SH2-171 - The Teddy Bear Nebula - Image Processing Walkthru

Nov 14

Written By Patrick A. Cosgrove

November 16 2025

My Image of SH2-157 - The Lobster Claw Nebula

🔭 Project Summary

Target: Sh2-157 – The Lobster Claw Nebula (+ vicinity of NGC 7635)

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

Constellation: Cassiopeia • Distance: ≈ 8–11 thousand light-years

Type: Emission Nebula / H II Region

Imaging Period: October 16–28, 2025 • Total Integration: 34 h 30 m (SHO + RGB)

Filters: Ha (6 nm) · O III (6 nm) · S II (6 nm) · RGB

Telescope: Askar FRA400 f/5.6 Quintuplet Astrograph (400 mm)

Camera: ZWO ASI1600MM-Pro (−15 °C; Gain 139 NB/RGB)

Mount: ZWO AM5 on Custom Steel Pier

Processing: PixInsight (SHO starless + RGB stars) & Photoshop

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

Acquisition notes: Narrowband 300-s subs; RGB 90-s 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 NoiseXterminator by RC-Astro

CC Cosmetic Correction

CT Curves Transformation Process

DBE Dynamic Background Extraction Process

ET Exponential Transformation

HT Histogram Transformation

NXT NoiseXerminator by EC-Astro

SCNR Subtractive Chromatic Noise Reduction Process

SPCC SpectroPhotometric Color Calibration

STF Screen Transfer Function

STF->HT Method - dragging the STF triangle to the base of the Histogram Transformation tool and

then applying this to the image as a way of going 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

Ha
- 4 frames removed - for thin clouds
O3
- 1 frames removed - for thin clouds. Note - strong gradients and weak signal
S2
- 2 frames removed, for thin clouds. Note - strong gradients and weak signal
Darks
- All looks ok
Dark Flats
- All looks ok
Flats
- all good

2. WBPP 2.8.9

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 run 1:29:49 - no errors

3. Load Master Images and Create Color Images

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

4. Initial Process of Linear SHO data

Run DBE for the SHO linear image. Use subtraction for the correction method. Choose a sampling plan that avoids the nebulae (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 = 2.25, Y = 2.10. 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 - see params from snapshot below.
Run SXT and don’t save the SHO stars.
Run NarrowbandNormalization (see screenshot below for parameters used)

Master SHO Image DBE Sampling Plan (click to enlarge)

Master SHO- Before DBE (click to enlarge)

Background Subtracted by DBE (click to enlarge)

Measuring Star Sizes with PFSImage Script (click to enlarge)

Before BXT (Copy)

BXT Fix Only (Copy)

Full BXT (Copy)

After NXT V3 (Copy)

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

Master SHO Starless Image (click to enlarge)

NarrowbandNormalization Parameters used.

After NBNormalization (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 the nebulae (see below)
Run BXT - correct only. This cleans up and stars at the corners. Not much to do in this image as the scope is very crisp.
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 the PFSImage script to measure star sizes. X = 2.34, Y = 2.19. 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 - see params from snapshot below.
Run SXT and don’t save the RGB starless image.

Master RGB DBE Sampling Plan (click to enlarge)

Master L Background subtracted (click to enlarge)

Master-RGB Before SPCC (click to enlarge)

Before BXT (Copy)

After BXT Fix Only (Copy)

Afer BXT Full (Copy)

After NXT V3 (Copy)

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

Final Master RGB image - Before Star Removal.

