Dithering with a DSLR and Star Tracker

September 21, 2020  •  16 Comments

Dithering with a DSLR and Star Tracker 

Dithering is a special process that should allow you to fix some annoying problems in your astro images.  Many sensors produce fixed pattern noise, which can include hot pixels, banding, color mottle, and purple glow.  Canon cameras tend to have very bad banding issues, along with color noise.  Nikon cameras often have problems with a purple glow around the edges or bottom of the frame.  All cameras have color mottle, which are splotchy areas of usually purple and green.  You will mainly notice these problems if you don’t capture enough light during each exposure.

Theoretically, Dithering can help with all of these problems!  With dithering enabled, your camera will move slightly in a random direction between each exposure.  This small motion, when averaged over dozens or hundreds of images, should smooth out and remove most sensor problems.  We're going to find out today just how well it works though...

All cameras produce a “white noise”, which is present at all times.  This “white noise” can be "covered up" by capturing more light.  Let’s take a look at a few examples.  The photos below were taken with a Nikon D750.  The first image was taken at ISO 12,800, 15 seconds, f/2.8.  You’ll notice a ton of grain and a bright purple glow at the bottom.  This is all the “white noise” generated by my camera.  The foreground was very dark, and there was not enough light to cover up that "white noise" pattern.  To fix this, I took a 4 minute long photo at f/2.8, ISO 800.  I’ve now captured enough light to overpower that "white noise", and it is no longer visible!

(Both images were brightened extensively in post-processing to clearly show the sensor problems)



Taking a longer exposure is the secret to capturing amazing photos at night!  Most of the information online tends to focus on using short exposures and high ISO's to capture night images.  As you begin to take longer exposures, ideally 3+ minutes, your photos will improve dramatically.  There's only one problem - the stars are moving!

A star tracker will allow you to photograph the Milky Way or even distant nebulae and galaxies.  Basically, the star tracker will move your camera very slowly to follow the motion of the stars.  You can now capture much longer exposures with sharp stars!  However, since your camera is moving, the foreground will blur out.  Therefore, you need to take 2 exposures - one for the foreground, and one for the sky.  You can then blend the two together to create a final image.  I cover this in my Star Tracker Tutorials.

We’ve seen how easy it is to fix your camera sensor’s inherent problems by simply taking a longer exposure.  However, this is only really effective for wide-angle nightscapes.  We can easily shoot longer exposures without visible star trails due to the wide Field Of View.  If I use my SkyGuider Pro and do a rough polar alignment, I can usually get 4+ minute exposures at 14mm.  However, if I use a telephoto lens I won’t be able to shoot very long exposures at all.  

When I use my Tamron 150-600mm lens, I normally max out around 30 second exposures.  If I try to shoot any longer, I get star trails.  This lens is also usually between f/5.6 – f/6.3, which doesn’t capture a lot of light, especially compared to an f/2.8 lens.  Due to the small aperture and short exposures, I can never capture enough light to overpower the "white noise" being generated by my camera.  Even if I take 300 photos and stack them together, the pattern noise will still be baked into the final image.  With my Nikon D750, that means I'll have an ugly purple glow at the bottom of my final image.  You could take Dark and Bias frames, which will help to remove those annoying problems.  However, I personally find Dark Frames to be a waste of time, especially if you are using a good camera from Nikon, Sony, or Canon.  This is where dithering comes in.


What You’ll Need to Dither

If you have a SkyGuider Pro or Star Adventurer and want to dither, you’ll need a few different pieces of hardware and software.  First, you’ll want to get an auto-guider.  This is a little camera that will drastically increase the accuracy of your star tracker.  Basically, the auto-guider camera will lock onto a star that you specify.  It is constantly watching this star while you are taking photos. If it notices the star drifting outside of a crosshair, the auto-guider will send a command to your star tracker.  If you use a SkyGuider Pro or Star Adventurer it will only be able to tell it two things – “Speed up!” or “Slow down!”.  For example, let’s say the star begins to drift to the right.  The auto-guider will see that, and send a command to the star tracker to “slow down”.  The star tracker will very briefly slow down, and the star will return to the center of the box.  If the star begins to drift to the left, the auto-guider will say “speed up!”.  The star tracker will then speed up a bit so that the star gets back in the center of the box.  The actual process is much more complicated, but that’s basically what’s going on.

In order to actually use the auto-guider, you’ll also need a guide scope.  This is just a small telescope that focuses the light for the auto-guider.  I usually recommend getting this combo here.  There are other options, but this is a perfect combo for the SkyGuider Pro and Star Adventurer.

Once you have your auto-guider and guide scope, you need to find a way to mount everything.  I originally used a hotshoe mount, but this was fairly flimsy.  I eventually learned that I could use a simple arca-swiss clamp to attach the guide scope directly to my camera’s L-Bracket.  This method is much more secure.  However, it will put more weight on one side of your camera, which can cause balance problems at night.  If I use my Space Cat telescope, then I can mount the auto-guider directly to the “saddle bar” (optional accessory).  This is the best method, and provides the most secure connection without creating any balance problems.  However, if you are using a normal telephoto lens, you won’t be able to do this.

