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19.1 Instrument Capabilities and Design

The Space Telescope Imaging Spectrograph (STIS) was built by Ball Aerospace Corporation for the Goddard Space Flight Center (GSFC) Laboratory for Astronomy and Solar Physics, under the direction of Bruce Woodgate (GSFC), the Principal Investigator (PI). STIS has been performing very well since its installation during the second HST servicing mission in February 1997. A basic description of the instrument, and of its on-orbit performance through the Servicing Mission Orbital Verification (SMOV) program is provided by Kimble, et al. (1997, ApJL, in press). We encourage all STIS users to reference this paper, and to review the related papers in this special ApJ Letters which describe the Early Release Observations, and demonstrate the realized scientific capabilities of STIS. Long-slit and slitless image spectroscopy of galactic nuclei and SN1987A are described in Bower et al. (1997), Hutchings et al. (1997), and Sonneborn et al. (1997); medium- and high-resolution UV echelle spectroscopy of stars and the interstellar medium are described by Heap et al. (1997), Jenkins et al. (1997), and Walborn et al. (1997); Schultz et al. (1997) describe visible and near-IR spectroscopy of a brown dwarf near a much brighter companion; Pian et al. (1997) and Sahu et al. (1997) describe deep CCD imaging of a Gamma Ray Burst transient and of gravitational lens arclets, respectively; and Gardner et al. (1997) describes the serendipitous detection of a high-redshift galaxy in a parallel observation.

STIS is a versatile instrument providing both imaging and spectroscopic capabilities with three two-dimensional detectors operating from the ultraviolet to the near-infrared. The optics and detectors have been designed to exploit HST's high spatial resolution. STIS has first order gratings, designed for spatially resolved long-slit spectroscopy over STIS's entire spectral range, and echelle gratings, available only in the ultraviolet, that maximize the wavelength range covered in a single spectral observation of a point source. The STIS Flight Software supports on-board target acquisitions and peakups to place science targets on slits and coronagraphic bars.

STIS can be used to obtain:

Spatially resolved, long-slit or slitless spectroscopy from 1150-11,000 Å at low to medium spectral resolution (R ~ 400-14000) in first order.
Echelle spectroscopy at medium to high spectral resolution (R ~ 23,500-100,000), covering a broad instantaneous spectral range ( ~800 or 250 Å, respectively) in the ultraviolet (1150-3100 Å). In addition to these two prime capabilities, STIS also provides:
A modest imaging capability using: the solar-blind far ultraviolet MAMA detector (1150-1700 Å); the solar-insensitive near ultraviolet MAMA detector (1700-3100 Å); and the optical CCD (2000-11,000 Å) through a small complement of narrow- and broad-band filters.
Objective prism spectroscopy (R ~1000-26) in the vacuum ultraviolet (1200-3100 Å).
High time resolution ( = 125 microseconds) imaging and spectroscopy in the ultraviolet (1150-3100 Å) and moderate time resolution ( ~10 -seconds) CCD imaging and spectroscopy in the optical and near IR (2000-11,000 Å).
Coronagraphic imaging in the optical and near IR (2000-11,000 Å) and bar-occulted spectroscopy over the entire spectral range (1150-11,000 Å). See Table 19.1 on page 19-6 and Table 19.2 on page 19-7 for a complete list of grating and filters, respectively.

The STIS Detectors

STIS uses three large format (1024 x 1024 pixel) detectors:

