Fortunately there are ways to capture both pinpoint stars and achieve low noise. You can either use a star tracker with very long shutter speeds to capture a lot of light and thus a high signal to noise ratio, or you can use star stacking, a method of capturing multiple photos with shorter shutter speeds that are then aligned and averaged in software to reduce noise.
Last Updated: 23rd October 2015
A common approach to astrophotography has become the use of Digital SLR cameras (DSLR). These are relatively cheap, can be used for astronomy and ordinary terrestrial photography, and produce surprisingly good astronomy images so have become quite popular.
There’s a few basic steps required for getting started in DSLR astrophotography. I would summarise them as: 1. Buy a camera 2. Buy a tripod, telescope or other tracking platform 3. Acquire a piece of software to help take long exposure photographs 4. Acquire a piece of software to process (including stack) the photographs you take.
The question often arises from the above of what piece of software to use for stacking and processing the resulting images that you take using your camera. Or, also often the case, people don’t realise that there is software available to make this easy. So here I am going to list a few options, hopefully making it easier for anyone who finds this page.
If you know of programs which are suitable for DSLR astrophotography image processing that are not on this list please let me know, also let me know if information here needs updating. Thank you.
Software suitable for stacking and/or processing astrophotography DSLR images:
Deep Sky Stacker
This is a free and very capable piece of software for aligning, combining and performing post processing of astrophotographs from digital SLR cameras. The best thing about this software is that it’s free, and amazingly capable for something that is free.
This software will read a wide variety of file formats including Canon RAW format, and process them. I have had some issues with processing canon RAW files with respect to getting good colour balance post-stacking so often choose to first convert the RAW files to TIF before processing. This may simply be a lack of experience on my part, as I do not use this software often.
The registering capabilities of Deep Sky Stacker are very good but do not match the capabilities of RegiStar or PixInsight when it comes to getting a good alignment of frames. There are often cases I find DSS will not correctly align frames where as RegiStar and PixInsight will.
I don’t tend to like the post-processing capabilities of Deep Sky Stacker so tend to finish my use of DSS at the point it has stacked the “Autosave.tif” and take that file in to PhotoShop from there to perform post-processing.
Deep Sky Stacker’s biggest advantage is probably it’s ease of use (very intuitive and easy to use interface) and it’s flexibility with it supporting all major file formats and handling various scenarios covering most astrophotography needs.
Find Deep Sky Stacker here: http://deepskystacker.free.fr/english/index.html
Starry Landscape Stacker
This is an Apple/Mac program and a great option for those who do not use Windows. It is effectively a good alternative to Deep Sky Stacker for those who use Apple PC’s.
PixInsight is an advanced astrophotography image processing piece of software. I now have some experience using PixInsight for processing CCD images from an SBIG ST8-XME camera and RAW CR2 files from a Canon 6D DSLR and can certainly see the potential of the software.
If you ant a one-stop-shop for astrophotography image processing and you are happy to spend the $250 on PixInsight, there’s a very good chance you need none of the other pieces of software listed on this page. Having said that, you will be up for a steep learning curve.
PixInsight operates in a very different way to other software. They even seem to put buttons on dialogue boxes around the opposite way to what is most common just to confuse the user. The difference in how processing is done and the user interface in PixInsight makes the learning curve very steep and troubling at first. There are video tutorials online which are almost essential for getting an understanding of how to use the software before you lose your hair trying, but once concerned it is proving to be very powerful. It took me a few attempts coming back to PixInsight over a few months before I became familiar enough with it and stopped hitting brick walls to be able to process FIT and DSLR images with some confidence.
Functions such as applying a LinearFit across LRGB frames, and the Dynamic Background Extraction function on any image to flatten image backgrounds are particularly useful and relatively easy to use once you understand the basics of the PixInsight user interface.
Where other processing software has failed to produce a good result of DSLR images (software such as using DSS, RegiStar and Photoshop) PixInsight has excelled and brought out more detail in images than I realised existed in the raw data.
There is no doubt to my knowledge that PixInsight is the most advanced piece of software for stacking astrophotography deep sky images. It’s set of processes and plugins is both extensive and powerful. The catch is only in it’s usability and how patient you must be to work through its steep learning curve to achieve good results.
I would suggest if you are going to use PixInsight, start with DSS and understand the basics of astrophotography image processing before you begin the daunting process of understanding how to use PixInsight. Also, if you have easy to align good quality images then you will likely get a very good result from DSS in a much quicker time frame than PixInsight which will require you to perform more steps.
