Dark Subtraction

Dark current arises mainly from the thermal electrons produced in electronic circuits including thermal noise from devices surrounding the detector. The dark current increases with the integration time. Dark frames must be obtained with the same integration time, non-destructive read-out number, and co-add number as the object frame when the dome and primary mirror cover of the telescope are closed. A reference dark frame is made of combining several dark frames with median filtering in order to exclude cosmic ray events. Figure 28 show a reference dark frame. Dark current is subtracted from each object frame by using the reference dark frame.

  

  

Figure 28: A reference dark frame made of combining several dark frames with median filtering. The integration time is 300 s.