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.\u00a0<\/p>

But when you\u2019re imaging a comet, increasing the image brightness by stacking aligned on the stars won\u2019t work; the comet just becomes blurred.<\/p>

More guides<\/strong><\/strong><\/h3>

How to photograph a comet<\/strong><\/em><\/a><\/li>
Image processing<\/strong><\/em><\/a><\/li>
Deep-sky image processing<\/strong><\/em><\/a><\/li> <\/ul> <\/div>

Credit: Jim Miller \/ Getty Images <\/figcaption> <\/figure> <\/div> <\/div>
Stacking aligned on the comet doesn\u2019t work either; that causes the stars to trail.<\/p>
Here I\u2019ll 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.<\/p>
$\"Comet$
Comet C\/2017 K2 Pan-STARRS meeting globular cluster M10. Credit: Martina McGovern <\/figcaption> <\/figure>
Stack and blend to create a sharp comet\u00a0<\/strong><\/h2>
I used DeepSkyStacker, PixInsight and Photoshop to stack, process and blend two images: one of the stars and M10, and one of the comet.<\/p>
All in all, 27x 120-second light frames went into the finished image.<\/p>
My start image was this data, already stacked in DeepSkyStacker, which was told to stack on the stars; I called this stack my \u2018Stars\u2019 file.<\/p>
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 \u2018Comet\u2019 file.\u00a0<\/p>
To create this Comet file, open DeepSkyStacker, click Open picture files<\/em><\/strong> and navigate to your dataset.<\/p>
Select them all (Ctrl + A) and click Open<\/em><\/strong>. Click on one of your frames to check the comet is visible in your stack and set the nucleus as the alignment point.<\/p>
If you want it brighter, you can use the slider (see image below). Select Edit Comet Mode<\/em><\/strong>.<\/p>
$\"Photograph$
In DeepSkyStacker, click on one of your comet frames and set your alignment point on the comet\u2019s nucleus. Brighten using the sliders (top right) then stack <\/figcaption> <\/figure>
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.<\/p>
When happy with the position, hold down the Shift key and click.<\/p>
Finally, under Options<\/em><\/strong> click Settings > Stacking Settings<\/em><\/strong> to access the Comet<\/em><\/strong> tab.<\/p>
Select Comet stacking<\/em><\/strong> and click OK<\/em><\/strong>. After stacking, save your Comet file in the same location as your Stars file.<\/p>
Data stretch and crop<\/strong><\/h2>
You now have a Stars file and a Comet file to process in PixInsight.<\/p>
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).<\/p>
You want to temporarily stretch the data to see the picture better \u2013 this is done via Screen Transfer Function (Process > All Process > Screen Transfer Function<\/em><\/strong>).<\/p>
Ensure the Link RGB channels<\/em><\/strong> is off (highlighted, below), then click the Nuclear<\/em><\/strong> icon underneath to stretch the data.<\/p>
$\"Photograph$
In PixInsight, open the \u2018Comet\u2019 and \u2018Stars\u2019 files. With \u2018Link RGB channels\u2019 off, click the \u2018Nuclear\u2019 icon to stretch the data. Use \u2018DynamicCrop\u2019 to crop both image files <\/figcaption> <\/figure>
Now you can see your data better, crop out stacking artifacts, ensuring you apply the same crop to each file.<\/p>
This is done via PixInsight\u2019s DynamicCrop. Click on the Stars file. Click Process > All Process > DynamicCrop<\/em><\/strong>.<\/p>
Hover your cursor over the Stars image and a +<\/em><\/strong> appears. Click and hold to highlight your crop region.\u00a0<\/p>
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.<\/p>
A Process01<\/em><\/strong> tab appears (highlighted, image above).<\/p>
Now click and hold this tab, dragging it anywhere over the top of your Comet image. By releasing your mouse, the crop is performed.<\/p>
To finish cropping your Stars image, click the green tick icon in the DynamicCrop window (highlighted).<\/p>
Balance colours\u00a0<\/strong><\/h2>
$\"Photograph$
Use \u2018ColorCalibration\u2019 to balance your colours. First create a \u2018Reference image\u2019 in your Stars image, then apply it. Repeat the same steps for the Comet image <\/figcaption> <\/figure>
You can now balance your colours via PixInsight\u2019s ColorCalibration.<\/p>
Click on Process > All Process > ColorCalibration<\/em><\/strong>.<\/p>
Starting with your \u2018Stars\u2019 image, first create a tiny \u2018Reference image\u2019 window that is a true sample of your background, with no bright stars inside.<\/p>
Use your mouse wheel to zoom in, press Alt + N<\/em><\/strong> simultaneously, then click and drag to create this small window in the Stars image, called \u2018Preview01\u2019.\u00a0<\/p>
Click on the dropdown menu next to the \u2018Reference image\u2019 box in the ColourCalibration window (highlighted, image above).<\/p>
A Select View<\/em><\/strong> window appears. Select Stars->Preview01<\/em><\/strong> followed by OK<\/em><\/strong> and then the Apply<\/em><\/strong> icon to complete the colour balance. Repeat ColorCalibration for the \u2018Comet\u2019 image.\u00a0<\/p>
3 quick tips<\/strong><\/h2>

