Scheduling a Twilight Flat

Creation of a twilight flat requires a clear, moonless twilight period. Here I describe the scheduling parameters necessary to create a twilight flat. For more details on using the scheduler, see Chapter 4.

1. Check that a twilight flat trigger is defined in astrod.conf, as described in Section 4.4.1.

   trigger    twi_flat         twi      0          1jm

2. In the schedule list, add the following lines:

   sched twi_flat "-t 5 -u -10 -a 90.0 -l 70.0 -e 2000.0 -i 5"

   sched twi_flat "-t 5 -u -10 -a 88.0 -l 70.0 -e 2000.0 -i 5"

   sched twi_flat "-t 5 -u -10 -a 86.0 -l 70.0 -e 2000.0 -i 5"

   sched twi_flat "-t 5 -u -10 -a 92.0 -l 70.0 -e 2000.0 -i 5"

   sched twi_flat "-t 5 -u -10 -a 94.0 -l 70.0 -e 2000.0 -i 5"

   sched twi_flat "-t 5 -u -10 -a 90.0 -l 72.0 -e 2000.0 -i 5"

   sched twi_flat "-t 5 -u -10 -a 90.0 -l 68.0 -e 2000.0 -i 5"

   sched twi_flat "-t 5 -u -10 -a 92.0 -l 68.0 -e 2000.0 -i 5"

   sched twi_flat "-t 5 -u -10 -a 94.0 -l 72.0 -e 2000.0 -i 5"

   sched twi_flat "-t 5 -u -10 -a 88.0 -l 72.0 -e 2000.0 -i 5"

   The first several images will probably be saturated. This is not a problem, as there are plenty to go around. We will need at least 30 to create a decent twilight flat. Each field is imaged once with a medium length (20 s) exposure, and the telescope returns to the same azimuth and elevation after at least five minutes. This allows the stars to drift so we do not image the same stars in the same pixel from image to image. I'm not sure if the azimuth should be changed from ${\sim 90^\circ}$ to ${\sim 270^\circ}$ for the southern hemisphere.

3. On a clear, moonless twilight, run the system.

   It is important that these images are taken on schedule. If they are taken late, the night sky lines will dominate and you will end up with a sky flat (see Figure 7.2) instead of a twilight flat.