If you want to process DSLR images with PixInsight you will need a beefy machine to run it on. It will easily consume all of my 16 gigabytes of RAM on my Core i7 64bit windows machine when processing a stack of 20 DSLR images. Programs such as RegiStar work in a significantly smaller footprint.
PixInsight is available as 45 day free trial.
Find PixInsight here: http://www.pixinsight.com/
StarStaX
Star Trail Photography Software
StarStaX is a multi-platform image stacking software. From their website: https://www.markus-enzweiler.de/StarStaX/StarStaX.html
StarStaX is a fast multi-platform image stacking and blending software, which allows to merge a series of photos into a single image using different blending modes. It is developed primarily for
Star Trail Photography where the relative motion of the stars in consecutive images creates structures looking like star trails. Besides star trails, it can be of great use in more general image blending tasks, such as noise reduction or synthetic exposure enlargement.
StarStaX has advanced features such as interactive gap-filling and can create an image sequence of the blending process which can easily be converted into great looking time-lapse videos.
StarStaX is currently under development. The current version 0.70 was released on December 16, 2014. StarStaX is available as a free download for Mac OS X, Windows and Linux.
CCDStack is one of a suit of products made by CCDWare aligned to advanced usage of telescopes.
I have used CCDStack a reasonable amount now for processing images from my ST8-XME astronomy camera and find it very usable and relatively powerful. I like features such as being able to see what data is being rejected by a sigma function on light frames and doing this very quickly and easily compared to PixInsight which shows you no preview before processing the full stack. This makes it very easy to tweak stacking parameters for a good result and apply different filtering to individual frames (such as when a satellite passes through a frame, applying harsher exclusion to that frame).
CCDStack will easily in only a handful of steps register your frames, normalise (apply weighting to) frames, apply data rejection to frames and combine frames in to a stack using weighting determined by the normalisation.
I found CCDStack to be a good and logical step up from CCDSoft. It is usable and has intuitive and useful functionality. The program seems relatively light weight also, working efficiently with a large number of files.
I have not tried CCDStack for DSLR images. It does apparently open CR2 RAW files (amongst other formats) however in my quick attempt it did not open CR2 files from my Canon 6D (I’m unsure why).
Astro Pixel Processor is a complete image processing software package: https://www.astropixelprocessor.com/
TBA on details – I’m still testing this one!
Maxim
I primarily use MaximDL for image reduction, as it’s image reduction process is very painless. Provide it with a directory of all your reduction .FIT files and it will nicely sort them in to a database of reduction groups to be applied to any image you open. Open the .FIT needing to be calibrated/reduced and it will apply the appropriate reduction frames without you choosing reduction files of the correct temperature, binning, etc. This is significantly easier than any of the other packages which all require you to do more manual work with reduction frames. The benefits of MaximDL’s reduction frame handling for .FIT files may or may not be transferred to use of DSLR raw files – I have not tried reduction of DSLR images in Maxim.
MaximDL’ stacking seems fair however I haven’t had need to use it for alignment and stacking. I also haven’t tried MaximDL for large images such as DSLR, with the largest I typically use in Maxim being those from my SBIG ST8-XME.
This is a fantastic piece of software for aligning and combining individual astrophotographs from digital SLR cameras. It works very efficiently with large files, is amazingly capable in aligning photographs and has quite good stacking algorithms built in as a bonus.
This software is primarily intended for simply the registering (aligning) of frames such that they can be combined. This piece of software is so good that you can combine old film images with new digital images, or digital images from different cameras with different focal lengths and all sorts. It will also easily handles field rotation (fixed tripod shots are OK) and pretty much any other distortion.
The problems I have with this software is that it does not read Canon RAW files, so conversion to some other format such as TIF is required first, that it does not handle reduction of the images which leaves you needing another piece of software (like PhotoShop) to do that manually first, and that when combining frames in to a stack it does not provide any weighting of frames or sigma exclusion of noise in frames leaving this piece of software primarily useful for registering frames and saving those registered frames, not stacking them.
RegiStar’s excellence at registering frames comes with a price, and in this case that’s about US$159.
The version of RegiStar that I am familiar with is 1.0, and it hasn’t been updated for some time (2004). This means it’s not up to date with current file types (RAW) but doesn’t detract from it’s excellent ability to align TIF images. Increasingly, as time ticks on and no further updates are published, you would be wise considering an alternative piece of software which is updated more regularly, such as PixInsight.
