Without a bias frame

Debiassing of CCD data can be performed reasonably well by the subtraction of values derived from the bias strips. If the data has two bias strips then an interpolation using a straight line or constant for each line is used. Alternatively a plane can be fitted to the whole of the bias strip data. If only one bias strip is present then extrapolation across the data is used. In this case a single value is derived for each line or one global value for the whole frame.

To subtract the bias using interpolation type something like:

%  debias in='"rdata/*,ffr/*"' out='*_tmp' bounds='[2,10,400,416]' rnoise=10
          adc=2

This will interpolate between each pair of lines in the bias strips using a constant. Before the interpolation occurs the bias strips are smoothed using a box filter (this aims to reduce the variation along the strips rather than across them, thus reducing the inter-line noise).

If you do not have any bias frames or bias strips then it is still possible to debias the data provided you know what the debias level actually is (this is also useful when the debiassing has already been done for you and you want to add estimates of the errors, such as in IR data). To debias using a constant try something like:

% debias in='"rdata/*,ffr/*"' out='*_tmp' usecon zero=100.0

This subtracts 100.0 from all the data before making error estimates applying data masks etc.

IR data that are bias subtracted using combination bias and sky frames should only be `debiassed' using this method if you actually know what the bias level is (this is usually an observing option) otherwise any errors generated will not be correct (it is not good enough to assume that the bias level is zero and then remove the bias plus sky frames later). If you do this remember to debias all frames so that the zero point remains the same.