A bias current is routinely applied to CCD detectors to ensure that, as near as possible they are operating in a linear manner. This current has the effect that a non-zero count is recorded in all pixels. The observer should ensure that a number of `bias frames' are obtained to adequately estimate the effect. The average of the bias frames should be taken, and the result subtracted from all other frames before using them in a reduction. Failure to account for the bias may have a detrimental effect on the signal-to-noise-ratio (S/N) in the final spectrum.
ECHOMOP does not provide any special facilities for the subtraction of the bias current from CCD frames. However, it does 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 you to check if this is the case. KAPPA or FIGARO can be used to subtract the bias if necessary.
% csub /inframe/ /nnn/ /outframe/ (KAPPA) % icsub /inframe/ /nnn/ /outframe/ (FIGARO)
will subtract bias /nnn/ from data /inframe/ and put the resulting data in /outframe/.
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 frames. The resulting error estimates must then be copied into the object data frames error arrays. ECHOMOP will incorporate these errors when calculating the variances on the extracted spectra.