May 19 2026

🔭 Project Summary

Target: Messier 94 – The Crocodile Eye Galaxy/ NGC 4736/ Cat’s Eye Galaxy / NGC 4736

Capture Dates: April 11, 20, and 21, 2026

Constellation: Canes Venatici • Distance: ≈ 16 million light-years

Type: Face-on spiral galaxy with bright central region, active inner ring, and faint extended outer disk

Imaging Period: April 11–21, 2026 • Total Integration: 12 h 49 m 30 s (LRGB)

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

Telescope: William Optics 132 mm f/7 FLT APO Refractor with P-FLAT7A 0.8× reducer

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

Mount: iOptron CEM60 on custom steel pier

Processing: PixInsight (LRGB) & Photoshop

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

Acquisition notes: L: 202 × 90 s; R: 103 × 90 s; G: 102 × 90 s; B: 106 × 90 s at −15 °C, Gain 0; total 12 h 49 m 30 s after culling bad or questionable subs.

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Special Note

Welcome to the image-processing page for this project. You got here by following a link from the main Messier 94 project report, and you can return to that page using the back button in your browser.

Abbreviations Used

BXT BlurXTerminator 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

Summary:

M94 Crocodile Eye Galaxy LRGB Processing Flow

Sequential summary of the M94 workflow, organized around the actual dependency chain from frame rejection through final PixInsight processing, star recombination, and Photoshop polish.

1. Data Review and Linear Integration

1. Blink / Cull Subs Screened L, R, G, and B for thin clouds and obvious defects; 10 frames removed during Blink

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2. SubFrameSelector Rejected additional subs based on FWHM and detail degradation; star-size trends appeared more atmospheric than dew-driven

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3. WBPP 2.8.9 Quality-first run: calibration, CosmeticCorrection, registration, integration, autocrop, no drizzle; completed without errors

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4. Build Master Images Loaded and renamed L, R, G, B masters; created the linear RGB image with ChannelCombination

2. Linear Luminance and RGB Preparation

Linear Luminance Branch

5. Prepare Linear L DBE subtraction → BXT correct-only → PFSImage star-size measurement

6. Refine Linear L Full BXT with enhanced star reduction → NXT V3 → SXT star removal; luminance stars were not retained

Linear RGB Branch

7. Prepare Linear RGB DBE subtraction → SPCC color calibration → BXT correct-only

8. Refine Linear RGB PFSImage star-size measurement → full BXT → NXT V3 → SXT star removal

Saved Star Branch

Saved RGB Stars RGB stars saved from StarXTerminator for later ScreenStars recombination

Star Stretch Branch

9. Create Star Options Used Seti Astro Star Stretch to create two nonlinear RGB star versions: 1.5 stretch / 1.5 saturation and 1.6 stretch / 1.5 saturation

3. Nonlinear Luminance Processing

10. Stretch Starless L Used the STF → HT method to take the luminance image nonlinear

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11. Build Core Masks Used GAME to create inner-core and mid-core masks for controlled work on M94’s bright center and ring structure

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12. Tone and Highlight Control Applied HDRMT, CurvesTransformation, and masked CT to open the core and manage the bright inner region

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13. Local Detail and Noise Applied LHE at multiple scales with MID_MASK, then NXT V3 before sharpening

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14. Sharpen L Applied MLT sharpening, followed by a final CurvesTransformation tweak

4. Nonlinear RGB Processing and LRGB Build

15. Stretch Starless RGB Used the STF → HT method to take the RGB image nonlinear

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16. Initial RGB Tone Work Applied HDRMT to open highlights, then CurvesTransformation to adjust tone scale, darken the core, and boost saturation

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17. Masked RGB Refinement Used MID_MASK and RangeSelection masks to control the bright core and bluish ring without overworking the outer structure

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18. Add Luminance Used LRGBCombination with default parameters to fold the processed luminance structure into the RGB image

5. Final Color and Star Recombination

19. Astro Color Mixer Used the prototype PixInsight Astro Color Mixer script for final color adjustments

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20. Red Balance Correction Applied an additional CurvesTransformation pass to back off a red color balance that developed during processing

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21. Add Stars Back Used ScreenStars to recombine RGB stars; tested two star-stretch levels and selected the larger-star version

6. Final Output

22. Export to Photoshop Saved the finished PixInsight image as a 16-bit unsigned TIFF for final polish

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23. Crop and Polish Cropped in closer so the smallish galaxy was more prominent in the final composition; used Camera Raw for light tonal polish

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24. Final Exports Added watermarks and exported clear, watermarked, and web-sized JPEG versions

Processing this Image

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

The main processing challenge for this image was balancing M94’s bright inner core and ring structure against the much fainter extended outer disk. I used a conservative LRGB workflow: careful frame rejection, separate luminance and RGB preparation, star removal for nonlinear work, luminance-driven detail enhancement, controlled masking around the core and ring, and final star recombination before a light Photoshop polish.

1. Blink

First, I screened the data for thin-cloud frames and obvious defects.

