ECHOMOP does not provide any facilities for the subtraction of the bias current from CCD frames. It does however, require that such subtraction be performed before processing takes place. The bias subtraction may often be done automatically at the observatory by the data recording software, but it is up to the user to check if this is the case. KAPPA can be used to subtract the bias if necessary. For example;
CSUB /frame/ /nnn/ /frame/
will subtract bias /nnn/ from data frame /frame/.
For cases where the bias subtraction represents a significant source of error it may be desirable to determine the error due to bias subtraction on a pixel-by-pixel basis using many bias frames.
This procedure must be performed by the user, and the resulting error estimates copied into the object data frame error array. ECHOMOP will incorporate these errors when calculating the variances on the extracted spectra.
Martin Clayton