Notes for reducing CIAO images
CIAO's raw frame is not in proportion to the exposure time, because
each of the non-destructive reads (NDRs; 16 at most) is summed to a frame.
When you reduce the CIAO images, please divide them by the number of NDRs (C_NDR is the fits keyword), then the images become proportional to total exposure time (fits keyword is EXPTIME).
If the frames are further divided by number of coadd (fits keyword is COADD), then they become proportional to each exposure time (EXP1TIME).