How to process a comet in PixInsight: the complete workflow

Comets are stunning celestial visitors, but they present a unique challenge in astrophotography: they move across the sky during your imaging session at a different rate than the stars. If you stack your frames normally, the comet smears into a streak. If you align against the comet, the stars smear. This guide shows you how to create a clean image with sharp stars and a crisp comet using PixInsight.

At the top, we followed the comet and let the stars slide. The bottom, we tracked the stars...

Overview of Steps

1. Weighted Batch Preprocessing

2. Star Extraction

3. Comet Alignment

4. Comet Integration

5. Post-Processing

Everything here is demonstrated in the video: How to Process a Comet in PixInsight.

The video companion to this post

1. Weighted Batch Preprocessing

Start by calibrating and integrating your data. You can use WBPP or do it manually.

Manual Calibration Workflow:

• Integrate bias frames → master bias per ISO/Gain.

• Integrate dark frames → master dark per exposure and ISO/Gain.

• Calibrate darks with master bias.

• Calibrate dark flats with calibrated darks.

• Integrate dark flats → master dark flat.

• Calibrate flats with master dark flat.

• Integrate flats → master flat.

• Calibrate lights: subtract master dark, divide by master flat.

• Optionally apply cosmetic correction.

• Debayer (de-mosaic) lights.

A de-bayered frame

• Star-align (register) lights.

• Integrate registered lights.

A registered (star-aligned) frame

Result: A star-aligned image with perfect stars—but a smeared comet. Call this "star-integrated".

A "star-integrated" image

2. Star Extraction

Next, isolate the stars. Use StarXTerminator or StarNet++ to create a star-only image. Save it as stars.fits or .xisf.

💡 Tip: If the comet’s bright core sneaks into the star image, use CloneStamp to remove it. We want only stars here.

The stars pulled out

3. Comet Alignment

Run CometAlignment on the registered frames:

• Load the registered frames.

• Mark the comet nucleus in the first frame.

• Mark the nucleus in the last frame. PixInsight calculates positions for intermediate frames.

Roger, the reticule is over the core.

• In the Subtract section, specify the star image and choose Stars Aligned. This removes stars from each frame, leaving just the comet. Residual star ghosts will average out during stacking.

The comet-aligned exposure after star removal

Result: Comet-aligned frames with minimal star trails.

4. Comet Integration

Use ImageIntegration on the comet-aligned frames. You’ll get the comet plus some edge noise from cropping.

The integrated comet

Now blend the stars back using PixelMath:

~(~$T*~stars)

Here $T is the integrated comet image. This formula smoothly reintegrates stars.

Result: A clean image with sharp stars and a crisp comet.

"It's so nice to have my stars back!"

5. Post-Processing

Treat the image like any deep-sky object:

• Dynamic Crop

• Automatic Background Extraction

• Dynamic Background Extraction

• Plate-solve if needed

• Spectrophotometric Color Calibration

• BlurXTerminator

• EZ Soft Stretch

• Curves, noise reduction, and masks to enhance comet detail

Final Result: A stunning comet image with pinpoint stars—no streaks, no trails.

The final result

The same technique benefitted me when I photographed the comet 12P/Pons-Brooks as it passed the bright star Hamal. This is "The Devil's Diamond."

Periodic comet 12P/Pons-Brooks was known as "The Devil Comet" due to cryovolcanic eruptions that resulted in plumes that resembled horns.

In summary

• Use WBPP or manual calibration for clean star stacks.

• Extract stars early for subtraction during comet alignment.

• CometAlignment + PixelMath = smooth integration.

• Finish with standard post-processing for best results.

Check out this photograph of a long-tailed comet in the skies over the Yaquina Head Lighthouse.