Files names and extensions used for STIS (O)

Understanding Associations

An association is created when repeated exposures are obtained through CR-SPLITs or REPEATOBS, and when wavecal exposures are linked to science exposures. The repeated exposures in an association will appear in a single FITS file. You can recognize a file as part of an association because there will be a zero in the last position of the rootname (e.g., o3tt01010_raw.fits). The rootnames of the individual exposures in an association are contained in the association file, which has suffix _asn (e.g., o3tt01010_asn.fits). An association file holds a single binary table extension, which can be displayed with the IRAF tasks tprint or tread. The information within an association table shows how the associated exposures are related. 

The association table above tells the user that the product, or data set, will have the rootname o3tt01010, that there will be two science exposures contained in the o3tt01010_raw.fits file that are CRSPLITs, and that a o3tt01010_wav.fits file should exist containing the contemporaneously obtained automatic wavecal. The o3tt01010_raw.fits file will contain six image extensions, one triplet of {SCI, ERR, DQ} for each exposure. The pipeline will calibrate these data as a unit, producing a single cosmic-ray rejected image (rootname_crj.fits), its data quality and error images, and rectified spectra. Similarly, for repeatobs observations, in which many identical exposures are taken to obtain a time series, all the science data will be stored in sequential triplet extensions of a single FITS file. These will be processed through the calstis pipeline as a unit, with each image extension individually calibrated. The set of images will also be combined to produce a total time-integrated calibrated image.

STIS FITS Image Extension Files

The STIS FITS image extension file contains:

• A primary header that stores keyword information describing the global properties of all of the exposures in the file (e.g., the target name, target coordinates, total summed exposure time of all exposures in the file, optical element, aperture, detector).
• A series of image extensions, each containing header keywords with information specific to the given exposure (e.g., exposure time, world coordinate system) and a data array.

Storage of STIS Two-Dimensional Data

All uncalibrated and calibrated ACCUM mode science data (with the exception of the extracted one-dimensional spectra, see below) are stored in FITS image extension files with the particular format. Each STIS readout generates three FITS images. SCI, ERR and DQ, as explained below:

• The first, of extension type SCI, stores the science values.
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• The second, of extension type ERR, contains the statistical errors, which are propagated through the calibration process.
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• The third, of extension type DQ, stores the data quality values, which flag suspect pixels in the corresponding SCI data.

The error arrays and data quality values are described in more detail in section 2.5. Each of these extensions can contain one of several different data types, including images, binary tables and ASCII text tables. The value of the XTENSION keyword in the extension's header identifies the type of data the extension contains.

Two-Dimensional Rectified Spectral Images: The rootname_sx2.fits and rootname_x2d.fits files, which hold the flux and wavelength-calibrated two-dimensional spectra for long-slit first-order observations, are stored as FITS images, as are the raw and calibrated imaging data. The units of the data in the extracted two dimensional spectra are ergs sec-1 cm-2 Å-1 arcsec-2. 

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 rootname_flt.fits or rootname_sfl.fits. The units of the data in these files are counts per pixel. The conversion of the counts to flux (or magnitude) is explained in section 5.3.1. 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:

Storage of Acquisition 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).

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:

• [SCI,1] is a subarray image (100 x 100 for point source acquisitions; larger for diffuse acquisitions) of the sky obtained after the initial blind pointing.
• [SCI,2] is an image of the same subarray after the coarse locate phase of the acquisition.
• [SCI,3] is an image of the 32 x 32 subarray taken during the slit-illumination phase of the target acquisition.

Storage of Acquisition/Peakup Images: An acquisition/peakup exposure will produce a single raw data file for a spiral search peakup, and one for each linear search peakup (that is, if you have performed a peakup that requires SEARCH=LINEARAXIS1 and SEARCH=LINEARAXIS2 scans, then two data sets will be produced-one for each scan). Keywords in the header of spectroscopic data identify the data set name of the acquisition/peakup images (in the ACQPEAK1 and ACQPEAK2 keywords). The rootname_raw.fits data file produced for an acq/peak exposure contains one science image extension:

• [SCI,1] is the confirmation image, taken at the end of the peakup, after the final move which places the target in the slit.
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• To examine the flux values of the individual steps in the ACQ/PEAK, list the pixels (using the listpix task) of the fourth extension, i.e., rootname_raw.fits[4]

Storage of STIS Tabular Data

All the time-tag and one-dimensional extracted STIS spectra are stored in binary tables, as described below. Time-Tag: Time-Tag mode is used for high time resolution spectroscopy and imaging in the UV (with the MAMA detectors only). Time-Tag event data are 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 the table below.

Columns of a Time-Tag Data Table

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 section 5.6). There will be a separate table extension for each associated exposure in an associated set. For example, if you specified NEXP=3 in your Phase II instructions, you will find the extracted spectrum from the second exposure in the second table extension.

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 quantity, such as wavelength or flux. The table below 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 be a separate table extension for each associated exposure in an associated set. For example, if you specified NEXP=3 in your Phase II instructions, you will find the extracted spectrum from the second exposure in the second table extension.

Columns of a STIS Extracted Spectrum Table