Lum
- 6 frames removed for thin clouds.
Red
- 0 frames removed for thin clouds.
Green
- 4 frames removed for thin clouds.
Blue
- 0 frames removed for thin clouds
Darks
- All looked OK.
Dark Flats
- All looked OK.
Flats
- all good.

A total of 10 frames were removed during Blink.

2. SubFrameSelector

Preserving fine detail was important, so I wanted to remove the subs that showed statistically significant detail degradation. To determine which frames to remove, 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 - star sizes increasing as the night progressed. I do not think this was dew; it was more likely atmospheric seeing. 25 frames marked for removal. A few frames were removed because their star sizes were too small. This is almost certainly due to clouds attenuating them - ones I missed during the Blink operation.

Now the red channel, where 11 frames were removed.

Now the Green Channel, where 7 frames were removed.

And finally the Blue Channel, where 9 frames were removed.

A total of 62 frames were removed during the SFS operation, for a total reduction of 1.55 hours.

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
Selected 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 correction options selected except linear defect correction.
Enabled for Autocrop
I chose NOT to use Drizzle processing.
WBPP completed in 1:40:28 - no errors

4. Load Master Images and Create Color Images

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

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

5. Initial Processing of Linear Luminance Data

Run DBE for the linear luminance 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 cleaned up the stars at the corners. There was not much to correct because the optics were performing well.
Run the PFSImage script to measure star sizes. X = 1.97 Y= 1.76. This will influence the values used in BXT.
Run full BXT. I used an enhanced set of values to shrink the stars more aggressively. 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.

Before BXT

BXT Fix Only

Full BXT

After NXT

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

Master Lum Starless Image (click to enlarge)

6. Initial Processing 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 parameters used.
Run BXT - Correct only. This cleaned up the stars at the corners. There was not much to correct because the optics were performing well.
Run the PFSImage script to measure star sizes. X=1.65 Y = 1.61. 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 before SPCC (click to enlarge)

Before BXT

BXT Fix Only

BXT Full

After NXT

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

7. Go Nonlinear

Using the STF → HT method, take the linear Luminance and RGB images nonlinear.
Using Seti Astro Star Stretch, take the RGB Star image nonlinear with two levels of star stretch:
- 1.5 stretch with 1.5 saturation boost
- 1.6 stretch, and 1.5 saturation boost
- so that later we can choose which size stars to use.

8. Process the Nonlinear Lum Starless Image

Use GAME script to create a mask of the inner core region
Use GAME script to create a mask of the mid-core region
Open up the highlights by running HDRMT with levels=6, “To Lightness” checked, and “Lightness Mask” checked.
Apply CT to darken the background sky
Apply LHE with a scale of 26, contrast limit of 2.0, amount of 0.3, and an 8-bit histogram and the MID_MASK. This helps bring out smaller details.
Apply LHE with a scale of 226, contrast limit of 2.0, amount of 0.2, and a 10-bit histogram and the MID_MASK. This helps bring out larger details.
Apply NXT V3 now (see parameters in the screenshot below). I wanted to do this before sharpening and bringing out more detail.
Apply HDRMT with levels=6, “To Lightness” checked, and “Lightness Mask” checked with the Core_Mask
Apply CT with Core Mask
Apply LHE with a scale of 104, contrast limit of 2.0, amount of 0.5, and an 8-bit histogram and the MID_MASK. This helps bring out smaller details.
Apply MLT Sharpening (see params used in screenshot below)
Applied a final CT tweak.

9. Now Process the RGB Starless Image

Open up the highlights by running HDRMT with levels=6, “To Lightness” checked, and “Lightness Mask” checked.
Adjust tone scale with CT - darken core and boost saturation
Apply CT with MID_MASK to adjust without touching the outer ring
Use RangeSelection to create a Range_Mask to cover the bright core area as well as the bright bluish ring around the core. See the params used below.
Running HDRMT with levels=5, checked, and “Lightness Mask” checked - use it WITH the Range_Mask
Apply CT with the Range_Mask
Use LRGBCombination to fold the L signal into the RGB. Default params used.
Use my brand new prototype PI script, Astro Color Mixer, to do the final color tweaks
Export as 16-bit TIFF to PS for a slight round of polishing.

The balance has gone somewhat red, so I wanted to back off on that, and I used another CT application to do so.

10. Add the Stars Back In

Using the ScreenStars Script, add stars back into our RGB starless image.
I tried using two levels of stars, and I found I like the look of the larger stars.

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 decided to zoom in a bit to allow the smallish galaxy (in the original field) to be more visually prominent.
I then did a little polishing using the Camera Raw filter.
Added Watermarks

Exported clear, watermarked, and web-sized JPEGs.

Back to the M94 Project Page

Alternatively, you can use the back arrow to return to the Messier 94 project page, or you can use the menu at the top of the page to continue your navigation.

Thanks,

Pat

Messier 94 - The Crocodile Eye Galaxy 2026 - Image Processing Walkthrough