LBC images shows a readout bias which shows gradients both along the x and y axis, as usual for CCD images, and also a characteristic zero level bias. When an Observing Block (OB) sequence of Bias is started this zero level changes at each exposure of the sequence and reaches a stable value after 20-25 exposures. From one Bias sequence to another the zero level can change, and it is possible that a jump occurs even within a Bias sequence. The gradients in the x and y directions are, however, much more stable. For this reason, the zero level bias must be removed image per image, using the corresponding overscan.
Correcting the Bias
The bias subtraction of flat and science frames is carried out usually in two steps. First of all, one has to combine all the bias frames in a masterbias image, which takes into account the gradient in x direction of the CCD and has the y-gradient and the zero level subtracted. Then, from each flat and science image the y-gradient and zero level are removed using the same procedure, and, after that, one can remove the x-gradient using the masterbias.
Alternatively, LBC users can correct their frames with IRAF.
In this case, the task is:
cl> ccdproc in out bpmasks overscan+ zerocor+ zero=masterbias
where bpmasks is the Bad Pixel Mask (BPM) image to be used, zero is the masterbias frame, overscan+ says to apply overscan or prescan bias correction and zerocor+ confirm to apply zero level correction.
Download MASTERBIAS frames
Here it is possible to download a masterbias referring to the observing night 2006-10-28.