Now that you’ve got your auto-guider and guide scope mounted, you need to download some software.  PHD2 will do all of the auto-guiding for you in just 4 easy steps!  Connect your equipment, begin looping exposures, pick a star, begin guiding.  That’s all there is to it!  The only downside with this method is that you’ll need a laptop out next to your star tracker.

My preferred way to use an auto-guider is with the ZWO ASIAir.  This is a nifty little device that does everything PHD2 and SharpCap do, but you can control it from your smartphone! 

Okay, so you’ve got your auto-guider, guide scope, PHD2 or ASIAir, DSLR, and star tracker.  At this point, you’re almost ready to dither!  However, there’s one final consideration – image acquisition software.  I normally just use a wired remote to control my DSLR.  This simplifies my shooting process, but it does not work with dithering.  For dithering to work properly, there needs to be communication between your DSLR, auto-guider, and star tracker.  Therefore, you will need some type of image acquisition software to dither.  

You can either use the ASIAir app on your smartphone, or Backyard Nikon/EOS on your laptop.  Both options will allow you to control your DSLR in real-time, you can even adjust the camera settings, shooting interval, and see a live preview.  Most importantly, this image acquisition software should have features that will allow you to dither!  The exact process will change depending on the software, but it’s generally the same workflow.


Dithering with PHD2 and Backyard EOS / Nikon

Backyard EOS and Backyard Nikon are two of the most popular image acquisition programs available.  These will allow you to control your camera, whether Nikon or Canon, from your laptop.  You will need a USB connection from the DSLR to your laptop for this to work.  Once the DSLR is connected to the laptop, and the auto-guider is connected to the laptop and star tracker, you can begin.

First, start up PHD2.  Now click on the little “Brain” icon towards the bottom of the screen.  In the “Global” tab you should see the Dither settings.  Be sure to choose “Random” for the mode, and check the box for “RA only”.  The SkyGuider Pro and Star Adventurer only have an automated Right Ascension (RA), so that needs to be checked.  You can also adjust the scale.  Since DSLRs usually have large sensors with a Bayer Array, we need a larger scale for the dithering to be effective.  Once you’ve configured these settings, click Ok at the bottom. 

Now we need to turn on the Server option in PHD2.  Go up to the Tools tab on the main screen.  You should see “Enable Server” towards the bottom.  Click on it, and it should now be checked.  This will allow Backyard EOS / Nikon to communicate with PHD2.

You can now adjust the PHD2 Dithering settings in Backyard EOS / Nikon.  Go to the Settings and find PHD Guider.  You may want to do your own research on how to setup Backyard EOS / Nikon.  I haven't personally used either application yet.

Once everything has been set correctly, dithering should occur!  Let’s break down what will actually happen though.  PHD2 will guide on a particular star, sending commands to the tracker to keep that star centered.  Your DSLR will be taking a photo.  Once the exposure ends, Backyard EOS / Nikon will relay that to PHD2.  PHD2 will now send a command to the star tracker to move slightly in a random direction.  Since we are using a SkyGuider Pro or Star Adventurer, it can only move the RA axis.  Once the tracker has moved, PHD2 will lock back on to the star and begin guiding again.  Your DSLR will now begin taking its next exposure.  This process will repeat for the rest of the night.  The end effect is that the stars will shift position between each photo very slightly.  The stacking software will align all the stars back to the same spot.  In that process, it should smooth out any color mottle and banding.  It will also remove hot pixels.


Dithering with the ASIAir

First, make sure all your cables are connected properly.  You need to connect the ST4 cable from the auto-guider to the "Guide" port on your Star Adventurer or SkyGuider Pro.  The USB cable needs to be connected from the auto-guider to the ASIAir Pro.  If you have a dedicated astro camera, make sure it's also connected to the ASIAir Pro.  For those using a DSLR, you should connect the DSLR to the ASIAir Pro using a USB cable.  Make sure your camera is actually supported too!  Unfortunately, Sony cameras aren't not currently supported on the ASIAir Pro, so this method will only work for some Nikon and Canon shooters.

The camera should be set to full Manual Mode, with Bulb Mode enabled.  For most people, scroll your shutter speed dial until you see Bulb.  For some Canon cameras, you may see a B on your top dial.  This may stand for Bulb Mode.  You should also set your Aperture to the widest possible f/stop.  In my case, that's between f/5 and f/6.3 with my Tamron 150-600mm lens.  You can always change the ISO and Shutter Speed via the ASIAir Pro app on your smartphone later.

At this point, I'm assuming you've connected the ST4 and USB cables properly from the auto-guider, the DSLR is connected to the ASIAir Pro via USB, you've done a polar alignment, you've balanced your camera with the counterweight, the lens is focused, and the object you are photographing is centered in the frame.  You should be ready to shoot!

Once you are ready to photograph your object, now we can talk about dithering with the ASIAir Pro.  First, go to the Guide Settings up top.  At the bottom of the Guide Settings you'll see "Dither Settings".  Once inside the "Dither Settings" menu, you can adjust a lot of variables including - Pixels, Stability, Settle Time, Interval, and RA Only.  Let's break down each one.

