A bias frame is a zero-time exposure with no light. The exposure with the shortest
possible exposure time provided by a device. One maps a pre-amplifier bias (offset)
of readout electronics. This routine creates an average bias frame,
sometimes named as a masterbias.
Because bias exposure times are really short (1/100s or less), it is
recommended to acquire and average a lot of exposures (>10).
An output bias Bij is computed as the average of every pixel of
input images (Ik)ij:
Bij = 〈 Ik 〉ij,
where i,j is an index of a pixel and k is an index of an image.
Both chip temperatures or exposure times are checked to be nearly the same
during processing when FITS_KEY_TEMPERATURE and FITS_KEY_EXPTIME
environment variables are set to values of
corresponding quantities.
Input and output
On input, a list of bias frames is expected.
On output, a single file representing of the mean bias frame is created.
Masked values (if parameter -bitmask is presented) are
set to the frame mean.
Parameters
-bitmask file
mask frame (see
phcorr for description).
Only pixels, marked by this mask are processed.
-st, --saturate s
Set saturate limit in
units of input frames.
Any pixel out of the range th < pixel < s
(see also --threshold) is rejected the processing.
This switch is useful mainly for over-exposed parts of frames
and elimination of non-linear parts of gradation curve.
If unset, the value is determined from FITS header
(by FITS_KEY_SATURATE, see).
If the keyword is not found and the frame contains
an integer number type, the maximum value 2**BITPIX-1 is
provided, otherwise the maximum value of given data-type
is used (which practically switch-off the saturation bound check).
The value is set in ADU (values reported by camera).
-th, --threshold th
Set threshold limit
in units of input frames.
Any pixel below the value is rejected from the processing.
If unset, the value is number one.
Threshold is minor significance parameter.
It should help for faulty values or filtering
of bad pixels.
The value is set in ADU (values reported by camera).
-B bitpix
set numerical type of output images,
see Common options (default -32)