Find RegiStar here: http://www.aurigaimaging.com/
ImagePlus
I cannot say much about ImagePlus as I have not used it for DSLR image processing. However many people do and it comes highly recommended. You can find out plenty of information about it around the web.
Find ImagePlus here: http://www.mlunsold.com/
The Milky Way rises over a fairly intimate canyon view in Canyonlands National Park in Utah. The green color in the sky is from airglow.
Introduction
If you’ve done any night photography, then you’re likely very familiar with the noise of exposures in low light using a high ISO. But the noise isn’t all just from the high ISO. If you exposure for a longer period of time, you’ll capture more light, and have a higher signal to noise ratio, meaning less noise. A higher signal will result in less noise even with a high ISO (depending on your camera, modern sensors are much better). However, when shooting the Milky Way you’re usually going for short star trails, or completely pinpoint (trail-less) stars, which means you’re limited in how long you can expose before the stars appear to move too much in the frame. So you’re stuck using a shutter speed that isn’t long enough to capture enough light for a high signal, and having to use a high ISO as well to boost the signal to a usable level, which can add additional noise to the image (although how much depends on your camera).
The result is that you’re usually shooting with a shutter speed that captures enough light for tolerable noise, but it’s also probably long enough that the stars trail in the frame. If you try shooting with a short enough shutter speed to capture completely motionless stars, the noise level can be so high (depending on your camera and lens f-stop) that the exposure is either unusable or needs a ridiculous amount of noise reduction, resulting in large size prints that can look very grainy or blocky due noise or noise reduction. Noise reduction tools these days are great, but they can’t fix everything all the time.
Fortunately there are ways to capture both pinpoint stars and achieve low noise. You can either use a star tracker with very long shutter speeds to capture a lot of light and thus a high signal to noise ratio, or you can use star stacking, a method of capturing multiple photos with shorter shutter speeds that are then aligned and averaged in software to reduce noise. Or you can combine a star tracker with star stacking for incredibly low noise.
Left: 20 seconds @ ISO 12,800 – Note the star trails Center: 10 seconds @ ISO 12,800 – Pinpoint stars but more noise (brightened to show detail) Right: Star stacked result of 10 exposures at 10 seconds each – Pinpoint stars and low noise All shots with the Nikon D810A and Nikon 14-24mm f/2.8 lens @ 14mm and f/2.8
The image above contains three 100% crops from the same night, all shot with the Nikon D810A and the Nikon 14-24mm f/2.8 lens @ 14mm, f/2.8, and ISO 12,800. On the left is a shot taken with a shutter speed of 20 seconds, the center is 10 seconds, and the right is the star stacked result of 10 exposures at 10 seconds each. Notice how the image on the left has lower noise than the 10 second shot in the middle, but the image on the left also has longer star trails. By star stacking, we get the result on the right, pinpoint stars and low noise. It’s hard to tell on these small images on the web, but the noise in the star stacked result is much less than the noise in the 20 second exposure.
What About a Star Tracker?
Before we get into star stacking, you might be wondering if you could just us a star tracker, a device that sits on top of your tripod and turns with the rotation of the earth so that your camera can follow the stars, capturing long exposures with no star movement. You can certainly do this, but that requires lugging around the star tracker, and polar aligning the tracker every time you move your tripod. This is fine if you’re just doing shots of the sky without a foreground, but if you’re capturing the foreground then it will blur in the star tracked exposures, so if you want a sharp foreground you’ll need a separate exposure (or more than one) of the foreground with the tracker turned off. I do this anyways even without a tracker so that I can get detail and low noise in my foreground. Blending the static foreground shot with the star tracker shot would require dealing with blending the blurred foreground of the star tracker shot with the sharp foreground of the static shot. So, the blending gets more complex.
Also, there is no real benefit to a star tracker for wide angle astro shots. Stacking 20 shots of 10 seconds each will result in essentially the same image as if you had taken a single shot for 200 seconds with a tracker (20 shots x 10 seconds = 200 seconds). The stacking shots combine to produce the accumulated shutter speed (amount of light hitting the sensor). The benefits of a tracker are when you are using long lenses and shooting deep space objects that require the camera to move in order for the object to stay within the field of view of the long focal length.
What is Star Stacking?
Star stacking is a method of overlaying multiple night sky exposures of the same composition, aligning the images so the stars all line up between each exposure, then averaging the brightness & color values of those exposures, producing a result with far less noise than a single exposure. The noise differs between each exposure, and even from pixel to pixel in the same exposure the noise differences can be high, so the averaging process greatly reduces that variation, resulting in a much smoother sky.
