NGC 2174 - The Monkey Head Nebula - Jan 2026 Image Processing Walkthrough

Written By Patrick A. Cosgrove

January 11, 2026

My 2025 image of NGC 2174!

🔭 Project Summary

Target: NGC 2174 — The Monkey Head Nebula

Capture Dates: October 27 & 28, 2025

Constellation: Orion • Distance: ≈ 6,400 light-years

Type: Emission Nebula / H II Region (active star-forming region)

Imaging Period: October 16–28, 2025 • Total Integration: 5 h 10 m (Ha + OIII + SII)

Filters: Ha · OIII · SII (Astronomik 36mm unmounted 6 nm)

Telescope: Astro-Physics 130mm f/8.35 Starfire APO (built 2003)

Guide Scope: Tele Vue TV-76 f/6.3 (480mm) APO doublet

Camera: ZWO ASI2600MM-Pro (−15 °C; Gain 100)

Mount: Custom steel pier (new) + iOptron Tri-Pier with column extension (new)

Rotator: Pegasus Astro Falcon Camera Rotator

Processing: PixInsight & Photoshop

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

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    Special Note

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    Abbreviations Used

    BXT BlurXTerminator by RC-Astro

    CC Cosmetic Correction

    CT Curves Transformation Process

    DBE Dynamic Background Extraction Process

    ET Exponential Transformation

    HDRMT HDR MultiScale Transform

    HDRC HDR Composition

    NXT NoiseXTerminator by RC-Astro

    MAS MultiscaleAdaptiveStretch

    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

    Summary:

    NGC 2174 (Monkey Head Nebula) — SHO Processing Flow (2026)

    Two-column swimlane aligned to your PixInsight → Photoshop workflow for narrowband Ha/OIII/SII (SHO), including NB-to-RGB star conversion, Warm/Cool masks, and a pre-star Photoshop polish.

    Shared upstream steps
    1. Blink / cull subs Ha: remove 3 • OIII: remove 5 • SII: remove 2 (clouds)
    →
    2. WBPP (2.8.9) Max quality • CC enabled • Autocrop • No drizzle
    →
    3. Load masters & build SHO Rename masters • CombineChannels → SHO master
    →
    4. Start linear SHO master DBE (subtractive) → BXT → NXT → SXT → NarrowbandNormalization
    SHO Nebula Lane starless / tone / texture
    5. Linear cleanup (SHO) DBE (subtractive) → BXT (Correct Only) → PSFImage (X=3.60, Y=3.89) → BXT (Full, enhanced shrink) → NXT
    6. Star separation + normalization SXT → save stars • keep starless SHO → NarrowbandNormalization (starless)
    7. Go nonlinear (nebula) MAS stretch (to taste) → begin nonlinear work
    8. Warm/Cool masks + initial color shaping WarmMask (ColourMask): Hue 313→65 • Blur 5 → boost with CT • CoolMask (ColourMask): Blur 5 → boost with CT
    9. Reduce magenta cast Invert → SCNR Green @1.0 → invert back
    10. Tone + local contrast (masked) CT (global) → CT + LHE (WarmMask: radius 64, CL 2.0, amount 0.6, 8-bit hist) → CT + LHE (CoolMask: radius 64, CL 2.0, amount 0.18, 8-bit hist)
    11. Cleanup + sharpening (nebula) CT (global) → NXT → SCNR Green @1.0 (shadow greens) → CT (global + WarmMask) → MLT sharpening (per your panel)
    12. Pre-star Photoshop polish (round-trip) Export 16-bit TIFF → Photoshop Camera Raw (global + selected areas, ~100px feather) → save TIFF → import back to PixInsight
    Stars Lane NB → RGB stars
    5. Keep the extracted SHO stars (linear) Saved during SXT: the SHO star image (linear)
    6. Convert narrowband stars to RGB (Seti Astro) NB-to-RGB Stars: export “r,g,g” → run the script → save the resulting nonlinear RGB stars
    7. Optional star touch-ups Minor halo control / gentle star reduction (if needed)
    Merge & finish
    13. Add stars back ScreenStars → combine nonlinear RGB stars with the polished starless SHO
    →
    14. Final Photoshop export / finish 16-bit TIFF → crop → subtle Clarify/Curves/Color Mixer → watermarks → export clear + watermarked + web JPEGs

    Processing this Image

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

    1. Blink

    I screened all subs and cal frames with Blink.

    • Ha: 3 frames removed for clouds

    • OIII: 5 frames removed for clouds

    • SII: 2 frames removed for clouds

    2. WBPP 2.8.9

    All frames were loaded into WBPP:

    • 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.

      WBPP run in 24:37 minutes - 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 color image

    Master Ha, O3, S2 300-second images

    Master SHO Color image (click to enlarge)

    4. Process the SHO Linear Master

    • Run DBE using Subtractive correction

    • 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 PSFImage script to measure star sizes.  X = 3.60  Y= 3.89. 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 1.3 times the measured stellar radii. See the BXT Panel Snapshot below.

    • Run NXT - see params used in screenshot below

    • Run SXT and save Star images.

