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Subsections
H. Bushouse
Space Telescope Science Institute, 3700 San Martin Drive,
Baltimore, MD 21218, Email: bushouse@stsci.edu
Abstract:
The Space Telescope Science Institute (STScI) has various software tools
available for calibrating and analyzing Near Infrared Camera and
Multi-Object Spectrometer (NICMOS) datasets. These include the tasks
used in the STScI pipeline, tools to generate the reference files used
by the calibration pipeline tasks, and general image analysis, manipulation,
and arithmetic tasks.
NICMOS, calibration, pipeline, software
All STScI NICMOS tasks can be run from within the IRAF/STSDAS environment and
are written either in the native IRAF SPP language, the ANSI C language, or
as IRAF cl scripts. The two main tasks used to calibrate and reduce NICMOS
observations are calnica, which applies instrumental calibrations, and
calnicb, which processes associated sets of observations. There are also
several tools available for use in generating the calibration reference files
needed by calnica, such as bad pixel masks, detector read noise images,
detector non-linearity corrections, and dark current and flat field
images. Any existing IRAF and STSDAS tools can be used for the
analysis of NICMOS datasets, but there are several that have been
written specifically for the five data array grouping (known as
image sets) that is used for storing NICMOS images. These tasks
include tools for displaying images with overlayed data quality masks,
image set arithmetic and statistics, and image set combining. There
are also prototype tools for measuring and removing residual DC
offsets from calibrated NICMOS images. Brief descriptions of these tasks
are given in the following sections.
The two calibration pipeline tasks are calnica and calnicb.
Calnica is used to apply instrumental calibrations to individual
datasets, while calnicb is used to process associated sets of NICMOS
images. The calibrations applied by calnica include the removal of
standard instrumental signatures, such as dark current subtraction, detector
non-linearity correction, and flat fielding, as well as NICMOS-specific
processing, such as subtraction of MultiAccum-mode zeroth read images, and
the combining of MultiAccum readouts into a single image.
Calnicb is applied to associated sets of NICMOS images, which must have
already been processed with calnica. Major steps performed by
calnicb include the combination of multiple images obtained at
individual sky positions, calculation and subtraction of the HST thermal
background signal, and mosaicing of images obtained in a dither pattern.
See Bushouse et al. (this volume) for more details about the calibration
pipeline tasks.
There are several tools available within the STSDAS package
hst_calib.nicmos that can be used to generate some of the
reference files used by the calnica pipeline task. They are:
-
msbadpix - Detects bad pixels in NICMOS images and produces a bad pixel mask
(MASKFILE reference file)
-
msreadnoise - Measures readout noise in NICMOS images and produces a readnoise
image (NOISFILE reference file)
-
msstreakflat - Processes Earth ``streak flat'' images and produces a destreaked
flat field image (FLATFILE reference file)
-
ndark - Builds a NICMOS DARKFILE reference file from individual dark current
images
-
nlincorr - Computes a polynomial fit to the non-linear NICMOS response and
produces a NLINFILE reference file
The STSDAS packages hst_calib.nicmos and toolbox.imgtools.mstools
contain the tasks listed below, which are useful for image analysis. The
tasks with names beginning with the letters ``ms'' are set up to handle the
five data array grouping of NICMOS images, known as ``image sets''.
-
markdq - Mark data quality (DQ) flags on a displayed image
-
ndisplay - Display an image and superimpose DQ flags
-
pstack - Plot a stack of pixel values from a MultiAccum image
-
msarith - Image arithmetic
-
mscombine - Combine images with rejection of outliers
-
msstatistics - Compute image statistics
In addition to the software tasks listed above, which are all available in
STSDAS v2.0.1, there are several tasks which, at the time of this writing,
are still under development. The most notable of these are various tasks that
measure and remove the quadrant-dependent DC offsets, often known as
``pedestal'', which appear in some NICMOS images. Two of these experimental
tasks are nped and pedsky, both of which are currently implemented
as IRAF cl scripts. They estimate the pedestal level in each image quadrant
by attempting to minimize the flat-field residuals that get imposed on the
image when the flat-field is applied to the constant pedestal level. Once
the algorithms are completely developed, they will be rewritten and released
as public STSDAS tasks.
Next: NICMOS Science
Up: NICMOS Data Calibration and
Previous: NICMOS Software at the
Norbert Pirzkal
1998-07-09