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STScI NICMOS Software

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

Introduction

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.

Calibration Pipeline Tasks

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.

Reference File Tools

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

Image Analysis and Reduction

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

Under Development

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.