Beginner tutorial – how to stack comet images in Deep Sky Stacker (my work flow)

First visit to this astronomy/astrophotography blog? Well, firstly a warm welcome to you and thanks for stopping by. After reading this post, if you want to find out more about me and this blog, why not visit my introductory page at https://undersouthwestskies.blogspot.com/2025/01/welcome-to-my-new-astronomyastrophotogr.html  You can also browse other posts by using the search bar or the blog post list on the right hand side and I'd really like to hear from you via the comment boxes. Drop me a tip, an observation or a question. Take care and clear skies to you.  Steve 

 Beginner tutorial – how to stack comet images in Deep Sky Stacker (my work flow)

Ever wondered how to turn your series of faint comet frames into one crisp, detailed image? In today’s post. I’ll walk you through my step-by-step work flow for stacking comet images in Deep Sky Stacker DSS) – a free, powerful tool that’s perfect for astrophotographers who want to capture the beauty of a comet without those distracting star trails.  

In summary, the work flow order goes like this:

1.      Open picture frames

2.      Then for each image use the green comet image to select the comet’s nucleus – saving the position for every frame

3.      Entering the stacking settings, choosing a comet stacking mode (Comet only, stars only, both)

4.      Starting the process of stacking

 

So, let’s dive straight into the more detailed work-flow.

Step 1: Register all images 

First, let’s get your light frames ready.

• Open Deep Sky Stacker and click ‘open picture files’
• Select all the comet frames you want to stack
• (Select all the bias, flats and dark frames you want to add in)
• Click ‘check all’ – and then choose ‘Register checked pictures’

·       In the pop-up window, make sure to:  –

o   check ‘register already registered pictures’

o   check ‘automatic detection of hot pixels’

o   DO NOT check ‘stack after registering’.

o   Then click ‘OK’ to start the registration process

This last step makes sure DSS knows exactly how to align and handle your images before we move on to the comet itself.

 

Step 2: Mark the comet’s nucleus in each image 

Now for the fun part — telling DSS where your comet is in each shot.

• Select the first image in your list
• Click the green comet icon on the right side of the screen
• hold down the Shift key and click directly on the comet's nucleus to mark its position. A pink circle should appear around it.
• Save the changes for that image. You can click ‘save all’ so DSS keeps track of the comet position across all selected frames OR
• Repeat this process for every light frame

This step can be a little repetitive, but it’s essential — it tells DSS how the comet moves across your frames, allowing it to align the comet while still handling the star background correctly.

Tip: If you’ve seen the tutorial “How to Stack and Process Comet Images with Deep Sky Stacker,” it shows a great visual example of this process. How to Stack and Process Comet Images with Deep Sky Stacker

 

Step 3: Stack the images

With everything marked, it  is time to stack your comet.

• Click ‘Stack checked pictures’.
• When dialogue box appears, choose ‘Stacking parameters’ appears
• go to the Comet tab and choose your desired stacking mode.
• "Stars" (comet trails)
• "Comet" (star trails) or
• "Stars + Comet" (both) – my go to choice  
• Click OK to start the stacking process. 
• If another pop-up window appears called ‘stacking steps’, keep the default settings and just click ‘OK’ again.

When DSS finishes, save your stacked image as a TIFF file, ready to edit in your favourite processing software — whether that’s Photoshop, Affinity Photo, PixInsight, or any other astrophotography editor.

Want to See It in Action?

If you prefer to follow along visually, these videos do a fantastic job of demonstrating how to select and use the comet stacking modes in DSS:

How to STACK Comet in Deep Sky Stacker - Beginners Astrophotography Tutorial

How to Stack Comet Images Without Star Trails in DeepSkyStacker

Final Thoughts

Stacking comet images in DSS might feel a bit technical at first, but once you’ve done it a couple of times, it becomes second nature. The reward is absolutely worth it — a clean, vibrant image that beautifully captures the motion and structure of your comet, without losing the stars in the background.

Give it a try with your next comet data set and watch your images transform! As always, if you have a tip or an observation about my work-flow, then drop me a comment so we can improve it for all and, it goes without saying, but I will anyway – stay safe, have fun and clear skies to you all

Steve

Beginner's guide to photographing comets

 First visit to this astronomy/astrophotography blog? Well, firstly a warm welcome to you and thanks for stopping by. After reading this post, if you want to find out more about me and this blog, why not visit my introductory page at https://undersouthwestskies.blogspot.com/2025/01/welcome-to-my-new-astronomyastrophotogr.html  You can also browse other posts by using the search bar or the blog post list on the right hand side and I'd really like to hear from you via the comment boxes. Drop me a tip, an observation or a question. Take care and clear skies to you.  Steve 

In today's blog, a 10-minute read, I 

• describe Comet Lemmon
• outline some beginner's tips for capturing this or another comet image
• suggest a way to post-edit your image of a comet

Detected first on 3^rd January 2025 by the Mount Lemmon Survey in Arizona, pre-discovery images from the Pan-STARRS survey date back to November 2024. Initially thought an asteroid, follow up observations revealed a condensed coma and faint tail. A long-period comet! With perihelion to the sun on 8^th November 2025, its inbound orbit period is around 1,350 years. And it comes from far out in the solar system, probably the outer Oort cloud. So, this is a relatively rare visit to the inner solar system. 

Like other comets, Lemmon has a s small rocky or icy body nucleus and a coma where solar heating has caused ice to sublimate, releasing dust and gases to form the fuzzy cloud around the nucleus. Its tail is broadly curved, of diffused ionised gas, bluish-green in appearance. As the comet gets closer to the sun, sublimation will increase, releasing more material and enhancing they coma and tail. Solar wind and radiation pressure, as it approaches the sun, will continue to ensure that its tail points away from the sun.

