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A wonderful pastime that enables us to take stunning photos of the night sky—that’s how I’d describe astrophotography. It can be tough to capture high-quality photographs of objects in space because of various issues, including atmospheric distortion, equipment limitations, and light pollution. Astrophotography dithering techniques help overcome these challenges. In this post, I’ll explain dithering, how it works, and why it’s an important astrophotography technique.
Understanding Astrophotography Dithering Techniques
To explain the astrophotography dithering technique, we must first understand why it’s required. Slight tracking inaccuracies and other problems can cause our images to slightly shift from one frame to the next when we take longer exposures of the night sky. The equipment we use and its flaws are what cause hot pixels, noise, and other visual defects.
The importance of noise reduction in astrophotography
Noise is always present in photography because of the nature of the electronics used in cameras1. Some cameras have less noise than others, but it is always something that we as astrophotographers have to deal with. Why do cameras produce noise and how does this affect our astrophotography images?
In electronic circuits noise is always produced. We can define noise in images as any unwanted random signal that is created by the camera electronics and appears as signal in the camera sensor. In digital photography there are four types of noise that will affect our images. These are:
- Shot noise
- Read noise
- Thermal noise
- Fixed pattern noise, (amp glow)
Let’s look closer at some of these sources of noise and how we can use dithering to reduce it.
Shot noise occurs because photons of light arrive randomly onto the sensor. If you use short exposures or try to image very faint objects, then this noise will be a large part of the signal your camera sensor sees. This is why when imaging deep sky objects which are very faint, we need to use long exposures and the more integration time we spend on a target this will reduce the noise in our image. Use my astro calculator to find out how long to expose for your target..
Read noise is produced every time the camera reads out the signal from the sensor. Each photograph you take has a certain level of read noise produced by the electronics.
Thermal noise increases with temperature as the electronics of your camera heat up. On summer nights my DSLR sometimes reached 35-40 degrees centigrade, and this produced a horrendous level of thermal noise ruining my image. This level of noise was so bad I could not use noise reduction techniques to reduce it. This is one reason why I bought a cooled camera and now take my images while cooling the camera sensor to a constant 0 degrees Celsius. This has reduced thermal noise in my images drastically.
Fixed pattern noise, (amp glow), is the background pattern of noise which covers your camera sensor due to the electronics being close to the sensor. In my camera, the ZWO ASI533 MCPRO there is almost no amp glow but with some cameras this amp glow has to be removed by taking dark frames and subtracting them from the image to remove the noise.
It is best to introduce as little noise as possible when we image, but in many cases, we have to further reduce noise early on in our processing workflow. Stacking also reduces noise in proportion to the number of images you stack together.
Why do we need to reduce noise in astrophotography images? The answer is that the signal-to-noise ratio will determine the quality of the picture and noise can hide details in our images. We can’t tolerate much noise in astrophotography because the signal is often low as the objects we photograph are so faint.
One form of noise reduction that dithering helps with is hot pixels. Hot pixels are pixels that do not give a correct reading, they appear as a bright spot in the image. With dithering these hot pixels change position and then when images are stacked they average out and are less obvious.
I first heard about dithering as I was spending time in the Cloudy Nights astronomy forum and posted an image of the Wizard Nebula which I was far from happy about. My image was looking very bad with way too much noise and not enough detail. This image was taken without any dithering applied and using a DSLR camera (a Canon 600D modified) through my Celestron 130mm SLt reflector telescope. I was taking two-minute exposures on a very hot night with temperatures of 30C.
I went on the forum and asked questions, and one person told me about walking noise and how dithering would cure that, so I watched several videos about astrophotography dithering on YouTube and put it into practise.
Another benefit of astrophotography dithering is that satellite trails or aeroplanes that appear in one or two frames can be eliminated in the stacking software after dithering. This is yet another reason to use it, as you don’t need to throw out those frames.
My first attempts at dithering were not very successful as I had some trouble setting it up in Astrophotography Tool, but a few days later I had figured that out and proceeded to capture images with no walking noise.
I now always use dithering as part of my normal astrophotography sessions and combined with auto guiding and using my new camera a ZWO ASI 533 MC PRO cooled to 0C I now have much cleaner data in my images and higher quality images.
I will return to this post and add some images to compare the effect that dithering has when I am able to conduct an experiment or find suitable images from previous imaging sessions.
