SH2-157 - The Lobster Claw Nebula 2025: Image Processing Walkthrough

Written By Patrick A. Cosgrove

November 7, 2025

My Image of SH2-157 - The Lobster Claw Nebula

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    Abbreviations Used

    BXT BlurXTerminator by RC-Astro

    CC Cosmetic Correction

    CT CurvesTransformation Process

    DBE Dynamic Background Extraction Process

    ET ExponentialTransformation

    HT HistogramTransformation

    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 Historgam Transformation tool and

    then applying this to the image to go nonlinear

    SXT StarXterminator by PC-Astro

    WBPP Weighted Batch Preprocessing Script

    Processing this Image

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

    1. Blink

    • General

      • Lots of satellite tracks - easy to handle

    • Ha

      • 3 frames removed - 2 for thin clouds, three more tracking

    • O3

      • 3 frames removed, three obstructions, six for clouds

    • S2

      • 4 frames removed, 1 for tracking, and three for clouds

    • Red

      • No frames removed!

    • Green

      • No frames removed!

    • Blue

      • No frames removed!

    • 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 2:10:50 - no errors

    WBPP Calibration View

    WBPP Post Calibration View

    WBPP Pipeline View

    3. Load Master Images and Create Color Images

    • Load all master images and rename them.

    • Using CombineChannels, create the Master SHO and RGB color images

    Master Ha, O3, and S2 Images

    Master SHO image.

    Master R, G, and B images

    Master RGB 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 and 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)

    Master SHO after DBE (click to enlarge)

    Background Subtracted by DBE (click to enlarge)

    Measuring Star Sizes with PFSImage Script (click to enlarge)

    BXT Settings Used. (click to enlarge)

    NXT Panel used. (click to enlarge)

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

    Final Master SHO Image

    Master SHO Starless Image (click to enlarge)

    NarrowbandNormalization Paramters used.

    Afer 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)

    Before DBE (click to enlarge)

    Master RGB after DBE (click to enlarge)

    Master L Background subtracted (click to enlarge)

    Measuring RGB star sizes.

    Params used for BXT

     

    SPCC Panel settings.

    SPCC Regression Results.

    Master-RGB Before SPCC (click to enlarge)

    Afer SPCC (click to enlarge)

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

    FInal Master RGB image - Before Star Removal.

    Master RGB Star image after SXT.

    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

    StarStetch Params for stars #2

    Starstretch Panel for Stars #2

    Nonlinear RGB Stars image #1

    Stars #2 Image

    Stars #3 Image

    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

    Initial WarmMask (click to enlarge)

    WamMask afer CT Boost (click to enlarge)

    The Params used to create the initial CoolMask

    The CenterMask created with GAME. (click to enlarge)

    Initial CoolMask (click to enlarge)

    CoolMask after CT boost (click to enlarge)

    The BubbleMask created with GAME. (Click to enlarge)

    The initial image(click to enlarge).

    After SBNR Green at 0.95 (click to enlarge)

    Global CT Adjust. (click to enlarge)

    After LHE with the WarmMask (click to enlarge)

    The bright central feature we will adjust with the CentralMask. 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)

    NXT Params used in the next step

    Another CT adjust (click to enlarge)

    A final CT with the CoolMask (click to enlarge)

    Invert the image (click to enlarge)

    Final Invert (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)

    CT with CenterMask (click to enlarge)

    The Bubble Region Behore 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,

    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!

    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.

    Feel free to use the back arrow to go back to the NGC 7380 Main page, or you can use the menu at the top of the page to continue your navigation.

    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/
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    NGC 7380 - The Wizard Nebula 2025: Image Processing Walkthrough