6. Take RGB Stars Nonlinear

Use Seti Astro Star Stretch Script to stretch Stars
I decided to create three versions of the nonlinear star image. I would start with the settings that were lower than the default and then go for those that were higher.
- Stars # 1 used a Stretch of 4.0 and a color boost of 1.4
- Stars #2 used a stretch of 5.0 (the default) and a color boost of 1.4
- Stars # 3 used a stretch of 6.16 and a color boost of 1.2

Star Stretch Script panel and params used for stars #1

7. Process the Nonlinear SHO Starless Image

Create the WarmMask
- Use the ColourMask Process with Star Hue 331 and end hue 68, with a blur of 5
- Apply CT to boost the mask
Create the CoolMask
- Use the ColourMask Process with Star Hue 166 and end hue 270, with a blur of 5
- Apply CT to boost the mask
Create the CenterMask with the GAME script. (We will use this to control the bright center feature in the claw)
Create the BubbleMask with the GAME script (We will use this to focus on the Bubble Nebula)
The initial color balance has too much magenta in it. To remove this, I am going to use my Invert Trick:
- Invert the Image (magenta is now green!)
- Run SCNR Green at 0.9 (removes the green!)
- Invert the Image
Apply CT to set the basic tone scale and color saturation
Apply the Center Mask (let’s make that bright feature look better)
- Apply CT to darken
- Apply HDRMT with levels of 4 and “to intensity” selected (This will compress bright values)
- CT to lighten after HDRMT
- Apply LHE with a factor of 26, contrast limit of 2.0, and an amount of 0.3, and a histogram of 8 bits (This will bring up some fine detail)
Apply the BubbleMask
- Apply HDRMT with levels of 7 and “to intensity” selected
- Apply CT

Apply NXT with the params shown in the screensnap below
Now a final round of tweaks:
- CT for the whole frame
- CT with the BubbleMask
- CT with the CoolMask
- UnsharpMask with default params with the WarmMask
- A final global CT

Params used to create the initial WarmMask

WarmMask after CT Boost (click to enlarge)

The Params used to create the initial CoolMask

The CenterMask created with GAME. (click to enlarge)

CoolMask after CT boost (click to enlarge)

The BubbleMask created with GAME. (Click to enlarge)

After SBNR Green at 0.95 (click to enlarge)

After LHE with the WarmMask (click to enlarge)

The bright central feature we will adjust with the CentterMask. No Adj. yet made. (click to enlarge)

HDRMT With CenterMask Applied (click to enlarge)

LHE with CenterMask Applied (click to enlarge)

After HDRMT With BubbleMask (click to enlarge)

A final CT with the CoolMask (click to enlarge)

After CT with the WarmMask (click to enlarge)

Apply CT with the CoolMask (click to enlarge)

CT with the CenterMask in place (click to enlarge)

The Bubble Region Before Enhancement (click to enlarge)

CT adjust with BubbleMask (click to enlarge)

After NXT Applied - no mask (click to enlarge)

A final CT with the BubbleMask (click to enlarge)

Unsharp Mask used with the WarmMask (click to enlarge)

After one more CT, the SHO Starless image is complete.

8. Add the Stars Back In

Using the ScreenStars Script, add stars back into our SHO starless image. Do this three times with the three star images created
Pick the best one. I went with star image #1 because I think the heavier stars detract from the nebulosity in the image.

The script used to add the images back in,

With Stars#1 (Copy)

With Stars#2 (Copy)

With Stars #3 (Copy)

Assessing the Three Star Images

Final Image ready for Photoshop Polishing!

9. Export the Image to Photoshop for Polishing

Save the image as a TIFF 16-bit unsigned and move to Photoshop
Make final global adjustments with Clarify, Curves, and the Color Mixer - slight tweaks really
Use the lasso tool with a feather of 100-pixel and select small detail areas of the image, and enhance with the Clarity, Texture, and color mixer tools
Added Watermarks

Export Clear, Watermarked, and Web-sized jpegs.

The Final Image!

10. Final Comments

On one level, this was a complicated process. The main complication was the need to process the RGB subframe separately from the SHO subframes. On the other hand, the processing steps used were simple and straightforward, and I had little trouble dealing with noise.

With the observatory now operational, I am finding it is easier than ever to extend my integration times.

With more integration, the SNR improves, and this, I believe, makes the processing relatively more straightforward to accomplish.

Back to the Main SH2-157 2025 Page

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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/