Linearising the Data

After BIAS subtraction, the signal from the detector array may be corrected for non-linearity by modifying it with a suitable polynomial of the form:

where x is the measured signal and is proportional to the light intensity falling on the detector. These coefficients may determined manually by examining FLAT fields taken at different exposure times as the software does not contain any automated way of doing this. Such coefficients are specified in ASCII text in a linearization file:

! +
! \$CGS4_MASKS/fpa61_linear.dat
! Linearization coefficients determined on 23-Jan-1991 by A CGS4 User
! Order given is A, B, C, D, E vertically.
! -
0.9980
0.0176
-0.0037
6.46E-5
0.0000

Note that this means that all pixels are calibrated with the same polynomial. The file type must be .dat and it is stored in the directory $cgs4_masks/. Note that, although every effort has been made to avoid arithmetic errors during linearization, floating point overflows can occur if the coefficients are set to ridiculous values. The higher order terms, in particular, should be << 1.00 to avoid such problems.

For the arrays used in CGS4, no measurable non-linearity has been found to within 98% of full well. For these reasons, this facility is normally turned off by setting the value to #.