You can extend this to get pinpoint stars by using a short enough shutter speed for your given focal length so that the stars do not appear to move in the frame. So you get the best of both worlds, pinpoint stars and low noise.
Low Noise Foreground
The star stacking method reduces noise in the entire image, not just the sky. Even though the sky is aligned separately from the foreground in the stacking process, the foreground area can still be stacked and averaged to reduce noise. Depending on the ambient light of the scene, this might mean that you don’t need to do any separate foreground exposures to get a foreground that has detail and is in focus with low noise. But, where I’m often shooting, this isn’t normally the case. I still will take separate foreground exposures at a lower ISO (normally ISO 1600), for longer shutter speeds (usually multiple minutes per exposure), and when needed I will change focus and take multiple foreground shots to get everything in focus.
In the image above you can see how stacking greatly reduces the noise in the foreground part of the image. The star stacked result is significantly cleaner and has more detail. However, the foreground is still too dark and too noisy because there simply just isn’t enough light collected from the foreground in 10 seconds, and while stacking cleans it up, it can’t bring out more detail that wasn’t captured in the first place. So I would take another exposure at ISO 1600 for several minutes, and blend that cleaner, brighter result with the star stacked result of the sky.
Exposure Settings for Star Stacking
Shutter Speed
Pick a shutter speed that will capture pinpoint stars with the focal length of the lens you are using. There is a complex formula that can be used to calculate this with respect to your location on earth, the direction your camera is pointing, your focal length, and the size of the photosites on your camera’s sensor, but an easier way is to use the “150 rule” (something I sort of made up, see below), or just start with half of the shutter speed you normally use that is generating noticeable star trails. Take a test shot and then zoom in on the LCD preview and see how the stars look. If they appear to be pinpoints, then try a longer exposure until they start to trail and go back from that, the goal is to find the longest possible exposure before the stars trail.
You may have heard of the “500 rule”, which is a simple way of determining your shutter speed for the stars so they don’t trail too much. You take your focal length (or the 35mm equivalent if you’re shooting on a crop sensor) and divide that into 500 to get the shutter speed. However, I find this rule to be far too sloppy, usually resulting in distractingly long star trails. If you’re looking for reasonable star trails and aren’t star stacking, try dividing into 400 or 300 instead.
For pinpoint stars, try dividing into 150 and then adjust from there to find the best shutter speed. For example, if you’re using a full frame camera with a 14mm lens, then 150 / 14 = 10.7 seconds, so I round down to 10 seconds. For 50mm, that would be 150 / 50 = 3 seconds, which may sound extremely short, but isn’t so bad if you have an f/1.4 lens. At f/1.4, you’re 1 stop away from f/2, and 2 stops away from f/2.8. Using the same shutter speed, going up an f-stop cuts the light in half, and going down an f-stop doubles the amount of light. So to capture the same amount of light when you increase your f-stop, you have to double your shutter speed. So, a 3 second shot at f/1.4 captures the same amount of light as a 6 second shot at f/2, or a 12 second shot at f/2.8.
Ultimately, just find the right shutter speed for your setup. Start with a shutter speed from one of the above methods and keep adjusting and testing from that point.
ISO
Generally speaking, you’ll need an ISO of 3200 or greater for star shots. The actual ISO you can use will depend on the amount of ambient light, your shutter speed, and your camera’s high ISO performance. Using a Nikon D5 or D850, I generally have no issue shooting as high as ISO 10,000 or 12,800 for night sky shots, but you also need to make sure you keep it low enough to avoid blowing out the stars.
In some cameras, if you’re star stacking with shutter speeds of around 10 seconds or less and your lens has strong vignetting at the aperture you’re using, you could end up with very strong magenta color noise issues along the edges of the frame when you apply vignette correction in your raw editor, which brightens the dark edges, revealing the color noise. This happens because the amount of light hitting the edges, in particular the corners, of the frame is much less than the amount of light hitting the center of the frame due to the vignetting of the lens, and if the light level is dark enough then you’re not overcoming the amplification/circuitry noise of the sensor and camera, so boosting the dark areas shows that noise. The solution here is to use a low enough ISO to prevent or at least minimize the amplification noise. Try ISO 3200 or less if you are experiencing this. If you can’t get rid of it, you can leave in the vignetting, or you can color correct the magenta fringing. An example of this issue and the solution is shown in my complete Milky Way photography editing workflow video tutorial.
