Analysis of Excessive Green in the Triangulum Galaxy

 

Analysis of Excessive Green Color in
Hα Regions of M33

An image of M33 (the Triangulum Galaxy), acquired on October 6–7, 2026, exhibited unexpectedly strong green coloration in several regions. This observation prompted a review of the source data and processing workflow to identify the cause.

The raw data were acquired using a William Optics GT102 telescope and a ZWO ASI1600MM camera at gain 139 and offset 21. The LRGB acquisition details are summarized below:

Filter	Exposure (sec)	Number of Subs
Luminance	60	58
Red	30	51
Green	30	54
Blue	30	54

Initial Analysis

My standard PixInsight workflow for LRGB data was used to produce the final image shown below. IC 132, one of the regions under investigation, is identified by the yellow box; NGC 604, the comparison region, is identified by the red box.

M33/LRGB

Cropped views of the two regions are shown below:

Normal	Annotated

IC 132 is one of the largest and brightest H II (ionized-hydrogen) regions in the Triangulum Galaxy (M33). Its primary emission is hydrogen-alpha (Hα), and it is among the galaxy’s most prominent star-forming regions, along with NGC 604.

In the image, NGC 604 displays the reddish-pink glow expected of an H II region. In a natural-color LRGB image, IC 132 should likewise appear red, pink, or red-orange—similar to the bright H II regions in M33’s spiral arms—rather than green.

The initial investigation considered several common causes, including overly aggressive background neutralization, inaccurate color calibration, and incorrect SPCC parameters. However, the overall galaxy image appeared well balanced; if any of these issues had been present, their effects would likely have been more widespread.

I then examined a single raw subexposure from each color channel and measured the same aperture in each linear master. The results are summarized below:

Region	R	G	B
IC 132	0.0103	0.0156	0.0127
IC 132 diffuse area	0.0094	0.0135	0.0114
NGC 604	0.0186	0.0254	0.0219

The measurements show that green dominates the core of IC 132 while red is comparatively weak. Green is also the strongest signal in NGC 604, but its red and blue values are proportionally higher than those of IC 132. This difference alone, however, does not fully explain IC 132’s pronounced emerald appearance.

Representative subexposures from the three RGB filters are shown below. The nebulosity surrounding the IC 132 knot is visibly more prominent in the green channel than in either the red or blue channel.

Red	Green	Blue

Looking Elsewhere

Further analysis was performed using the RGB masters and nonlinear processing. The table below summarizes the RGB contributions for the same 14 × 11-pixel region of interest centered on IC 132.

	Red	Green	Blue	G/R	B/R	G/B
IC 132 Chan Combo only	629	938	766	1.491	1.218	1.225
After SPCC	140	179	162	1.279	1.157	1.105
After stretching (MAS)	14216	20168	16786	1.419	1.181	1.201

Interpretation

Before SPCC, IC 132 exhibits a substantial green excess: the green signal is approximately 49% higher than the red signal.

After SPCC, the color balance improves considerably. Green is approximately 28% higher than red and 10.5% higher than blue, indicating that SPCC is appropriately correcting the expected camera and filter response.

After MAS, the green disparity increases again. Green rises to approximately 42% above red and 20% above blue, indicating that the modified arcsinh stretch amplifies the existing green-cyan coloration.

This result does not necessarily indicate that MAS independently boosts the green channel. Because each channel begins at a different intensity, a nonlinear stretch can alter their relative ratios. Since green is initially brighter, the transformation changes its separation from red and blue.

Applying the same analysis to NGC 604, as shown below, yields green-to-red and green-to-blue ratios that are more consistent with expectations.

	Red	Green	Blue	G/R	B/R	G/B
NGC 604 Chan Combo	1012	1357	1157	1.34	1.14	1.17
After SPCC	140	179	162	1.16	1.41	0.83
After stretching (MAS)	14216	20168	16786	1.10	1.30	0.85

This comparison is decisive: IC 132 and NGC 604 exhibit genuinely different calibrated color ratios, and the MAS stretch accentuates that difference.

Region	R	G	B	Dominant channel
IC 132	140	179	162	Green
NGC 604	395	460	556	Blue

Normalized to the green channel:

• IC 132: R 0.782, G 1.000, B 0.905
• NGC 604: R 0.859, G 1.000, B 1.209

IC 132 therefore exhibits a genuine green peak, whereas NGC 604 contains a pronounced blue component. This difference explains why the two regions appear markedly different despite both having an elevated raw green response.

For IC 132, the green signal is:

• 42% above red
• 20% above blue
• 30% above the average of red and blue

For NGC 604, the green signal is below the average of the red and blue signals.

Evaluation of SCNR

Although I generally avoid applying SCNR to astrophotography images because it can remove valid signal, the preceding analysis indicates that its use is justified in this case. SCNR Green with Average Neutral protection should primarily affect IC 132, where green exceeds the average of red and blue. NGC 604 should be affected minimally, if at all, because its green signal is already below that average. A script I occasionally use simplifies this operation by reducing the green contribution only when it exceeds the average of the red and blue values; otherwise, the pixel remains unchanged. The script then rescales the overall luminance based on the values measured before and after the green reduction.

Applying the modified SCNR produces a more natural and visually balanced result.

M33/LRGB w/Modified SCNR applied

The enlarged comparison below shows IC 132 and NGC 604 before and after correction. IC 132 is corrected as intended, while NGC 604 remains effectively unchanged.

IC 132 before	IC 132 after
NGC 604 before	NGC 604 after