Check that DeepSkyStacker has accurately identified the comet\u2019s nucleus in all frames before stacking your Comet files.<\/li>
In PixInsight, \u2018Automatic Background Extraction\u2019 can remove gradients before proceeding to ColorCalibration.\u00a0<\/li>
After performing ColorCalibration in PixInsight, save your files as 32-bit TIF files to preserve the dynamic range of your data.<\/li> <\/ol>
Now I\u2019ll explain how to extract the stars from the Stars file and combine them with our now-sharp comet.<\/p>
Blend both elements to create one sharp image<\/strong><\/h2>
$\"photograph$
Martina\u2019s original blurry image of C\/2017 K2 Pan-STARRS meeting globular cluster M10 in July 2022. Credit: Martina McGovern <\/figcaption> <\/figure>
We’ve stacked our subframes, creating two files \u2013 one aligned on the stars (the Stars file), the other aligned on C\/2017 (the Comet file) \u2013 and begun processing them in PixInsight.<\/p>
Now we\u2019ll finish processing and then combine our files.<\/p>
First, we\u2019ll perform a histogram stretch on both, before extracting the stars from our Stars file to create a \u2018Stars Only\u2019 file minus the blurry comet. We\u2019ll then combine the two files.<\/p>
Let\u2019s start by stretching the Stars file \u2013 which currently includes a blurred comet (see the before image, above).<\/p>
We previously performed a temporary automatic stretch with the Screen Transfer Function in PixInsight, but now we click Process > All Processes > HistogramTransformation<\/em><\/strong> (see image below).<\/p>
$\"Photograph$
Open your Stars image in PixInsight. Use HistogramTransformation to stretch your data before adjusting the colour channels. Repeat for your Comet image <\/figcaption> <\/figure>
Open the Stars image, then select it in the dropdown menu of the HistogramTransformation window.<\/p>
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).<\/p>
Click and drag the right anchor point under the histogram leftwards to start stretching the data. Once you\u2019re happy, click the blue square Apply<\/em><\/strong> button, followed by the X<\/em><\/strong> icon on the bottom right.\u00a0<\/p>
Next, drag the left anchor point to the right, closer to the histogram but without clipping the curve.<\/p>
Click Apply, then X<\/em><\/strong>. You can make further stretches, but don\u2019t go too far as we\u2019ll adjust again later.<\/p>
Once satisfied, close the preview window. Repeat these HistogramTransformation steps for the Comet image.<\/p>
Remove the stars<\/strong><\/h2>
$\"Photograph$
The StarXTerminator PixInsight plugin will create a starless image and a stars-only image. Use HistogramTransformation again to stretch the stars-only image <\/figcaption> <\/figure>
Now we can extract the stars from the Stars image. I use Russ Croman\u2019s StarXTerminator plugin for PixInsight.<\/p>
Once installed, click Process > All Processes > StarXTerminator<\/em><\/strong> (see image above).<\/p>
Tick Unscreen Stars<\/em><\/strong> and then the blue square icon to run the star removal script.<\/p>
This creates a starless image and a \u2018Stars Only\u2019 image (highlighted).<\/p>
Close the unneeded starless image and continue stretching the Stars Only one using the HistogramTransformation tool.<\/p>
You now have Stars Only and Comet files, ready to merge in Photoshop. Save them as 16-bit TIFs.<\/p>
Click File > Open as Smart Object > file location<\/em><\/strong>, so they appear side by side.<\/p>
In the Layers section of each file (highlighted, image below), rename the layer. Name the Stars Only layer \u2018Stars\u2019 and the Comet\u2019s layer \u2018Comet\u2019.<\/p>
$\"Photograph$
Using Layers in Photoshop, rename each file\u2019s layers then move the Stars layer on top of the Comet layer. With a mask and the brush tool (try size 400, hardness 0, Normal mode, opacity at 100 per cent), blend the two before flattening and saving <\/figcaption> <\/figure>
Select the Comet image and the Move tool. Drag the Comet layer onto the header bar of the Stars Only image.<\/p>
Still holding the mouse button, bring the cursor to the centre of the image and release the mouse button.<\/p>
Now, re-click and hold to move the Comet image so that it covers the Stars Only image.<\/p>
You now have two layers in Stars Only: a Stars layer and a Comet layer. Make sure Stars is above Comet.<\/p>
Now blend your comet into the Stars using a layer mask.<\/p>
To add this to the Stars layer, select it and click Layer mask<\/em><\/strong> (highlighted, image above).<\/p>
Click the thumbnail that\u2019s appeared next to the Stars layer and invert the mask by pressing Ctrl + i<\/em><\/strong>.<\/p>
Your Comet image is now visible and the Stars Only image is \u2018hiding\u2019 behind it.\u00a0<\/p>
Using the Brush tool (highlighted), wipe over the Comet image background to allow the stars to \u2018pop\u2019 through.<\/p>
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.<\/p>
3 quick tips<\/strong><\/h2>

