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Tutorial:
Can dithering replace dark calibration?


Dithering

Page I, Frame

  1. What is dithering?
  2. My hardware and software

1: What is dithering?

When you talk about dithering in astronomy it's normally a small movement of the mount between each sub photo you take. What happens then is that when align and stacking the images the images will be shifted a bit to align the stars. But then also any static pattern from the sensor, hot pixels, column defects and alike will be placed in different position relative the stars on each sub image.

If the the sub images are just aligned and added the hot pixel will move around by random and look like this. The blurring look comes from it's a color camera and I used deBayer technique and above that zoom factor of x20 that take the sub image from 40x40 pixel to 800x800:

Hot red pixel

This image show a defect hot red pixel. It's only one pixel but it has been moving around by the dithering process. Hot pixels are normally cancelled out by dark calibration.

But dark calibration is very time consuming, especially if you don't have a temperature regulated cold camera, and your cameras lifetime will be shortened because of all the extra dark sub images you have to take. To my old Canon 5D camera I have 40 GigaBytes of sub dark images. And every time we add or subtract calibration images it adds random noise.

Can we use dithering to skip the dark calibration?

Yes, if the camera has very low static pattern (this is not noise, noise are random not static), I have read modern Sony cameras are good. I did this test on my own camera, a Canon EOS 6D.

If we use a function called Sigma clipping/reject instead of just add the sub images in combination with the dithering function.

It works like this:
After aligned the sub images the Sigma clipping (or Median) function looks on the same pixel coordinates of each sub image. Let say 29 of 30 sub images has the level from 800 to 880 and the the 30th has 10000. The last is a hot pixel. The sigma clipping function will then exclude say everything that's outside mean value 840 +/- 30%, the pixel with value 10000 is outside and will be excluded and maybe replaced with the mean value of the others.

The result could look like this:

Hot pixel canceled

The hot pixel has almost disappear, it could be better than this if optimizing the parameters for dithering and Sigma clipping.

My camera is a color camera and then I have to do a deBayering process before I align the sub images, that will smear out the defect pixels and it cause the Sigma reject function to be less effective. A mono chrome camera had been a better choice.

It's important that the dithering move the mount large enough to separate the hot pixels enough. But at the same time, big movements makes the auto guiding more complicated, it take more time after a dithering until the mount stabilize again.

Update:
Now I use an another technic without debayering, I demosaic the image first to its color planes, red, green1, green2 and blue. After that align and median/sigma stack. Much better and I believe I don't have to do that strong dithering in the future. With smaller dithering the auto guiding don't need that long time to stabilize. Normally it's recommended to do 5 to 10 pixels dithering on a monochrome camera. If you have a color camera as I have I have found 10 to 20 pixels is enough. Maybe I can reduce that now with my own process.

You can read here how I use it, I have test it on a couple of images (Canon EOS 6D) and it looks to work perfect:
../tutorial-astroimagej/ tutorial-aij-01-introduction.html

Dithering

  • Pros:

  • Reduce hot pixel and static pattern disturbance
  • Save camera lifetime
  • Save hard disk space
  • Save a lot of time
  • Eliminate aircraft and satellite tracks
  • No additional random noise from dark/bias calibration images
  • Make me happy :-)
  • Cons:

  • Auto guiding after a dithering take longer time to stabilize but also give the camera time to cool down (DSLR)
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2: My equipment and software to do dithering

My equipment:

  • Canon EOS 6D as main camera
  • Mount EQ6 Synscan goto controlled by EQMOD
  • QHY5 as guide camera, connected by a off-axis adapter.
  • An astroserver (Windows PC) that is controlled by remote.

Here is a list of what software I use and where you can download them, all of them are free.

This is just what I use, there are a lot of other hardware and software that can do this.

You have to be familiar with these software or what other software you use otherwise it will be difficult to follow.

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