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20.2 STIS File Structures

All STIS data products are FITS files. Images and two-dimensional spectroscopic data are stored in FITS image extension files, which can be directly manipulated, without conversion, in the IRAF/STSDAS environment. These FITS image extension files allow an associated set of STIS science exposures, processed through calibration as a single unit, to be packaged into a single file. Accessing the images in the FITS image extension files in IRAF follows a simple convention explained in detail in Chapter 2. The catfits task can be used to display the complete contents of the primary and extension headers of the data file. Section "Types of STIS Files" on page 20-6 describes the contents of these files and how to access them.

Tabular STIS information, such as extracted one-dimensional spectra or the TIMETAG mode event series are stored as three-dimensional FITS binary tables. The tables can be accessed directly in the IRAF/STSDAS environment using tasks in the tables.ttools package as described in Chapters 2 and 3.

20.2.1 STIS FITS Image Extension Files

Figure 20.1 illustrates the structure of a STIS FITS image extension file, which contains:

Storage of STIS ACCUM Mode Science Data: Raw and Calibrated Two-Dimensional Spectroscopic and Imaging Data

All ACCUM mode uncalibrated science data from STIS and all calibrated STIS data, with the exception of the extracted one-dimensional spectra (see below) are stored in FITS image extension files with the particular format shown in Figure 20.1.

A triplet of FITS image extensions corresponds to each exposure in a STIS data file:

Two-Dimensional Extracted Spectra

The rootname_sx2.fits, rootname_x2d.fits files which hold the flux and wavelength-calibrated two-dimensional spectra for long-slit first-order observations are stored as FITS image, as are the raw data and the calibrated imaging data. The units of the data in the extracted two dimensional spectra are ergs sec-1 cm-2 A-1 arcsec-2 pixel-1. "Working with Two Dimensional Extracted Spectra" on page 23-3 describes how to work with these data to derive flux information and wavelengths.

Imaging Data

The final calibrated output product for CCD imaging data is the rootname_crj.fits file, and the final calibrated product for MAMA data is either the rootname_flt.fits or rootname_sfl.fits files. The units of the data in these files is in counts per pixel, just as in WFPC2 data, and in general these data can be manipulated just like WFPC2 data. See "Converting Counts to Flux or Magnitude" on page 3-15.

Storage of Acquisition and Acquisition/Peakup Images

Almost all STIS spectroscopic science exposures will have been preceded by an acquisition and possibly an acquisition/peakup exposure to place the target in the slit. Keywords in the header of spectroscopic data identify the dataset name of the acquisition (in the ACQNAME keyword) and acq/peak images (in the ACQPEAK1 and ACQPEAK2 keywords).

An acquisition exposure produces a raw data file (rootname_raw.fits) containing three science image extensions corresponding to the three stages of the acquisition procedure:

20.2.2 Tabular Storage of STIS Data

Time-Tag

Time-tag event data is contained in a binary table extension in which each row of the table corresponds to a single event in the data stream and the columns of the table contain scalar quantities that describe the event, as shown in Table 20.1.


Columns of a Time-Tag Data Table

Column Name

Units

Description

TIME

s

Elapsed time in seconds since the exposure start time

AXIS1

pixel

Doppler-corrected location of the event, AXIS1

AXIS2

pixel

Location of the event, AXIS2

DETAXIS1

pixel

Location of the event, raw AXIS1 (no Doppler correction applied)

The STIS pipeline collapses a time-tag event series into a single time-integrated image and processes it as if it were an ACCUM mode image. Outside of the pipeline the raw time-tag event stream can be manipulated to produce two-dimensional images which are integrated over user-specified times or manipulated directly (see "TIME-TAG Data" on page 23-5).

One-Dimensional Extracted Spectra

In the STIS pipeline, two-dimensional STIS echelle spectra are aperture extracted, order by order, and each extracted spectral order from a single spectral image is stored in a single table, one order per row. Each column of the table contains a particular type of quantity, such as wavelengths or fluxes. Table 20.2 shows the contents of the different columns in a STIS extracted spectrum table. Each table cell, corresponding to a particular spectral order and type of quantity, can contain either a scalar value or an array of values. For example, each cell in the WAVELENGTH column contains a one-dimensional array of wavelengths corresponding to a spectral order given by the scalar in the SPORDER column on the same row.

There will a separate table extension for each associated exposure in an associated set. For example, if you specified NEXP=3 on your proposal logsheet, you will find the extracted spectrum from the second exposure in the second table extension.



Columns of a STIS Extracted Spectrum Table

Column Name

Contents

Units

Description

SPORDER

scalar

Spectral order number

NELEM

scalar

Number of valid elements in each array

WAVELENGTH

array

Angstroms

Wavelengths corresponding to fluxes

GROSS

array

counts s-1

Extracted spectrum before subtracting BACKGROUND

BACKGROUND

array

counts s-1

Background that was subtracted to obtain NET

NET

array

counts s-1

Difference of GROSS and BACKGROUND arrays

FLUX

array

erg s-1 cm-2 -1

Flux calibrated NET spectrum

ERROR

array

erg s-1 cm-2 -1

Internal error estimate

DQ

array

Data quality flags



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Copyright © 1997, Association of Universities for Research in Astronomy. All rights reserved. Last updated: 11/13/97 17:34:12