Image trailing

From time to time we see images returned by the telescope which have trailing of the stars. With colour images we see the trailing in the red colour channel. The telescope images each colour channel separately in the order red (R), visible (V) and finally blue (B). The trailing in the R channel suggests that it is occurring at that start of imaging for the job.


We also commonly see that the trailing is faint and the star points have pixels values many times higher than that of the trails; from this we can deduce that the trailing is happening over a period of time shorter than the normal galaxy camera exposure time.

Colour image showing trailing in R channel

R channel of image showing trailing

At the current time the cause of the trailing is not at all clear. We currently wait for the mount to report that it has stopped slewing and then we wait for a further period of time for the mount to settle. Why we are still seeing trailing at this point is very baffling.

My current thoughts on a solution to this problem lie in changing the order in which we image jobs. In the current system we image each colour channel first, and then image the dark frame that is applied to each of the colour channels. This design was to help reduce the memory footprint whilst processing a job and also with the idea that a dark frame could be imaged whilst the mount is slewing to the location of the next job.

Comparison of old and new imaging order
(left: old, right: new)

The proposed change would see the order of exposures changed to dark, R, V, B. Exposing the dark channel first should give the mount enough time to settle and any trailing to finish before we actually start imaging the light frames. As shown in the diagram, I also intend to have the dark frame start imaging as soon as mount starts slewing to its target. This should help to reduce the time taken for each job and allow more requests to be completed each night. I am hoping that I can bring these changes on line early in the new year, and I will report on the development at it goes live.