chose for the original HSS/SHO prototype images: the default stretch and a higher color boost.

Using the Prootype Astro Star Crafter Tool to establish the star stretch and then output the final image.

The Stars and Starles images are reunited!

13. Export the Image to Photoshop for Polishing

Exported the combined image from PixInsight as a 16-bit unsigned TIFF
For the final image, I did little to no polishing.
I did do a slight crop - I did not like the star on the bottom being split by the edge of the frame. I cropped the bottom of the frame and a bit of the left side of the frame
Added the watermark
Exported the final JPEG versions: Clear, Watermarked, and Web-sized image.

14. The Final Image!

15. Final Comments

This image is unusual for a typical SHO narrowband image. It seems somehow softer and more muted. But I like images that surprise me a bit when I finish them.

I still have a hard time seeing the Phantom’s Mask here, but I sure do see a puppy dog, so maybe my kids will like this image!

I found this to be a fun project, and I was pretty happy to see how my Astro Star Crafter prototype is coming together. It really simplified my handling of RGB stars and reduced the steps it takes for me to finish an image!

Back to the Main SH2-173 2026 Page

Alternatively, you can use the back arrow to return to the Main SH2-173 Region page, or you can use the menu at the top of the page to continue your navigation.

Thanks,

Pat

SH2-173 - The Phantom of the Opera Nebula Image Processing Walkthrough