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5.4. Space Telescope Imaging Spectrograph (STIS)

STIS (4) was installed on board HST during the second servicing mission in February 1997. STIS can obtain high resolution imaging and spectroscopy from the UV to the IR. Its echelle gratings are capable of obtaining medium to high spatial resolution spectra in the UV. Long slit or slitless spectroscopy is available for the whole wavelength range at low to medium resolution. As it name implies, STIS can also be used to obtain images both in the optical and in the UV.

5.4.1 Detectors

STIS uses three large detectors, a "classic" CCD, used for imaging and target acquisition and two Multi-Anode Microchannel Array (MAMA) detectors for UV observations. Tables 9 and 10 summarize some of their characteristics.

Table 9. STIS detector characteristics


pixel size (") 0.05 0.024 0.024
field of view (sq. arcsec) 51 x 51 25 x 25 25 x 25
wavelength range (Å) 2000-11,000 1650-3100 1150-1700

STIS has a slit wheel that contains the apertures and slits. It also includes the grating wheel (called the Mode Selector Mechanism - MSM) that contains the gratings, the echelles and prisms for spectroscopy and mirrors for imaging. To achieve the full wavelength coverage, there are specific MSM positions (grating tilts) at specific central wavelengths that can be specified.

5.4.2 MAMA detectors

The MAMA detectors are photon counting detectors sensible to UV radiation. They can be used to take data in an accumulate mode or as a time series. They offer high spatial resolution and two-dimensional imaging over a large field of view.

Incident photons strike the photocathode creating photo-electrons that pass into the curved Micro-Channel Plates (MCPs). They are multiplied to a pulse that is recorded by an anode array behind the photocathode and detected by the detector electronics. In front of the photocathode two different materials have been deposited that make each detector more sensible to near and far-UV radiation.

The peak photocathode response of the FUV-MAMA occurs at Lyalpha while for the NUV is almost flat between 1800Å and 2600Å. Other properties of the detectors are listed in the table below.

It is very important to remember the observational limit brightness when designing STIS observations with the MAMAs as brighter objects might damage the detectors. If the source exceeds these limits, it is necessary to specify an alternative configuration of the instrument.

Table 10. STIS MAMA properties


Photocathode Cs2Te CsI
Quantum Efficiency 10% at 2537 Å 25% at 1216Å
Dark Count  1.24 x 10-4 cts/pix/sec 6.25 x 10-5 cts/pix/sec
Absolute peak count rate 50 cts/pix/sec 25 cts/pix/sec
Absolute global count rate 300,000 cts/sec 3000,000 cts/sec

MAMA detectors are capable of recording data in highres mode producing 2048 x 2048 images with a 10-30% increase in resolution (but smaller S/N per pixel). This mode also suffers from a lack of flat field reproducibility making it difficult to be of use for high S/N observations.

5.4.3 Spectroscopic Modes

A detailed description of the 15 spectroscopic modes can be found in Chapter 4 of the Instrument Handbook (Version 2.0 1998), in particular the summary in Table 4.1. The expected limiting magnitude, after integrating an orbit, per spectral resolution element and a S/N = 10 for an A-type star is V ~ 21 using the G750L grating, V ~ 14.9 for the G230M and V ~ 11.4 for the E140H echelle (the complete list is in table 4.2).

5.4.4 Imaging Modes

Selecting STIS instead of FOC or WFPC2 to obtain images has several advantages the detector has a higher throughput over a wider spectral range, a lower read noise and dark current (resulting in higher sensitivity images), true solar blind UV observations with the MAMAs are possible as well as high time resolution UV images

The imaging capabilities with the CCD were designed for target acquisitions. Images can be obtained without a filter, with a longpass, an [OIII]lambda5007 and an [OII]lambda3740 filter. With the MAMAs, the filters available include two longpass, two continuum (lambda2700, lambda1820), a MgII lambda2800, a CIII] lambda1909 and a Lyalpha filter. More details can be found in the instrument handbook.

5.4.5 What constitutes a STIS observation

All STIS data products are FITS files and should remain this way. They do not need to be read into GEIS or other formats. As any FITS file, the header stores the global properties of all the exposures in the files. A series of image extensions contain the data and a specific header to describe it.

