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15.3 Working with NICMOS Files

The quickest way to learn how each observation was performed is to use the iminfo task in the STSDAS toolbox.headers package to look at the headers of the science data. The output from iminfo summarizes on one screen the relevant information about an observation (Table 15.7) and the instrument configuration during the observations (Table 15.8), by reading and reporting the value of the appropriate keywords.


Observation Information in iminfo Listing

Field Descriptor

Header Keyword Source

Rootname

ROOTNAME

Instrument

INSTRUME

Target Name

TARGNAME

Program

ROOTNAME (positions 2-4)

Observation set

ROOTNAME (positions 5-6)

Observation

ROOTNAME (positions 7-8)

File Type

FILETYPE

Obs Date

DATE-OBS or FPKTTIME

Proposal ID

PROPOSID

Exposure ID

PEP_EXPO

Right Ascension

CRVAL1

Declination

CRVAL2

Equinox

EQUINOX



NICMOS-Specific Information in iminfo Listing

Field Descriptor

Header Keyword Source

Image type

IMAGETYP

Number of extensions

NEXTEND

Camera number

CAMERA

Aperture

APERTURE

Filter name

FILTER

Observation Mode

OBSMODE

Number of initial/final reads

NREAD (ACCUM)

Number of intermediate samples

NSAMP (MULTIACCUM or RAMP)

MULTIACCUM sequence

SAMP_SEQ

Exposure time (sec)

EXPTIME

Readout speed

READOUT

Association ID

ASN_ID

Number of Iterations

NUMITER

Calibration steps done

Switches whose values are set to "PERFORMED". Switches are: ZOFFDONE, MASKDONE, BIASDONE, NOISDONE, DARKDONE, NLINDONE, FLATDONE, UNITDONE, PHOTDONE, CRIDDONE, BACKDONE, WARNDONE

The entire suite of keywords from any header can be listed with the IRAF task imheader. Given that NICMOS data files contain multiple extensions, the number of the desired extension must always be specified. For example, to list the primary header content of a calibrated image, you type

cl> imheader n0g70106t_cal.fits[0] long+ | page

where [0] identifies the primary header. To list the header of the second science image in a MULTIACCUM sequence (the sixth extension):

cl> imheader n0g70106t_cal.fits[6] long+ | page

An example of a header listing is shown in Figure 15.3 below.

Figure 15.3: Long imheader

Chapter 2 describes in detail how to work with FITS file extensions. Here we will recap the essentials. In order to simplify access to NICMOS FITS image extensions, each extension header contains the two keywords: EXTNAME (extension name) and EXTVER (extension version number). The EXTNAME keyword identifies the nature of the extension (SCI, ERR, DQ, SAMP, TIME, see Table 15.7). The EXTVER keyword contains an integer value which is used to uniquely identify a particular imset (quintuple of image extensions). For example, the five image extensions (single imset) contained in the science data file for an ACCUM, RAMP, or BRIGHT OBJECT observation will all usually be assigned an EXTVER value of 1 because there will only be one group of extensions in the file. In a MULTIACCUM science data file, each group of extensions associated with a given readout will have a unique EXTVER value, running from 1 up to the total number of readouts in that particular file.

To list the header of the second science image in a MULTIACCUM sequence, in addition to the command line above, one can give:

cl> imheader n0g70106t_cal.fits[sci,2] long+ | page

In general, to access a particular image extension, append the name and version number of the desired extension in square brackets to the end of the file name. The EXTNAME value is specified first, then the EXTVER value, separated by a comma. Indeed, the use of the keywords EXTNAME and EXTVER is not limited to the task imheader, but can be used in all IRAF tasks.


The primary header data unit in a NICMOS FITS file does not contain the EXTNAME or EXTVER keywords. The absolute extension number 0 (zero) refers to the primary header.

If a calibration keyword needs to be changed, the IRAF/STSDAS chcalpar task can be used. For instance, to modify the flatfield calibration from PERFORM to OMIT in a given data file, the following command can be given:

cl> chcalpar n0g70106t_raw.fits

The pset list appropriate for the image will appear, and the calibration keyword can be modified. The operation performed with chcalpar can be equivalently performed (although in a more cumbersome way) with the general IRAF task hedit; in this case, the extension [0] of the primary header must be explicitly specified:

cl> hedit n0g70106t_raw.fits[0] flatcorr OMIT 


Do not try to edit a keyword in an extension header unless you are certain that the keyword does not reside in the primary header. (See "Header Keywords and Inheritance" on page 2-6).

Image sections can be specified in the case of NICMOS data with the same syntax as all IRAF images. For example, to specify a pixel range from 101 to 200 in the x direction and all pixels (denoted by an asterisk) in the y direction from the second error image in a file, the complete file name specification would be n0g70106t_cal.fits[err,2][101:200,*].


If you use both extension and image section syntax together, the extension name or number must come first enclosed in one set of brackets, and the image section specification in a second set of brackets.



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