When light frames are registered and integrated, and the tracking wasn't spot on, these hot pixels end up as streaks or "rain" in the final image.
|Extreme crop of an unprocessed (but stretched) integrated image|
There are various processing tools that can be used on the raw image frames to remove hot pixels. These tools rely on filters that remove intensity values from either the individual frames, or from the stack of images that are to be integrated. By careful use of these tools, most of the noise can be reduced. The noise that remains in the final master image, can be further reduced during post processing.
|Crop of the same area after processing|
If the camera is moved a few pixels in a random way, no streaks will form and the noise will not be visible. This technique is called dithering and was suggested by astrophotographer Tony Hallas.
Cameras are usually controlled by software running on a laptop, and many of these have the option to apply dithering.
However, most software is written for either Canon or Nikon cameras, and may not work with other brands. I use a Pentax K20D for all my astro work, and this camera has issues when trying to connect to a computer, so software controlled dithering is not possible for me.
The only way for me to use dithering was to sit next to the camera and manually move the camera in RA or DEC between exposures. Doing this during winter, trying to get 50+ exposures, was not my idea of a fun time.
The solution to this problem was to build my own hardware device for camera control; a ditherbox.
The device intercepts the trigger signal from the intervalometer and sends it on to the camera. In between exposures, it also tells the mount to move in either RA or DEC. It does nothing else. I have to figure out at what speed to move the mount (and program this into the handcontroller), and for how long. The box only sends a "move" signal for the whole time between exposures. This time is set in the intervalometer and is determined by the number of pixels to be moved, the pixel size, and the focal length of the lens or scope.
The heart of the ditherbox is a small microcontroller from Atmel, the ATtiny 84. All inputs and outputs are isolated through optocouplers, and it receives its power from the SynScan handcontroller. So there are no extra batteries or power cables involved.
More information on the device can be found on Stargazers Lounge. The software can be found on my Github site.
And here is an example of the benefits of dithering.
(NB: this image, while showing the same area of the sky, was taken with another focal length and under different conditions.)
|Same area in the sky, imaged using dithering;|
unprocessed (stretched) integrated image