Each STIS ACCUM data file will contain the following extensions:

As explained in the section above, STIS (and NICMOS) FITS files will contain multiple science exposures when an associated set is taken. Associations are recognized because they have a zero as the last character in the rootname. The following table summarizes the different naming suffixes. Note that the final calibrated spectra are the _x1d, _x2d, _sx1 and _sx2 ones.

Selected STIS File Names

suffix type contents

uncalibrated data
_raw image uncalibrated data
_tag table timetag event list
_spt image planning and telemetry support information
_wav image associate wavecal exposure
_asn table association file

calibrated data
_flt image flat fielded data
_crj image flat fielded and cosmic ray rejected data
_sfl image summed flat-fielded data
_x1d table aperture extracted, background subtracted, flux and wavelength

calibrated 1-d spectra
_x2d image rectified, wavelength and flux calibrated spectra, or geometrically corrected image
_sx1 table summed 1-d extracted spectra
_sx2 image summed 2-d extracted spectra
_trl table trailer file

The following table (which as the previous one was extracted from the latest edition of the HST Data Handbook) details the different extensions that can be found for different observation types. Example of STIS products for different observation types:

Observation type uncalibrated files calibrated files

ACQ, ACQ/PEAK _raw -
Image ACCUM (crsplit or repeatobs) _raw, _spt, _asn _flt, _crj,_sx2, _trl
Image ACCUM single exposure _raw, _spt, _asn _flt, _x2d, _trl
First order spectro scopy, ACCUM mode (crsplit or repeatobs) _raw, _wav, _asn, _spt, _wsp _flt, crj, _sx2, _trl
First order spectro scopy, ACCUM single exposure _raw, _wav, _asn, _spt, _wsp _flt, _x2d, _trl
Echelle spectroscopy, ACCUM mode _raw, _wav, _asn, _spt, _wsp _flt, x1d, _trl
TIMETAG mode (image or spectra) _tag + ACCUM extensions ACCUM extensions

To determine the different files in an association table the STSDAS task tread can be used. It will not "read" the FITS file, just display the information. The task infostis produces an output similar to iminfo for STIS data.

Information on the header keywords can be obtained from the HST Data Handbook or by querying the on-line Keyword Dictionary (

5.4.6 STIS Pipeline

This section will briefly describe how an idea gets transmogrified into a STIS spectrum printed in a scholarly paper.

Astronomers from all over the world submit proposals at each HST's cycle Call for Proposals. They are peer-reviewed and the Time Allocation Committee submits the suggested list of scientifically meritorious proposals to the STScI Director who makes the final selection.

Successful proposers submit then a Phase II proposal. This file describes in detail the observations that will be performed. They use a graphical editor to create and check this file and RSP2 to test the feasibility and efficiency of the orbits. Through RPS2 this Phase II is submitted.

At STScI the proposal is run through an operational RPS2. It translates the Phase II syntax into other files that will be used to create the commands that the telescope will execute. The structure of the orbits and other information can be analyzed in detail at this point. All the visits from the submitted proposals are entered into the Proposal Library and a program program is run that selects the most appropriate Plan Windows when the observations should be made. After this process (which also entailed the selection of the Guide Stars to be used for the observation) the Project Master Data Base (PMDB) is updated with the information about the visits to be executed for this proposal. These "flight ready" visits are then incorporated in the week-long Science Mission Specification and the commands are loaded to the spacecraft.

Once the observations are executed, they are transmitted to the ground where they are run through Generic Conversion and FITS files created. These are the raw data referred to before. During this stage the headers are populated with the calibration information. The program calstis is run on the raw data and the calibrated data is generated. After the pipeline the data is compressed and ingested into the optical disks that constitute the HST Data Archive. The header information is used to populate the archive tables. After archiving, the data is then distributed to the astronomer.

A parallel pipeline exists for the engineering files.

After the astronomer receives the data, it is analyzed and the final product, a scientific paper is submitted for publication.

4 Please refer to the STIS Intrument Handbook for complete details of the instrument. It can be found at Back.

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