NICMOS Data Handbook

3.2.1

The science data that an observer receives are calibrated in the pipeline by the following STSDAS calibration routines: CalTempFromBias, calnica, calnicb and runcalsaa. These routines perform different operations:

1.	CalTempFromBias: This routine calculates the temperature of the detector from the measured bias levels. It runs only on the MULTIACCUM observations and runs on the *_raw.fits files.

2.	calnica: This routine removes the instrumental signature from the science data. It is the first calibration step, and is applied to all NICMOS datasets individually. Calnica operates on the raw science data files.

calnicb: This routine operates on associations: it co-adds datasets obtained from multiple iterations of the same exposure, mosaics images obtained from dither patterns, and background-subtracts images obtained from chop patterns. Calnicb is applied to the calibrated science data files (output from calnica), and requires association tables (*_asn.fits) and the telemetry and engineering data files (*_spt.fits)

4.	Runcalsaa: This routine measures and removes an estimate of the SAA persistence signal and Bright Earth Persistence signal in a NICMOS image

These routines determine which calibration steps are to be performed by looking at the values of the calibration switch keywords in the primary header of the input science data files (see Table 2.4). The tasks select the reference files to use in the calibration of the data by retrieving the reference file names from the reference file keywords, also located in the primary header of the input data files. The appropriate values of the calibration switches and reference file keywords depend on the instrumental configuration used, the date when the observations were taken, and any special pre-specified constraints. They are set in the headers of the raw data file in the pipeline during the generic conversion process. The calibration indicators keywords record which steps have been performed on the data, and get updated after processing. In particular, the indicators for completed steps will have been assigned the value “performed”, while the indicators for the steps that were not performed will have been set to “omitted” or “skipped”. If the calibration switch keyword is set to “omit,” then the calibration indicator keyword will be “omitted” after running the calibration routine. If the calibration switch keyword is set to “perform,” and the reference file is a dummy, then the calibration indicator keyword will be “skipped.” The calibration indicators keywords should be examined in the primary header of the calibrated science data (*_cal.fits) to determine what calibration steps were applied to the data.

The CalTempFromBias, calnica, and calnicb tasks are available in STSDAS in the hst_calib.nicmos package. The runcalsaa routine in the pipeline comprises of two PyRAF tasks, saaclean and nic_rem_persist which are also available in the hst_calib.nicmos package. By using these tasks, observers can recalibrate data using the same software as the routine calibration pipeline at STScI. As an alternative, users who want to combine dithered or mosaiced NICMOS images can use the MultiDrizzle software:

(http://stsdas.stsci.edu/multidrizzle/)

3.2.2

Software for Grism Data Reduction

The NICMOS grism mode permits multi-object, slitless spectroscopy at low resolution. The calnica pipeline will process such grism images but will not apply the flat-fielding step. NICMOS flat fields strongly depend on wavelength, flat-fielding of grism data therefore needs to take the wavelength of extracted spectra into account. Software to extract spectra from calnica processed images is available from the Space Telescope European Coordinating Facility (ST-ECF).

Two software packages are available:

NICMOSlook. This is an interactive GUI driven IDL program to extract spectra from pairs of NICMOS grism images and corresponding undispersed images. It has a range of options to address specific properties of the NICMOS detector. It is the recommended program to extract individual spectra from NICMOS grism data. Program and documentation are available for reference at:

http://www.stecf.org/instruments/NICMOSgrism/nicmoslook/nicmoslook

aXe. This is a non-interactive general purpose package to extract spectra from slitless spectroscopy data. It can either be run stand-alone or within PyRAF. A version of aXe which includes correction for NICMOS specific effects and NICMOS configuration files is available. aXe is the program of choice to extract a large number of spectra from several images simultaneously. It is available at:

http://www.stecf.org/instruments/ACSgrism/axe/

Both packages require special NICMOS grism calibration data which are distributed with software. The packages are supported by the ST-ECF and questions should be directed to stdesk@eso.org

Because NICMOS grism data processing and extraction are not really "pipeline" procedures, this handbook will defer an overview of grism reduction methodology and the software tools calnicc and NICMOSlook until Chapter 5, where the discussion of NICMOS Data Analysis is presented.


	You cannot run NICMOSlook unless you have an IDL licence.