Correcting the chopped frames

Correcting the chopped frame for the array non-linearity is more complicated and less accurate than correcting the half cycle frames.

• Determine the mean level of the half cycle frames in the first exposure. Call this m1
• Determine the mean level of the dark frame with the same detector set up. Call this m2
• Compute m1-m2 (an artificial background) and add it to all frames that were taken with the telescope at position A, i.e frame 1,4,5,8,9, etc.
• Apply the correction with Eq. to these frames.
• Multiply all frames that were taken with the telescope at position B, i.e. frames 2,3,6,7,10, etc. by -1 and add m1-m2
• Apply the correction with Eq. to these frames.
• Subtract these frames from each other to create a chopped frame

This procedure corrects the central image only. To correct the other images also, one needs to process the ON and OFF beams separately.