| CADC filenames | Suffix | Type | Contents | Examples |
|---|
| Uncalibrated |
| oxxxxxxxx_raw | _raw | image | Raw science1 | O3X002050_RAW |
| oxxxxxxxx_tag | _tag | table | Timetag event list | O3X002050_TAG |
| oxxxxxxxx_spt | _spt | image | Support, planning & telemetry information | O3X002050_SPT |
| oxxxxxxxx_wav | _wav | image | Associate wavecal exposure | O3X002050_WAV |
| oxxxxxxxx_wsp | _wsp | image | The _spt file for _wav (wavecal) | O3X002050_WSP |
| oxxxxxxxx_asn | _asn | table | Association file | O3X002050_ASN |
| oxxxxxxxx_trl | _trl | table | Trailer file (input); historical record of generic conversion | O3X002050_TRL |
| oxxxxxxxx_jit | _jit | table | Contains spacecraft pointing data averaged over 3 s intervals | O3X002050_JIT |
| oxxxxxxxx_jif | _jif | image | Two-Dimensional histogram of the _jit file | O3X002050_JIF |
| oxxxxxxxx_pdq | _pdq | table | Post observation summary and Data Quality | O3X002050_PDG |
| Calibrated |
| oxxxxxxxx_flt | _flt | image | Flatfielded science | O3X002050_FLT |
| oxxxxxxxx_CRJ | _crj | image | Cosmic ray-rejected, flatfielded science | O3X002050_CRJ |
| oxxxxxxxx_sfl | _sfl | image | Summed flatfielded science | O3X002050_SFL |
| oxxxxxxxx_X1D | _x1d | table | 1-D extracted spectra: aperture extracted, background subtracted, flux and wavelength calibrated spectra | O3X002050_X1D |
| oxxxxxxxx_x2d | _x2d | image | 2-D extracted data: rectified, wavelength and flux calibrated spectra or geometrically corrected imaging data. | O3X002050_X2D |
| oxxxxxxxx_sx1 | _sx1 | table | Summed 1-D extracted spectra | O3X002050_SX1 |
| oxxxxxxxx_sx2 | _sx2 | image | Summed 2-D extracted spectra | O3X002050_SX2 |
| oxxxxxxxx_trl | _trl | table | Trailer file (output); historical record of pipeline processing Raw data from isolated wavecals, biases, darks, and flats, as well as from ACQs and ACQ/PEAKs, also have the _raw suffix. | O3X002050_TRL |
Typical STIS Output Products by Observation Type
| Observation Type | Uncalibrated Files | Calibrated Files |
|---|
| ACQ, ACQ/PEAK | _raw | none |
| IMAGING, ACCUM MODE, ASSOCIATED SET (crsplit or repeatobs) | _raw, _spt, _asn, _trl _flt | _sfl (MAMA only), _crj (CCD only) |
| IMAGING, ACCUM MODE, Single Exposure | _raw, _spt, _asn, _trl | _flt |
| FIRST ORDER SPECTROSCOPY, ACCUM MODE ASSOCIATED SET (crsplit or repeatobs) | _raw, _wav, _asn, _spt, _wsp | _trl,_flt,_sx2, _sx1,_crj (CCD only) |
| FIRST ORDER SPECTROSCOPY, ACCUM MODE Single Exposure | _raw, _wav, _asn, _spt, _wsp, _trl | _flt, _x2d, _x1d |
| ECHELLE SPECTROSCOPY, ACCUM MODE single exposure or ASSOCIATED SET | _raw, _wav, _asn, _spt, _wsp, _trl | _flt, _x2d, _x1d |
| TIMETAG IMAGING and SPECTROSCOPIC | _tag + ACCUM extensions | ACCUM extensions |
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. Figure 2.1
illustrates the contents of the association table for a CRSPLIT=2
observation, with an associated wavecal.
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 (see section 2.3.1). 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. Chapter 3 gives
more information about the pipeline processing.
STIS FITS Image Extension Files
Figure 2.2 illustrates the structure of a STIS FITS image extension file, which 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 shown in figure
2.2. Each STIS readout generates three FITS images. SCI, ERR and DQ,
as explained below:
- The first, of extension type SCI, stores the science values.
- The second, of extension type ERR, contains the statistical errors, which are propagated through the calibration process.
- 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. The procedure to derive flux information from these
data is described in section 5.4.1. Discussion of the one dimensional
extracted spectra is presented in section 2.3.3.
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.
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.
- 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 table 2.3.
Columns of a Time-Tag Data Table
| Column Name | Units | Description |
|---|
| TIME | seconds | Elapsed time in seconds since the exposure start time |
| AXIS1 | pixel | Pixel coordinate along the spectral axis, with Doppler-correction |
| AXIS2 | pixel | Pixel coordinate along the spatial axis, (no Doppler correction) |
| DETAXIS1 | pixel | Pixel coordinate along the spectral axis, prior to Doppler correction |
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).
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. Table 2.4 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
| Column Name | Contents | Units | Description |
|---|
| SPORDER | scalar | | Spectral order number |
| NELEM | scalar | | Number of valid elements in each array |
| WAVELENGTH | array | Å | 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 Å-1 | Flux calibrated NET spectrum |
| ERROR | array | erg s-1 cm-2 Å-1 | Internal error estimate |
| DQ | array | | Data quality flags |
|
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