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16.6 Recalibration

This section is intended to help you decide whether your data were calibrated with optimal calibration reference files and to help you decide whether you need to recalibrate your data.

16.6.1 Why Recalibrate?

Many users will find that the calibrated data produced by the OPUS pipeline-the standard calibration-are adequate for most scientific applications. The calnica pipeline calibrated your data with the most up-to-date, instrument configuration-specific reference files available at the time of the observation. However, updated or timely reference files sometimes do become available after your data were processed. Improved software for calibration (both calnica and calnicb) may also become available as our understanding of the instrument performance increases with on-orbit experience. This is especially true for data obtained during the first few months of the NICMOS lifetime in orbit. Particularly if you notice unusual features in your data, or if your analysis requires a high level of accuracy, you may wish to determine whether a better set of calibration reference files exist than were used to process your data. If better files are available or the calibration software changes significantly, you may choose to recalibrate your data using the new files or software.

Finding that a calibration reference file has changed since your data were calibrated doesn't always mean that you have to recalibrate. The decision depends very much on which calibration image or table has changed, and whether that kind of change to your data is likely to affect your analysis in a significant way. Before deciding to recalibrate, you might want to retrieve the new recommended reference file or table and compare it to the one used to calibrate your data at STScI in order to determine if the differences are important. You can use the table tools in the STSDAS ttools package to manipulate and examine calibration tables; images can be manipulated in the same way as your science data.

16.6.2 Recalibrating the Data

This section describes the mechanics involved in actually recalibrating a dataset. The basic steps in recalibrating a dataset are:

Assemble any necessary reference files or tables and your raw data files.
Set the desired calibration switches and reference file name keywords in the primary header of your raw (*_raw.fits) data file. These determine which steps will be executed by the calibration software and which reference files will be used to calibrate the data.
Run the calibration software.

Assembling the Input Files

In order to recalibrate your data, you need to retrieve all of the reference files and tables that are used by the calibration steps you want to perform. The source of these files is the Calibration Database (CDBS) at STScI. A complete description of how to retrieve the reference files is given in Chapter 1.

Setting the Calibration Parameters

The calibration software is completely data-driven, meaning that the calibration steps to be carried out are determined by the values of the calibration switches and the calibration reference files keywords contained in the primary header of the file to be processed. An important step is then to set the calibration switches and reference file keywords in the primary header of your raw data file (*_raw.fits) to reflect how you want the data recalibrated and which reference files you want to use at each step in the process. This is done most easily with the chcalpar task in the hst_calib.ctools package of STSDAS or with the hedit task in the IRAF images package.

The calibration switch keywords reside only in the primary header of NICMOS FITS files: it is critically important to specify extension number zero explicitly when passing file names to tasks like chcalpar or hedit to modify these keywords. For example, to modify calibration keywords in the file n3xe01bhm_raw.fits, be sure to use the complete name n3xe01bhm_raw.fits[0] as input to these tasks. If you do not specify extension zero explicitly, the keywords you modify will end up getting written into the first extension header instead, where calnica will not find them.

The chcalpar task takes a single input parameter-the name(s) of the raw data files to be edited. When you start chcalpar, the task automatically determines that the image data are from NICMOS and opens a NICMOS-specific parameter set (pset) that will load the current values of all the calibration-related keywords. To edit the calibration keyword values:

Start the chcalpar task, specifying the image(s) in which you want to change keyword values. If you specify more than one image, for example using wildcards, the task will read the initial keyword values from the first image in the list. For example, you could change keywords for all NICMOS raw science images in the current directory (with initial values from the first image), using the command: ct> chcalpar n*raw.fits
An example of using chcalpar on a single NICMOS file is shown in Figure 16.6.
After starting chcalpar, you will be placed in eparam-the IRAF parameter editor, and will be able to edit the set of calibration keywords. Change the values of any calibration switches, reference files or tables to the values you wish to use for recalibrating your data.
Exit the editor when you are done making changes by typing :q two times. The task will ask if you wish to accept the current settings. If you type "y", the settings will be saved and you will return to the IRAF cl prompt. If you type "n", you will be placed back in the parameter editor to redefine the settings. If you type "a", the task will abort and any changes will be discarded.

Figure 16.6: Editing Calibration Keywords with chcalpar

Running the Calibration Software

After you change the header keyword values for your raw data files, you are ready to recalibrate your data. To run calnica, type the name of the task followed by the names of the input raw data file and desired output calibrated data file. For example, to recalibrate the dataset n0g70106t, you could type:

ni> calnica n0g70106t_raw.fits n0g70106t_cal.fits

or:

ni> calnica n0g70106t " "

To run calnicb the name of the association table must be given as input:

ni> calnicb assoc_id_asn

To run calnicb on a subset of the *_cal.fits files, it is sufficient to edit the *_asn.fits table and remove the undesired files.

The calibration routines calnica and calnicb will not overwrite an existing output file. If the calibration tasks are run in the directory where the original calibrated files are located, a different output file name must be specified.

16.6.3 Calculating Absolute Sensitivity

The PHOTCORR step in calnica does not alter the values of the data (which are always in countrates in the calibrated file), but only writes the information necessary to convert countrates to fluxes into the header of the file, i.e., sets the values of PHOTFLAM, PHOTFNU, PHOTPLAM, PHOTBW, PHOTZPT, and PHOTMODE. Unless you wish to recalculate the absolute sensitivity for your observation (e.g., because a more recent throughput estimate exists for your observing mode), there is no need to recompute these values and you can set PHOTCORR=OMIT in the recalibration.

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