Step 2 - Making a bias calibration frame

If you intend to debias your CCD data using bias frames, then the next move is to combine all these into a `best bet' low noise frame; a `master bias'. There are probably only two ways in which you'd like to do this:

• Just combine them.
• Zero the mean level first and then combine them (leaving the mean at zero).

The second option may seem strange (if you're not used to it), but it has a good rationale behind it and is the default method. Using this method requires that your data have bias strips. These are used as a monitor of the bias level at readout time and the master bias is offset to them so that any small variations in the zero point are tracked.

You make a master bias by running the program:

• MAKEBIAS

If you want to make a master bias using the first method, then use a command like:

% makebias in='bias/*' out=master_bias rnoise=10 zero=false

for the second method use:

% makebias in='bias/*' out=master_bias rnoise=10

The IN specification bias/* means get all the frames in the subdirectory bias/. The RNOISE parameter specifies the readout-noise (in ADUs) of the CCD you're using (if you've set up a global value for this using CCDSETUP then this need not be supplied). MAKEBIAS shows an estimate of the readout-noise which it derives from the data, use this to check your value, or use this value if none other exists. The nominal readout-noise value can usually be found in the technical descriptions issued by the observatories. CCDPACK uses the readout noise value to generate error estimates, you may specify GENVAR=FALSE to disable this option if your destination analysis package does not make use of data errors, your data format doesn't support the storage of this information, or if disk space is tight (the addition of error components to your data will double the disk space needed).