Lemmon, with its eccentric and retrograde orbit as it approaches the sun, is probably an early relic from the formation of the solar system and so it gives astronomers a chance to study material relatively unaltered since then. Like many of our visiting comets remaining unchanged as they arrive in our area of space, our sun heats them causing their icy nuclei to release gas and dust on tails and comas that reveal their composition. Studying this material allows us to learn what the early solar system was made of and how planets such as ours may have formed; delivering water and organic materials, the building blocks of life.  Lemmon carries fresh samples of this primordial material – its chemistry and dynamics is of critical interest.

I have only photographed one comet so far and I found it one of the most fun bits of astrophotography I have done so far on my learning journey. The comet changed its appearance every night I went after it and you can read more details here at this previous post:  https://undersouthwestskies.blogspot.com/2025/01/chasing-comet-c2023-tsuchinstan-atlas.html

The positives - I located the comet and managed to image it. The 'future learning'? Getting my editing skills better developed, particularly on white balance!! 😆

I used Sequator to stack several images. 

Clearly at the time I hadn't then discovered 'freeze' ground' and also 'stack for comet' options!

I'm aiming to do much better on Comet Lemmon - if the cloud ever actually clears away long enough in the south west 😱 

Anyway, moving on - what are my ‘beginner’s’ tips for capturing this latest cosmic visitor, if you haven't caught one before?

1.      21^st – 26^th October, when it makes its closest approach to earth and is at its brightest. It is also when there is no or only a faint presence of a new moon

2.      In the northern hemisphere, it will appear in the NW sky after sunset (30 – 90 minutes), once twilight has faded.

3.      Find a clear unobstructed horizon in a dark sky site away from light pollution and be aware that as the night progresses this comet will appear lower and lower in the sky;

4.      Look for the constellation Boötes and its bright star Arcturus as lead in points to its location.

5.      Because it is low on the horizon, try to include some foreground elements such as trees, hills, silhouettes to add context and scale

6.      Its tail will probably point roughly away from the sunset west direction and somewhat downward

7.      A wide to moderate focal length e.g. 24 – 70mm on a full frame camera, or a longer lens/telescope if you want detail in the coma and tail; try ISO 800 – 1600, aperture F/2.8 – F/4 on a fast lens. Exposures 10 – 30” depending on your focal length and whether your mount tracks or not. Shoot in RAW

8.      You could use a star tracker with a 200 – 500mm lens or small refractor for greater tail structure, again ISO 800 – 1600, 20 – 60” and aperture F/4 – F/5; if you are not using a tracker – take multiple 1 – 5” shots and stack them later in software but align on the comet rather than the stars or do both and merge, to keep the nucleus sharp. 

 

Because of its low elevation, there may be atmospheric issues to deal with:

1.      Pick nights with good transparency, if possible, to combat reduced contrast and also haze issues

2.      Try for a clear horizon line under such circumstances; avoid shooting towards sunset glow

3.      Mitigate atmospheric blur and tail motion by taking many short exposures

4.      Use a light pollution filter if you are under lights

 Composition and framing

My Achilles heel in astrophotography and I do try hard to improve it. For comets, which are highly dynamic subjects, composition matters; go for foreground elements such as trees and mountains in wide angled shots. Comets tails always point away from the sun, so that helps in framing the subject. Try to align the comet’s tail diagonally through the image for a dramatic composition. Look for potential juxtapositions – a bright star, a star cluster, an element of the milky way.

How do you do the post processing of your comet image?

1.      In the past I have combined stacks – one aligned on the stars and one aligned on the comet – blending them afterwards in something like deepskystacker or SIRIL - masking carefully to retain both sharp stars and a crisp comet nucleus.

          Combine the two images by blending them together in photoshop or affinity photo

2.      Levels and curves adjustments should aim to extract tail detail without blowing out the coma; use noise reduction but preserve the faint structure in the tail

3.      Colour balance, try to preserve the greenish hues

4.      Crop and sharpen GENTLY to bring out structure without over processing to get artefacts.

What about some beginner tips for shooting comets generally?

1.      Do your research using sources like Sky and Telescope, Sky at Night, etc to learn where and when the comet will appear.  More often than not, best times are when the comet is far from the sun in the sky – so typically visible before dawn or after dusk and when the moon is absent or below the horizon. Choose nights when it is higher in altitude above the horizon to avoid atmospheric haze and light pollution. Stellarium and Sky Safari apps will help you visualise where the comet will appear relative to your horizon, and constellations above.

2.      I have already mention equipment choices above but to build on that:

a.      A DSLR or mirrorless camera that shoots in RAW format. An astro-camera will get deep detail but isn’t essential.

b.      Lens or telescope? Tough question! Wide angle (14 – 50mm) will capture the comet in context with the landscape or milky way. Telephoto or small refractors (100 – 300mm etc – will reveal coma structure and tail detail

c.       A tracking mount such as a skywatcher star adventurer 2i (other brands are available) will allow you to go beyond the limit of shorter exposure times

d.      Tripod and intervalometer enable stability and no vibrations.

e.      Camera settings – how long is a piece of string? If I were doing a TRACKED image then I’d probably go somewhere with ISO 800 – 1600; F/2.8 to F/4 for wide angle lenses and F/4 to F/6 for telephotos; exposures 20 – 60” if tracked and 2 – 10 “if not.  White balance daylight or 4000K

f.        Multiple frames rather than one long exposure are better, it improves signal-to-noise ratio and allows stacking later.

g.       And always shooting in RAW

 

Photographing comets is a unique challenge that combines astrophotography, artistry and timing. Prepare well, bring the right equipment, master your post editing workflow and remember, you are capturing a rare celestial event and ancient cosmic visitor.