A Scientific Image Technologies (SITe) CCD, called the STIS/CCD, with 0.05 arcsecond square pixels, covering a nominal 51 x 51 arcsecond square field of view (FOV), operating from ~2000 to 11,000 Å.
A Cs₂Te Multi-Anode Microchannel Array (MAMA) detector, called the STIS/NUV-MAMA, with 0.024 arcsecond square pixels, and a nominal 25 x 25 arcsecond square field of view (FOV), operating in the near ultraviolet from 1650 to 3100 Å.
A solar blind CsI MAMA, the STIS/FUV-MAMA, with 0.024 arcsec -pixels, and a nominal 25 x 25 arcsecond square FOV, operating in the ultraviolet from 1150-1700 Å. The basic observational parameters of these detectors are summarized in Table 19.1 on page 19-6 and Table 19.2 on page 19-7.
The CCD provides high quantum efficiency and good dynamic range in the near-ultraviolet through near-infrared, and it produces a time integrated image in the so-called ACCUM data taking mode. As with all CCDs, there is noise (read noise) and time (read time) associated with reading out the detector. Time resolved work with this detector is done by taking a series of multiple short exposures. The minimum exposure time is 0.1 sec, and the minimum time between successive identical exposures is 37 seconds for full-frame readouts and 11 seconds for subarray readouts. CCD detectors are capable of high dynamic range observations, which are limited for a single exposure by the depth of the CCD full well, roughly ~120,000 to 170,000 e^- for the STIS CCD. This number is the maximum amount of charge (or counts) that can accumulate in any one pixel during any one exposure, without saturation. Cosmic rays affect all CCD exposures, and observers will generally want to CR-SPLIT their observations to allow cosmic ray removal in post-observation data processing.
The two MAMA detectors are photon counting detectors which provide a two-dimensional ultraviolet imaging capability. They can be operated either in ACCUM mode, to produce a time-integrated image, or in TIMETAG mode to produce an event stream with fast (125 µsec) time resolution. Doppler correction for the spacecraft motion is applied automatically on-board for data taken in ACCUM high spectral resolution modes.
The STIS MAMA detectors are subject to both scientific and absolute brightness limits. At high local (>50 count sec^-1 pixel^-1) and global (>250,000 counts sec^-1) illumination rates, counting becomes nonlinear in a way that is not correctable. At only slightly higher illumination rates, the MAMA detectors are subject to damage.

STIS Physical Configuration

The STIS optical design includes corrective optics to compensate for HST's spherical aberration, a focal plane slit wheel assembly, collimating optics, a grating selection mechanism, fixed optics, and focal plane detectors. An independent calibration lamp assembly can illuminate the focal plane with a range of continuum and emission line lamps.

The slit wheel contains apertures and slits for spectroscopic use and the clear, filtered, and coronagraphic apertures for imaging. The slit wheel positioning is repeatable to very high precision: +/- 7.5 and 2.5 milli-arcseconds in the spatial and spectral directions, respectively.

The grating wheel, or Mode Selection Mechanism (MSM), contains the first-order gratings, the cross-disperser gratings used with the echelles, the prism, and the mirrors used for imaging. The MSM is a nutating wheel that can orient optical elements in three dimensions. It permits the selection of one of its 21 optical elements as well as adjustment of the tip and tilt angles of the selected grating or mirror. The grating wheel exhibits non-repeatability which is corrected in post-observation data processing using contemporaneously obtained comparison lamp exposures (i.e., wavecals).

For some gratings, only a portion of the spectral range of the grating falls on the detector in any one exposure. These gratings can be scanned (tilted by the MSM) so that different segments of the spectral format are moved onto the detector for different exposures. For these gratings a set of pre-specified central wavelengths, corresponding to specific MSM positions, i.e., grating tilts, have been defined.

STIS has two independent calibration subsystems, the HITM (Hole in the Mirror) system and the Insert Mechanism (IM) system. The HITM system contains two Pt-Cr/Ne line lamps, used to obtain wavelength comparison exposures and to illuminate the slit during target acquisitions. Light from the HITM lamps is projected through a hole in the second correction mirror (CM2), so light from the external sky still falls on the detector when the HITM lamps are used. The IM system contains flatfielding lamps and a single Pt-Cr/Ne line comparison lamp. When the IM lamps are used, the Calibration Insert Mechanism (CIM) is inserted into the light path, blocking all external light. Observers will be relieved to know that the ground system will automatically choose the right subsystem and provide the necessary calibration exposures.

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