    • Run NarrowbandNormalization on the SHO starless image (see params used below)

    Master SHO Sampling Plan (click to enlarge)

    Master SHO before DBE (click to enlarge)

    Master SHO after DBE (click to enlarge)

    Master SHO Background removed (click to enlarge)

    PSFImage panel showing star sizes.

    BXT Panel showing parameters used.

    NXT Params used.

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

    SHO Image before SXT.

    SHO Starless initial Image (click to enlarge)

    Initial SHO Stars Image (click to enlarge)

    Params Used.

     

    SHO Starless image after Narrowband Normalization (click to enlarge)

    5. Process Stars and Go Nonlinear

    • Since these are Narrowband stars and a bit funky in color, I am going to try the Seti Astro’s NB-toRGB Stars script to correct them

      • Extract the “r, g, b” images and save them as files (this seems like a dumb first step but exernal files for each image are required for input. I have no idea why it work this way)

      • Run the Script.

      • Final Image saved as the nonlinear Stars

      • The script produced a noticeable improvement in star color

    Extracted SHO stars -(labeled as R,G, and B)

    The script panel showing the params used.

    6. Take SHO Image Nonlinear and Process!

    • Use MAS to stretch the SHO image (note - forgot to grab screenshot of the panel - but adjust to get the image you want).

    • Create the WarmMask using the ColourMask process. (We will use these to set and adjust colors in the image later)

      • Set starting Hue of 313 and Ending Hue of 65

      • Set Mask Blur of 5

      • Take the resulting mask and boost with CT

    • Create the CoolMask Using the ColourMask process:

      • Set starting Hue of 158 and Ending Hue of 256

      • Set Mask of 5

      • Take the resulting mask and boost with CT

    • The image has some magenta areas I don't like. So let’s remove them!

      • Invert the image (magenta areas become green)

      • Run SCNR Green at 1.0 to remove the green

      • Invert the image again!

    • Apply CT to tweak the overall tone scale

    • Apply the WarmMask

      • Apply CT to tweak the color and tone of just the warm areas.

      • Apply LHE with a radius of 64 and a contrast limit of 2.0. and the amount of 0.6 and an 8-bit histogram

    • Apply the CoolMask

      • Apply CT to tweak the color and tone of just the Cool areas.

      • Apply LHE with a radius of 64 and a contrast limit of 2.0. and the amount of 0.18and an 8-bit histogram

    • Apply Global CT to tweak things

    • Apply NXT to reduce noise; see the screenshot for the parameters used.

    • There are some weird green areas in the shadows, so let’s run SCNR Green at 1.0 to remove this.

    • Apply Global CT

    • Apply CT with WarmMask

    • Now lets sharpen the image. Apply MLT with the parameters shown in the screenshot below.

    • I am pretty happy with the image now. In the past, I would add in stars and then go to a polish step in Photoshop. I found that doing the polish step here, before the stars are re-added, works best! I like to use the Raw Camera Filters (Texture, Clarity, and Dehaze) and the color mixer tools.

      • Export image to a 16-bit-unsigned TIFF file

      • Import to Photoshop

      • Run the Raw Filter and do global adjustments with Effects and the color mixer

      • Select the areas of interest using the lasso with a 100-pixel feather, then apply the Raw Camera filters to those regions.

      • Save the resulting image as TIPP

      • Import back to PixInsight.

    Nonlinear Starting Lum Image (click to enlarge)

    Parameters used to create the WarmMask

    Parameters used to create the CoolMask

    Initial WarmMask (click to enlarge)

    Boost with CT (click to enlarge)

     

    CoolMask - no boost needed. (click to enlarge)

    Invert the Image (click to enlarge)

    Invert the image again. (click to enlarge)

    Apply CT with the WarmMask (click to enlarge)

    Adjust CT with the CoolMask (click to enlarge)

    Apply a global CT (click to enlarge)

    Apply NXT V3 (click to enlarge)

    Apply CT (click to enlarge)

    SCNR to remove the green, (click to enlarge)

    After CT (click to enlarge)

    Apple LHE with the WarmMask (click to enlarge)

    Apply LHE with the CoolMask (click to enlarge)

    Params used on the next step.

    Apply SCNR (click to enlarge)

    CT with the warmMask (click to enlarge)

     

    Sharpening Params used.

     

    After MLT Sharpening (click to enlarge)

    After A polish in Photoshop (click to enlarge)

    7. Add the Stars Back In

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

    The final Starless Image (Click to enlarge)

    The star image (click to enlarge)

    The script used to add the images back in,

    Stars now added - Image ready for Photoshop Adjustments

    8. Export the Image to Photoshop for Polishing

    • Save the image as a TIFF 16-bit unsigned and move to Photoshop

    • Crop the image a bit

    • Make any 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!

    The First Cropped Image

    Back to NGC 2174 - The Monkey Head Nebula Project Page

    Alternatively, you can use the back arrow to return to the B33.NGC 2024 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/
    Next

    B33/NGC 2024 - The Horsehead and Flame Nebula - Dec 2025 Image Processing Walkthrough.