ASIAir Dithering

 

 V1.5.3

So for my first post here I'm going to start with dithering.  We see a lot of questions come up on the FB forum regarding how to set up dithering on the ASIAir.  Dithering is an important tool used to randomize fixed sensor noise and prepare sub frames for better statistical averaging and noise reduction during stacking and processing. Dithering is commonly used to eliminate what is called walking noise but it also helps improve signal to noise in general by ensuring that any fixed noise elements are not part of the signal being recorded. Walking noise is noise in the background that shows as lines or streaks due to poor polar alignment and from minor defects in imaging sensors. It often manifests itself in images that have not been exposed long enough or have low signal to noise due to excessive light pollution. Dedicated astro cameras have much less noise to start with and therefore suffer less from this effect than DSLRs. DSLRs require heavy amounts of dithering to suppress walking noise in final stacked images. Dithering, randomly moves the image by a set number of pixels so that stacking software can better reject the noise when using noise rejection options such as Sigma or Windsorized Sigma Clipping.

Dithering is accessed and configured under "Guide Settings" within the ASIAir app. There are four main settings that require adjustment based on your camera type, degree of mount backlash, and seeing conditions.

Pixels - This is the number of pixels that the image will be randomly shifted between exposures. The range is 1-10 pixels. Most dedicated astro cams can benefit from 1-3 pixels in movement. However DSLRs require the max between each frame to properly eliminate walking noise.

Stability - This setting is the required distance in seconds of arc that the target must be within after a dither before beginning the settlement period. In order to determine this setting you need to watch your guide graph for a while and determine what the current seeing conditions will allow. For example, if your guide graph is regularly swinging between +1" and -1" than your stability would be 2" which is the distance covered between 1 and -1 through the zero line. You should set this parameter just higher than your noted stability. In this example a stability of 3" should be used.

As another example, if the stability on your guide graph is varying between 0.5" and -0.5" then your stability is 1". Your stability setting should be set for 2". One setting higher than what you observed.

You can use a lower setting but in doing so you may not reach a point where guiding will resume as your stability will rarely be within the range selected long enough. It should never be set below your observed stability as shown on your guide graph or imaging may never resume.

Settle Time - This setting is the time that must pass once the guiding is within the stability distance configured. The guide must remain within the stability distance for the set period of time before the imaging sequence will continue. For example the guide graph is regularly varying between 1" and -1" on the guide graph and a stability of 3" is configured along with a settle time of 3s. Once dithering has moved the image by the desired number of pixels, guiding will move out of the range but slowly settle back into the range 1 to -1. Once the guide has moved within that range the settle time will start counting down. If the guide moves back out of the stability range, the countdown will reset and wait for guiding to move back into the range. Once guiding has stabilized between 1" and -1" and has remained there for 3 seconds, the imaging sequence will resume.

The desired settle time is mainly based on how much backlash exists in your mount and how quickly it can recover and stabilize. A well tuned mount can begin guiding after 1s has passed. A poorly tuned mount with a lot of backlash may take longer to clear. If it takes more than 15s to settle then dithering should be disabled until the backlash between the gears can be corrected.

Timeout - If the dither doesn't settle out for one reason or another, this sets the maximum time it will wait before starting the next exposure.  It is always good to set this to something reasonable like 60s.

Interval - This is the number of images that can be taken before dithering is executed. It is always good to dither after every frame but with some astro cams it may not be needed every frame. For DSLRs, the interval should always be set to 1 such that every frame is dithered.

RA Only - This option is really only useful when using a tracking platform that only guides in RA via ST4.  This allows those mounts to dither in one axis which can drastically improve the signal to noise ratio.

Dithering is an important tool to help noise reduction algorithms eliminate fixed noise in stacked images. There is generally no additional consideration needed during processing to benefit from this other than to make sure you are using Sigma Clipping or Windsorized Sigma Clipping during stacking. There is no cost to using it other than a few extra seconds between exposures. The benefits far out way the extra time needed and should be used if your mount allows it every time you image.