Starnet2 is a standalone program that can be used to extract stars, instead of the StarXTerminator plugin.<\/li>
PixInsight\u2019s CurvesTransformation tool can be used to further improve the colours of your Stars and Comet.<\/li>
Check the progress of your layer mask by highlighting the Stars layer, holding the Alt key and clicking the \u2018Layer mask\u2019 icon.\u00a0<\/li> <\/ol>
Have you captured an image of a comet? Share your best shots with us by emailing contactus@skyatnightmagazine.com<\/a><\/em><\/strong><\/p>
This 2-part guide appeared in the October and November 2024 issues of BBC Sky at Night Magazine<\/em><\/strong><\/p> <\/body><\/html>\n
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By Martina McGovern Published: Monday, 04 November 2024 at 13:51 PM 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.\u00a0 But when you\u2019re imaging a comet, increasing the image brightness by stacking aligned on the stars won\u2019t work; the […]<\/p>\n","protected":false},"author":24,"featured_media":0,"template":"","categories":[34],"acf":{"readingTimeMinutes":"9"},"uagb_featured_image_src":{"full":false,"thumbnail":false,"medium":false,"medium_large":false,"large":false,"1536x1536":false,"2048x2048":false},"uagb_author_info":{"display_name":"importmanagerhub@sprylab.com","author_link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/author\/importmanagerhubsprylab-com\/"},"uagb_comment_info":0,"uagb_excerpt":"By Martina McGovern Published: Monday, 04 November 2024 at 13:51 PM 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.\u00a0 But when you\u2019re imaging a comet, increasing the image brightness by stacking aligned on the stars won\u2019t work; the…","_links":{"self":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/rss_feed\/65487"}],"collection":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/rss_feed"}],"about":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/types\/rss_feed"}],"author":[{"embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/users\/24"}],"wp:attachment":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/media?parent=65487"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/categories?post=65487"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}