Advanced HOO Processing Techniques in PixInsight – A Case Study with NGC 2736 (Pencil Nebula)

Introduction: NGC 2736 and the Vela Supernova Remnant

NGC 2736, commonly known as the Pencil Nebula, is a slender, glowing filament of ionized gas located in the southern constellation Vela, about 800 light-years away from Earth. It’s part of the vast Vela Supernova Remnant, the expanding debris field from a massive star that exploded approximately 11,000 years ago. The shockwaves from this ancient supernova continue to interact with the interstellar medium, creating a complex web of emission structures across the region.

The Pencil Nebula is one of the most visually striking features within this remnant. Its sharp, needle-like appearance—resembling a streak of light across the sky—is the result of the shockwave slamming into surrounding clouds of gas and dust. These interactions generate strong hydrogen-alpha (Ha) and oxygen-III (OIII) emissions, making it an ideal target for narrowband astrophotography, especially using the HOO palette (Ha mapped to red, OIII to green and blue).

Unlike many targets that require all three narrowband filters (Ha, OIII, and SII) for a full SHO (Hubble) palette, NGC 2736 can produce a compelling image using just Ha and OIII. This simplifies the imaging process and makes it perfect for experimenting with HOO processing workflows in PixInsight, allowing astrophotographers to explore different methods of balancing and blending the two channels for artistic and scientific effect.

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Common HOO Processing Techniques in PixInsight

When processing HOO images, there are several main workflows used to combine and balance the Ha and OIII data. Each method has advantages and limitations, and the choice often depends on the object’s emission structure and the photographer’s goals.

1. Combine First, Then Stretch

In this approach, the linear Ha and OIII images are first combined into a color image using a channel combination tool (e.g., PixelMath with Ha -> R, OIII -> G+B). Afterward, the resulting RGB image is color calibrated and then stretched. This method preserves the original balance of intensity between channels and can be useful when you want to see how the data naturally combine.

Pros:

• Straightforward pipeline.
• Maintains natural emission strength between channels.

Cons:

• Can be difficult to emphasize specific structures or features if one channel is much weaker than the other.

2. Stretch Separately, Then Combine

In this method, the Ha and OIII images are each processed independently through background extraction (e.g., DBE), deconvolution (e.g., PSF image), BXT/NXT denoise, and then individually stretched using tools like MaskedStretch or HTF. Once non-linear, the channels are combined into RGB.

Pros:

• Greater control over each channel’s contrast and detail.
• Easier to match brightness between channels before combining.

Cons:

• Risk of mismatched color balance that may require further calibration.

3. Stretch Unequally to Highlight a Channel, Then LinearFit

This hybrid technique begins similarly to the second, where each channel is processed and stretched separately. However, one channel (e.g., Ha) is intentionally stretched more to highlight red features. Then, before combining, the LinearFit tool is used to balance the brightness across channels, creating a more color-rich and visually striking result.

Pros:

• Ideal for emphasizing certain emissions (e.g., red from Ha).
• Maintains aesthetic control and scientific accuracy via normalization.

Cons:

• Requires careful balancing to avoid introducing false color artifacts.

4. Tone Mapping (Separate Stars and Nebula)

Tone mapping is an advanced method where starless versions of the Ha and OIII channels are created (via StarNet2 or StarXTerminator), stretched and combined, and then the stars are re-added later. This gives total control over the nebula processing.

Pros:

• Allows complete freedom to adjust nebula color, contrast, and saturation.
• Star size and color can be controlled independently.

Cons:

• Time-consuming and more complex.
• Requires accurate star separation and recomposition.

5. PixelMath Color Mapping with Controlled Ratios

Some astrophotographers use custom PixelMath formulas to control the contribution of each channel. For example:

R = Ha
G = 0.8 * OIII + 0.2 * Ha
B = OIII

This technique blends channels more subtly, which can produce pleasing results and reduce overpowering colors.

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My Processing Workflow for NGC 2736 in PixInsight

For this project, I initially tried the commonly recommended method of combining the H and O channels first, followed by color calibration and then stretching. However, I found that the resulting image lacked the strong red tones typically associated with the Hα emission. The hydrogen structures appeared muted, and the oxygen signal visually dominated the scene. This result led me to reconsider the workflow.

To address this, I switched to an alternative approach where I processed each channel (H and O) individually. This included:

• DBE (Dynamic Background Extraction) to remove gradients,
• PSF-based star removal to improve subsequent deconvolution (optional),
• BlurXTerminator and NoiseXTerminator to enhance sharpness and reduce noise, respectively,
• Followed by individual histogram stretches, with more aggressive stretching on the H channel to enhance its visual presence.

After the stretches, I used LinearFit to normalize the two channels before recombining them into an HOO color image. This allowed me to retain the dynamic range in the hydrogen signal and bring out the red filamentary structures more prominently.

Final Touches and Cosmetic Enhancements

After combining and calibrating the image, I proceeded to separate stars and nebula for independent processing:

• Nebula: I manually removed green hues using ColorMask, exported the cleaned nebula to Photoshop as a 16-bit TIFF for final contrast and tone adjustment.
• Stars: Using RangeSelection masks, I shrank the stars slightly and adjusted their colors to match the overall aesthetic of the final composition.

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Conclusion

NGC 2736 is a beautiful and approachable narrowband target that offers an excellent opportunity to experiment with HOO processing in PixInsight. Different methods yield different results, and there is no one-size-fits-all solution. By understanding the strengths and limitations of each workflow, astrophotographers can better tailor their processing strategy to both the data and their artistic vision.

Whether you prefer a more natural balance or want to emphasize particular structures or colors, PixInsight provides powerful tools to explore the full potential of HOO imaging.

Happy processing!