How to photograph comets and stars while keeping your image crisp and blur-free

Comets are faint objects that move faster than our background stars. When a comet passes a deep-sky object, long-exposure astro photos can produce fantastic results. 

But when you’re imaging a comet, increasing the image brightness by stacking aligned on the stars won’t work; the comet just becomes blurred.

Stacking aligned on the comet doesn’t work either; that causes the stars to trail.

Here I’ll explain how to create an image with a sharp comet and stars, using my photo of comet C/2017 K2 Pan-STARRS passing globular cluster M10 in July 2022 to explain the composite techniques behind it.

Stack and blend to create a sharp comet 

I used DeepSkyStacker, PixInsight and Photoshop to stack, process and blend two images: one of the stars and M10, and one of the comet.

All in all, 27x 120-second light frames went into the finished image.

My start image was this data, already stacked in DeepSkyStacker, which was told to stack on the stars; I called this stack my ‘Stars’ file.

C/2017 was blurry, so I had to re-stack my dataset, this time telling the software to stack on the comet. I called this stack my ‘Comet’ file. 

To create this Comet file, open DeepSkyStacker, click Open picture files and navigate to your dataset.

Select them all (Ctrl + A) and click Open. Click on one of your frames to check the comet is visible in your stack and set the nucleus as the alignment point.

If you want it brighter, you can use the slider (see image below). Select Edit Comet Mode.

Using your mouse wheel, zoom in to view the comet nucleus better. To set your alignment point on the nucleus, hover the cursor over the centre, producing a red target cross.

When happy with the position, hold down the Shift key and click.

Finally, under Options click Settings > Stacking Settings to access the Comet tab.

Select Comet stacking and click OK. After stacking, save your Comet file in the same location as your Stars file.

Data stretch and crop

You now have a Stars file and a Comet file to process in PixInsight.

Open both files and drag the two image windows so that the Stars file is on top and the Comet file is below (see image below).

You want to temporarily stretch the data to see the picture better – this is done via Screen Transfer Function (Process > All Process > Screen Transfer Function).

Ensure the Link RGB channels is off (highlighted, below), then click the Nuclear icon underneath to stretch the data.

Now you can see your data better, crop out stacking artifacts, ensuring you apply the same crop to each file.

This is done via PixInsight’s DynamicCrop. Click on the Stars file. Click Process > All Process > DynamicCrop.

Hover your cursor over the Stars image and a + appears. Click and hold to highlight your crop region. 

To transfer this crop to the Comet file, click and hold down the blue triangle icon in the DynamicCrop window, drag it to a blank part of the main PixInsight window and release.

A Process01 tab appears (highlighted, image above).

Now click and hold this tab, dragging it anywhere over the top of your Comet image. By releasing your mouse, the crop is performed.

To finish cropping your Stars image, click the green tick icon in the DynamicCrop window (highlighted).

Balance colours 

You can now balance your colours via PixInsight’s ColorCalibration.

Click on Process > All Process > ColorCalibration.

Starting with your ‘Stars’ image, first create a tiny ‘Reference image’ window that is a true sample of your background, with no bright stars inside.

Use your mouse wheel to zoom in, press Alt + N simultaneously, then click and drag to create this small window in the Stars image, called ‘Preview01’. 

Click on the dropdown menu next to the ‘Reference image’ box in the ColourCalibration window (highlighted, image above).

A Select View window appears. Select Stars->Preview01 followed by OK and then the Apply icon to complete the colour balance. Repeat ColorCalibration for the ‘Comet’ image. 

3 quick tips

1. Check that DeepSkyStacker has accurately identified the comet’s nucleus in all frames before stacking your Comet files.
2. In PixInsight, ‘Automatic Background Extraction’ can remove gradients before proceeding to ColorCalibration. 
3. After performing ColorCalibration in PixInsight, save your files as 32-bit TIF files to preserve the dynamic range of your data.

Now I’ll explain how to extract the stars from the Stars file and combine them with our now-sharp comet.

Blend both elements to create one sharp image

We've stacked our subframes, creating two files – one aligned on the stars (the Stars file), the other aligned on C/2017 (the Comet file) – and begun processing them in PixInsight.

Now we’ll finish processing and then combine our files.

First, we’ll perform a histogram stretch on both, before extracting the stars from our Stars file to create a ‘Stars Only’ file minus the blurry comet. We’ll then combine the two files.

Let’s start by stretching the Stars file – which currently includes a blurred comet (see the before image, above).

We previously performed a temporary automatic stretch with the Screen Transfer Function in PixInsight, but now we click Process > All Processes > HistogramTransformation (see image below).

Open the Stars image, then select it in the dropdown menu of the HistogramTransformation window.

Select RGB/K to adjust the red, green and blue colour channels simultaneously before clicking the orange circle to open a Real-Time Preview window (highlighted).

Click and drag the right anchor point under the histogram leftwards to start stretching the data. Once you’re happy, click the blue square Apply button, followed by the X icon on the bottom right. 

Next, drag the left anchor point to the right, closer to the histogram but without clipping the curve.

Click Apply, then X. You can make further stretches, but don’t go too far as we’ll adjust again later.

Once satisfied, close the preview window. Repeat these HistogramTransformation steps for the Comet image.

Remove the stars

Now we can extract the stars from the Stars image. I use Russ Croman’s StarXTerminator plugin for PixInsight.

Once installed, click Process > All Processes > StarXTerminator (see image above).

Tick Unscreen Stars and then the blue square icon to run the star removal script.

This creates a starless image and a ‘Stars Only’ image (highlighted).

Close the unneeded starless image and continue stretching the Stars Only one using the HistogramTransformation tool.

You now have Stars Only and Comet files, ready to merge in Photoshop. Save them as 16-bit TIFs.

Click File > Open as Smart Object > file location, so they appear side by side.

In the Layers section of each file (highlighted, image below), rename the layer. Name the Stars Only layer ‘Stars’ and the Comet’s layer ‘Comet’.

Select the Comet image and the Move tool. Drag the Comet layer onto the header bar of the Stars Only image.

Still holding the mouse button, bring the cursor to the centre of the image and release the mouse button.

Now, re-click and hold to move the Comet image so that it covers the Stars Only image.

You now have two layers in Stars Only: a Stars layer and a Comet layer. Make sure Stars is above Comet.

Now blend your comet into the Stars using a layer mask.

To add this to the Stars layer, select it and click Layer mask (highlighted, image above).

Click the thumbnail that’s appeared next to the Stars layer and invert the mask by pressing Ctrl + i.

Your Comet image is now visible and the Stars Only image is ‘hiding’ behind it. 

Using the Brush tool (highlighted), wipe over the Comet image background to allow the stars to ‘pop’ through.

Adjust the brush size and opacity the closer you get to the comet. Once happy with your results, flatten the image and save the file.

3 quick tips

1. Starnet2 is a standalone program that can be used to extract stars, instead of the StarXTerminator plugin.
2. PixInsight’s CurvesTransformation tool can be used to further improve the colours of your Stars and Comet.
3. Check the progress of your layer mask by highlighting the Stars layer, holding the Alt key and clicking the ‘Layer mask’ icon. 

Have you captured an image of a comet? Share your best shots with us by emailing contactus@skyatnightmagazine.com

This 2-part guide appeared in the October and November 2024 issues of BBC Sky at Night Magazine