What is Dithering?
Dithering is a process initiated by your imaging software between captures that sends a message to your guiding software to set a new position for the guide star and move the mount accordingly to recenter the guide star, effectively moving the the image a few pixels randomly between captures. This randomizes the noise between captures, allowing it to be cancelled out during stacking.
I’m not the right person to be giving a tutorial on dithering and image scales, or what aggressiveness you should be using, but read on and check out some examples of what dithering during your imaging sessions can do for you. There is a lot of contradictory information out there on what settings to use both on your imaging software and guiding software, but I figured out on my own what works best for me.
All I had remembered from when I tried astrophotography the first time in 2005 was about taking darks, and at that time it was poorly done for me by Meade’s imaging software. This time around, I learned about properly taking darks and adding flats and bias frames, which are an enormous help in my lit-up skies. It’s not that I didn’t want to dither. Initially I just didn’t know what it was. After an afternoon of reading about it, I wanted to try it out, but I couldn’t get it to work, even though it said it was. So for a couple months I kept imaging and taking my calibration frames with disappointing results. I had a Saturday afternoon to figure it out ahead of an imaging session, and it turned out there was an error in the port entry for dithering. My imaging software said it was talking to my guiding software, but I wasn’t seeing the action being taken in the guiding. I finally got that straightened out and I think I have an excellent example in the Rosette Nebula as to why everyone should be doing this now if they aren’t already. Without it, you’re just stacking noise on top of noise and wasting hours trying to pull good data from bad.
All of the images below were taken with similar sky conditions from the same location on the identical setup, well focused with a Bahtinov mask and processed as similarly as possible.
This first image is over four hours of data on the Rosette, with Dark, Flat and Bias calibration frames added, but not dithered. It is very noisy, blurry and poorly balanced because of the noise.
The second image is just one hour and twenty minutes of dithered data with Flats and Bias applied, but no darks. This is not even close to a complete image, but the data in this integration was much easier to pull out and work with.
The third image is a two hour and fifty-four minute combination of the first and second image to see what I could get away with before the noise took over. The answer was, not much.
Below is a zoomed section of the same part of each image. This illustrated the differences more than the small images above. You can see that the four-plus-hour image on the left without dithering is blurry due to the noise. The middle one hour twenty minute dithered image is crisp, and the combined image on the right falls between, but in my opinion is on the edge of tolerable.
I’m not saying that dithering bypasses the need for darks, but I understand in theory why it would. That’s one of the many great astrophotography debates, and I haven’t experimented enough with both yet to say one way or the other. That said, the results of dithering alone are undeniable in the cropped comparison.
Dithering does add about 30-40 seconds between images for me, but the added time is well worth the results. Check out your imaging and guiding software to see if, and how they can be integrated to dither. BackyardEOS/Nikon have dithering with PHD/2, Metaguide and direct ASCOM available in all license levels. If you’re not already dithering, it’s time to start.