- TEC cooled camera
- 11.7 MP CMOS
- With WIFI
- Minimizes noise
- Great value approx. $730 (price may change)
How to dither for astrophotography
Types of Dithering
There are a few different methods commonly used.
Frame-to-frame and sub-pixel dithering are the two main types. Astrophotography dithering occurs when the telescope is shifted from frame to frame, usually by a few pixels2. Hot pixels and other fixed-pattern noise can be greatly decreased with this technique. On the other hand, sub-pixel dithering involves shifting the camera sensor by a few hundredths of a pixel between shots. This technique, which uses software to accomplish it, is more effective in reducing random noise.
I understand that the movement between each frame helps the stacking software to average out random noise, so the picture has a better signal to noise ratio. It really improves the quality of the image!
To properly dither, one needs to move the telescope or camera in a precise manner, which requires the use of the appropriate tools. Astrophotography dithering routines that can be adjusted to your needs are already available in the majority of existing astrophotography software.
Check this if you need more explanation of how the astrophotography dithering method works3.
Step-by-Step Dithering Guide for Astrophotographers
Follow these steps for perfect astrophotography dithering and the benefits that brings for noise reduction:
- Understand the idea and practice of dithering and how astrophotographers use this technique to reduce noise.
- Setup your equipment correctly to maximize the effectiveness of dithering.
- Experiment with your dithering settings to get the best results.
To dither you’ll need to have a connection between your telescope mount and the computer or laptop so that the astrophotography dithering software can move your mount between each exposure. I use APT, (astrophotography tool)4, for the software which controls the dithering.
Although you can dither without guiding, an autoguiding scope and camera will enable your system to track very accurately and maintain the small movement for each exposure so the noise can be reduced.
Dithering Guide: Important Steps
There are a few basic practices you should adhere to if you want to dither with the best outcomes. The use of a suitable dithering algorithm that introduces unpredictable movements between frames is first and foremost essential. Secondly, you should check out for excessive movement in your dithering routine since it could lead to distorted or blurry images. The number of frames required to average out any remaining noise after dithering is also significant.
So by how much and how often should you set your software to dither?
That is up to you but in my experience, you don’t need to dither by more than 10 pixels and I usually do a bit less than that. As to how often some people recommend dithering every fifth image but I have found it easier to dither after each image and I’ve seen this work fine. It really doesn’t add too much extra time to your imaging so I think it’s worth it.
Advanced Dithering Techniques
Astrophotography dithering is an important technique, and there are a number of more advanced methods you might use to achieve even better results. Use a dithering algorithm, for instance, that adjusts to the signal-to-noise ratio of the image, introducing more noticeable movements in places with more noise.
Which Software Should I Use to Dither?
Well, there are quite a few choices. NINA, PHD2 (auto-guiding software), APT (Astrophotography Tool), and Sharpcap are just a few of the more popular options. What do I use? I set up the dithering in APT, and it works in conjunction with PHD2 auto-guiding software. You need the two programmes to talk to each other because, if not, the dithering would be automatically corrected by the auto-guiding system and therefore cancelled out.
- Amazon Choice
- The same guiding telescope that I use
- For planetary and deep-sky use
- Accurate guiding setup for long exposures
Conclusion
Astrophotography dithering is a technique used to reduce noise, hot pixels, and other artifacts. Dithering helps to create smoother, higher-quality images of the night sky by creating controlled random motions between frames. This reduces the noise in your image.
Understanding and utilizing astrophotography dithering should enhance your photographs, whether you’re just beginning astrophotography or are a seasoned pro.
So, there you go! Get dithering next time you’re out imaging! Why wouldn’t you?
References
[1] No author (2024-08-25). Noise in Digital Photography: Understanding and Managing It. Retrieved from https://proedu.com/blogs/photography-fundamentals/noise-in-digital-photography-understanding-and-managing-it
[2] N.I.N.A. contributors (2024-08-25). Dithering – Nighttime Imaging ‘N’ Astronomy. Retrieved from https://nighttime-imaging.eu/docs/master/site/advanced/dithering/
[3] No author (2024-08-25). Should I Dither Astrophotography – ShutterHow. Retrieved from https://www.shutterhow.com/dither-astrophotography/
[4] No author (2024-08-25). APT – Astro Photography Tool. Retrieved from https://www.astrophotography.app/
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