Pixels - This refers to how many pixels the auto-guider will move its guide star.  If you are shooting with a wider focal length (70-200mm), you'd want a higher number here, maybe 5 or 10.  Since the field of view is wider, you need more movement for the dithering to work well.  If you are shooting 400mm+, you can probably use a Pixel value of 1, 2, or 3.  I'd recommend doing your own research on this topic for more information.  Personally, I always use a higher value (5 or 10).

Stability - This value is marked in seconds - 0.6", 1", 2", 3", 4".  These are arc-seconds, and they are referring to the error on your guiding graph.  In other words, how accurate do you want the guiding to be before you begin taking your next exposure?  If you set this to 0.6", 1", or 2", you may have to wait a long time before the ASIAir Pro will allow your DSLR to take the next photo.  I normally leave this on 4".

Settle Time - These values range from 1s, 3s, 5s, 10s, and 15s.  After your auto-guider has settled to the error you set above (in my case 4" arc seconds), then it will wait a set amount of time before taking your next image on your DSLR or dedicated astro camera.  I personally use a value of either 3s or 5s here, so that I'm not wasting too much time between each photo.

Interval - Very simply, leave this to 1.  That ensures the auto-guider will move the star tracker after every single exposure.

RA Only - If you are using a SkyGuider Pro or Star Adventurer be sure to turn this On.  If you are using a different mount that has an automated declination axis, turn this off.

This image shows the Dithering process. The first photo has completed, so the Dithering moves the star tracker slightly.  It then waits until things have settled down.  Once the arc-second error rating falls to an acceptable range, it will allow the camera to take another photo.


Once you've chosen all of your Dither Settings, you can begin Guiding.  Remember, you need to click the "Begin Looping" arrows on the right.  This will cause the auto-guider to begin taking photos.  Once you see some stars, you may need to refocus your guide scope.  Provided the stars are sharp, you can tap on one.  A green box will be drawn around the star you chose.  Now click the "Begin Guiding" crosshair button on the right.  It will begin a calibration.  This usually takes about 2 minutes.  After the calibration completes and the lines appear on the graph, we can start the final process of adjusting our Auto-Run settings.  I cover this process in my YouTube Course.

If you click the arrow in the top left of the screen, that will bring you back to the camera shooting interface.  From here, click Preview on the right.  Now choose Auto-Run.  When you enter the Auto-Run interface, click the button that has three dots and three lines on the right.  You are now in the Shooting Schedule menu.  I recommend deleting any schedules that are there, and starting fresh.  Click the + button to add a new schedule.  You'll be able to choose the Shutter Speed and Number of Photos.  I recommend capturing at least an hour of total exposure time.  Since you are using an auto-guider, you can probably get sharp stars up to 5 minutes now.  

Now that you've got your Auto-Run settings dialed in (shutter speed, object name, filters, number of photos, etc...) you can finally start taking your images!  Click the circular button on the right to begin your shooting sequence.

If everything is working correctly, your first photo should complete as normal.  Then the Guide Graph window will show the Dithering (as seen in the photo above).  Once the star tracker has settled, it will automatically have the camera begin taking another photo.  This process will repeat until the software has taken as many photos as you specified in the Auto-Run.

This brings me to my big problem with dithering (using a Star Adventurer or SkyGuider Pro).  You may be waiting up to 30 seconds between each photo!  The auto-guider needs to wait for the graph to settle before it will allow your camera to take another photo.  This can waste a lot of time!  During my Andromeda test, I wasted half of my night waiting for the dithering to settle.  That's a huge loss of light!

You'll need to do your own tests to determine if Dithering is right for you.  In my experience, it's a waste of time.  I'd rather spend my time capturing as much light as possible, rather than spending half the night just waiting on the dithering to complete.  I also noticed that my DSLR's battery seemed to drain a lot faster when connected to the ASIAir Pro.  For those using a Go-To mount, you might have much more success with dithering!




Final Thoughts

Dithering is a popular technique that should allow you to capture amazing photos, without needing Dark Frames!  The Dithering process moves the star tracker, and ultimately your camera, very slightly between each exposure.  This random movement, when spread over dozens or hundreds of photos, will help with the stacking process.  Since the stars shift between each frame, things like hot pixels, color mottle, and some fixed pattern noise should be removed during the stacking process.

However, as we saw today, dithering does have serious drawbacks.  With a SkyGuider Pro or Star Adventurer, and average guiding, you might spend up to half the night waiting for your guiding to settle.  That means rather than capturing 2 hours of data, you'll only capture 1 hour.  There's another factor to consider.  Since the SkyGuider Pro / Star Adventurer can only Dither in RA, we aren't getting "true" dithering.  Ideally, the stars would move in a random spiral direction between each photo.  In our case though, the stars will only move up/down or left/right, not both.  In other words, we aren't getting the full benefit since we are limited to RA Only.

If you want to give dithering a try, you'll need a star tracker, auto-guider and guide scope, PHD2, and some type of image acquisition software.  Alternatively, you can also use an ASIAir Pro to control everything from your smartphone.  If you've got the gear, it's worth a try!  Just remember the drawbacks I mentioned today.

Picture saved with settings embedded.


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