lrisbias -- Subtract overscan from LRIS images
- List of input CCD images to process. The list may
include processed images (in which case no action is taken).
- interactive = no
- Fit the overscan vector interactively? If yes and the
overscan function type is one of the icfit types then
the average overscan vector is fit interactively using the
icfit package. If no then the fitting parameters given
below are used.
- function = "legendre"
- Line-by-line determination of the overscan is specified by:
The smoothed overscan vector may be fit by one of the functions:
- average - the average of the biassec columns at each line
- median - the median of the biassec columns at each line
- minmax - the average at each line with the min and max excluded
- legendre - legendre polynomial
- chebyshev - chebyshev polynomial
- spline1 - linear spline
- spline3 - cubic spline
- order = 1
- Number of polynomial terms or spline pieces in the
- sample = "*"
- Sample points to use in the overscan fit. The string "*"
specified all points otherwise an icfit range string is
- naverage = 1
- Number of points to average or median to form fitting
points. Positive numbers specify averages and negative
numbers specify medians.
- niterate = 1
- Number of rejection interations to remove deviant points
from the overscan fit. If 0 then no points are rejected.
- low_reject = 3., high_reject = 3.
- Low and high sigma rejection factors for rejecting
deviant points from the overscan fit.
- grow = 0.
- One dimensional growing radius for rejection of neighbors
to deviant points.
- CCD readout noise in electrons per pixel. If specified,
the value is written to the image header as the keyword
- CCD inverse gain in electrons per ADU. If specified, the
value is stored in the image header as the keyword gain.
- dispaxis = -1
- Image axis along which the spectra dispersion run. The
dispersion axis is 1 when the dispersion is along lines so
that spectra are horizontal when displayed normally. The
dispersion axis is 2 when the dispersion is along columns so
that spectra are vertical when displayed normally. If set to
a positive value, that value is stored in the image header as
the keyword DISPAXIS. The default value of -1 causes no
action to be taken.
- observat = "keck"
- Name of the observatory at which the data were taken. If
defined, this value is stored in the image header in the
- noproc = no
- List processing steps only? (i.e., take no action)
The lrisbias task automates the removal of the overscan
signal from LRIS images. The program will remove bias from
images taken in either one- or two-amp readout modes.
lrisbias begins by reading the image header to obtain
values for the keywrods prepix, postpix, numamps, and window.
These are used to compute the image sections containing the
image data and overscan signal for the left and/or right
The CCDSEC and DATASEC keywords are removed from the image
header if present, and the newly-defined data section for the
first amplifier is stored in the DATASEC keyword. The ccdproc
task is invoked using the first overscan regions to subtract
overscan from the data. If two-amp mode was used, new data
and overscan sections are defined and ccdproc is invoked a
second time to remove the remaining overscan signal.
The LRISBIAS keyword is written to the output image header to
record the time of overscan removal. The keywords CCDSEC,
CCDSUM, OVERSCAN, DATASEC, TRIMSEC, and BIASSEC are also saved.
If legal values were provided for the parameters OBSERVAT,
DISPAXIS, RDNOISE, and GAIN, then corresponding header keywords
are saved, too.
Gregory D. Wirth, W. M. Keck Observatory
- lrisbias v0.0 1999-Feb-24
- Original release.
- lrisbias v1.0 2000-Jun-06
- Fixed Attempt to access undefined local variable `upp1'
bug with numamps=1. Revised treatment of 2-amp mode to better
deal with the case of all data on RIGHT amp. Also fixed
treatment of binned images.