F-Stop
You want to use the brightest (lowest f-stop number) aperture possible on your lens while still getting sharp stars. If you see coma distortion of your stars or general star flare, try stopping down a little bit at a time until it goes away or is minimized. You may have to live with some amount (or a lot) of coma distortion depending on your lens.
Number of Exposures & Intervalometer
Star stacking requires capturing multiple shots at the same shutter speed so that they can be aligned and averaged later. The more shots you have, the less noisy the star stacked result will be, but only up to a point before sensor pattern noise starts being visible, or you completely exhaust the signal to noise ratio.
In general, I’ve found 10 exposures to be plenty to give a result with very low noise. I’ve done as much as 20, but sensor pattern noise starts to become an issue around there. But even 5 stacked exposures will make a big difference in noise. Experiment and see what works for you.
You can take the shots manually by just hitting the shutter button multiple times, but a much easier way is to use an intervalometer that can be programmed to shoot X amount of shots at Y seconds each. Some cameras, like many of the full frame Nikons, have a built in intervalometer that can be used, or you can get an external remote/intervalometer. I love the Promote, but the Vello Shutterboss or Neewer intervalometers work well, but note that almost any intervalometer seems to have an issue with cable reliability. I used to use the Shutterboss as my primary remote, but I went through 3 of them in a year because the cables kept breaking. The Promote has replaceable cables, and I’ve gone through a few of them, but in the long run the Promote is cheaper than continuing to buy less expensive remotes. Just make sure you get the correct remote (or cable) for you camera. Use the Nikon 10-pin connection for pro full frame Nikons that support the 10-pin connection, other Nikons (including some full frames) need the USB cable. There are separate cables for Sony, Canon, etc.
Example Settings for Star Stacking
Here are some example exposure settings for star stacking at various focal lengths. ISO 3200 or higher would normally be used for all of these. On my Nikon D850, I’m often using ISO 6400 or higher. Remember that you need to take several exposures for star stacking, at least 5, but I normally take 10.
Focal length: 14mm
Shutter speed: 6 seconds
Focal length: 50mm
Star Stacking Software
Star stacking isn’t anything new, pure astrophotographers (space images without a landscape) have been doing this for a long time, and use programs such as Deep Sky Stacker, among many others, to do the stacking. But landscape astrophotography requires the alignment of the stars in the exposures to happen without the static foreground objects messing up the alignment. This can be done manually in Photoshop, but there are a couple of software programs that are designed for star stacking with landscape astrophotographs, and they make the process much, much easier.
Mac: Starry Landscape Stacker
Starry Landscape Stacker is the program I use, it is incredibly good at automatically finding the stars to make a mask for the sky, so that the foreground is not included in the alignment process. You can help the program find the sky by adding red dots that indicate where the sky boundary is located, and you can fine tune the auto generated mask with a paintbrush. Starry Landscape Stacker has some other great features, like letting you pick which frame to align with, this is great for when you want to align the stars to a particular frame where the stars are situated nicely against the foreground. Believe it or not, the stars will move enough in the 100 seconds (10 seconds x 10 exposures, or however long your stacking is) such that this is a very handy feature.
Star Trails Mac
Starry Landscape Stacker is available in the Mac App Store, and a free trial is available that has all the features but outputs images with a watermark.
You can read more about it here, and Ralph Hill, the author of Starry Landscape Stacker, has a great collection of video tutorials on how to use the program here.
Windows: Sequator
I do not have any experience with Sequator, but it is the star stacking program available for Windows that can deal with landscape foreground masking, like Starry Landscape Stacker above.
Sequator is free and available here, and this is a good video walk-though of Sequator.
Photoshop
Star stacking with landscape foregrounds can be done in Photoshop, but it is a tedious pain, and in my past experience Photoshop sometimes fails to align all the stars, particularly towards the edges of the frames. You can learn how to do it via example in my Milky Way photography editing workflow video, or with Lonely Speck’s video, but I strongly suggest using one of the programs above instead.
Blending Sky and Foreground
After you complete the star stacking steps, you can take the resulting noise-reduced sky image and blend it in Photoshop with foreground exposures from the same scene to produce your final image that will have the entire scene well exposed, sharp, and with low noise from the foreground to the stars. I cover the basics of this in my Milky Way photography editing workflow video tutorial.
Practice!
Now that you know what star stacking is all about and how to do it, head out on the next clear moonless night and give it a try! Figure out the best shutter speeds for your favorite focal lengths, and practice with one of the software tools mentioned above.
