Name

lrisbias -- Subtract overscan from LRIS images

Usage

lrisbias images

Parameters

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:

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

The smoothed overscan vector may be fit by one of the functions:

legendre - legendre polynomial
chebyshev - chebyshev polynomial
spline1 - linear spline
spline3 - cubic spline

order = 1

Number of polynomial terms or spline pieces in the overscan fit.

sample = "*"

Sample points to use in the overscan fit. The string "*" specified all points otherwise an icfit range string is used.

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.

Detector Parameters

rdnoise: CCD readout noise in electrons per pixel. If specified, the value is written to the image header as the keyword rdnoise.
gain: 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 OBSERVAT keyword.
noproc = no: List processing steps only? (i.e., take no action)

Description

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 amplifiers.

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.

Side Effects

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.

Author

Gregory D. Wirth, W. M. Keck Observatory

Revisions

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.