1.1 Instrument Capabilities and Design

The Space Telescope Imaging Spectrograph (STIS) was built by Ball Aerospace Corporation for the NASA’s Goddard Space Flight Center (GSFC) Laboratory for Astronomy and Solar Physics, under the direction of Bruce Woodgate, the Principal Investigator (PI). STIS performed very well upon 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 first Servicing Mission Observatory Verification (SMOV) program is provided by Kimble et al. (1998, ApJL, 492, L83). We encourage all STIS users to reference this paper. The Early Release Observations are also presented in this special ApJ Letters issue.

STIS is a versatile instrument providing two-dimensional imaging and spectroscopic capabilities with three detectors operating from the ultraviolet to the near-infrared. The optics and detectors were designed to exploit the high spatial resolution of the HST. STIS has first-order gratings, designed for spatially resolved long slit spectroscopy over its 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 center science targets on slits and coronagraphic bars.

STIS can be used to obtain:

• Spatially resolved, long slit or slitless spectroscopy from 1150–10300 Å at low to medium spectral resolution 
  (R ~ 500–17000) in first order.
• Echelle spectroscopy at medium to high spectral resolution 
  (R ~ 30000–110000), covering a broad instantaneous spectral range (Δλ ~ 800 or 200 Å, respectively) in the ultraviolet (1150–3100 Å).

In addition to these two prime capabilities, STIS also provides:

• Modest imaging capability using: the solar-blind CsI far-ultraviolet Multi-Anode Micro-channel Array (MAMA) detector (1150–1700 Å); the solar-insensitive Cs₂Te near-ultraviolet MAMA detector (1650–3100 Å); and the optical CCD (2000–11000 Å) through a small complement of narrow- and broad-band filters.
• Objective prism spectroscopy (R ~ 500–10) in the vacuum ultraviolet (1200–3100 Å).
• High time resolution (Δt = 125 microseconds) imaging and spectroscopy in the ultraviolet (1150–3100 Å) and moderate time resolution (Δt ~20 ­seconds) CCD imaging and spectroscopy in the optical and near IR (2000–10300 Å).
• Coronagraphic imaging in the optical and near IR (2000–10300 Å), and bar-occulted spectroscopy over the entire spectral range (1150-10300 Å).

A complete list of gratings and filters are given in Table 1.1 and Table 1.2, respectively, with references to the STIS Instrument Handbook for more details.

Table 1.1: STIS Spectroscopic Capabilities; for more information, refer to Section 13.3 of the STIS Instrument Handbook.

¹ See "Line Spread Functions in Section 13.6 of the STIS Instrument Handbook for detailed estimates.

² Number of exposures at distinct tilts needed to cover spectral range of grating with 10% wavelength overlap between adjacent settings.

³ For a complete list of supported and available-but-unsupported apertures for each grating, see Appendix A of the STIS Instrument Handbook.

⁴ Naming convention gives dimensions of slit in arcseconds. E.g., 52X0.1 indicates the slit is 52 arcsec long perpendicular to the dispersion direction and 0.1 arcsec wide in the dispersion direction. The F (e.g., in 52X0.2F1) indicates a fiducial bar to be used for coronagraphic spectroscopy.

⁵ For MAMA first-order modes, only ~ 25 arcsec of a long slit’s length projects on the detector. (See also Section 4.2.2 in the STIS Instrument Handbook.).

⁶ Full-aperture clear (50CCD or 25MAMA), longpass-filtered (F25QTZ or F25SRF2 in UV), and neutral-density-filtered slitless spectroscopy are also supported with the appropriate first-order and echelle gratings, as well as the PRISM.

⁷ The following slits are also supported for all echelle gratings. The 6X0.2 and 52X0.05 long slits are intended for use with extended emission line objects; order overlap must be considered when using these slits. Also the high S/N multi-slits 0.2X0.2FP(A-E) and 0.2X0.06FP(A-E) (see Chapter 12), the very narrow 0.1X0.03 slit for maximum spectral resolution, and the 0.2X0.05ND, 0.3X0.05ND, and 31X0.05ND(A-C) neutral density slits.

⁸ The 0.1X0.09 and 0.1X0.2 slits are supported with E230H only. F25MGII is supported with all NUV-MAMA gratings and the PRISM.

⁹ The 0.2X0.2 aperture is also supported with all first-order gratings. It is available-but-unsupported with the PRISM.

¹⁰ The F25SRF2 aperture can be used with the prism to filter out (geocoronal) Lyman-α emission.

¹¹  Resolution of 200,000 or greater is possible when used with the 0.1X0.03 slit and special observing and data reduction techniques.

¹² Resolution of 200,000 or greater is possible when used with the 0.1X0.03 slit and special observing and data reduction techniques.

 Table 1.2: STIS Imaging Capabilities; for more details refer to Section 14.3-14.5 of the STIS Instrument Handbook. 

 ¹ The dimensions are 28 arcsec on AXIS2=Y and 50 arcsec on AXIS1=X. See Figure 3.2 and Figure 11.1 in the STIS Instrument Handbook.
²  The MAMA plate scales differ by about 1% in the AXIS1 and AXIS2 directions, a factor that must be taken into account when trying to add together rotated images. Also, the FUV-MAMA uses a different mirror in the filtered and unfiltered modes. In the filtered mode, the plate scale is 0.3% larger (more arcsec/pixel). Further information on geometric distortions can be found in Chapter 14 of the STIS Instrument Handbook.

1.1.1 The STIS Detectors

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

• A Scientific Image Technologies (SITe) CCD, called the STIS/CCD, with 0.05 arcsec square pixels, covering a nominal 52 × 52 arcsec square field of view (FOV), operating from ~2000 to 10300 Å.
• A Cs₂Te MAMA detector, called the STIS/NUV-MAMA, with 0.025 arcsec square pixels, and a nominal 25 × 25 arcsec square FOV, operating in the near-ultraviolet from 1650 to 3100 Å.
• A solar blind CsI MAMA, called the STIS/FUV-MAMA, with 0.025 arcsec ­pixels, and a nominal 25 × 25 arcsec square FOV, operating in the far-ultraviolet from 1150–1700 Å.

The basic observational parameters of these detectors are summarized in Table 1.1 and Table 1.2.

The CCD: The CCD provides high quantum efficiency and good dynamic range in the near-ultraviolet through the near-infrared. 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 45 sec for full frame readouts and 20 sec for subarray readouts. CCD detectors are capable of high dynamic range observations; for a single exposure taken with GAIN=4, the depth (the maximum amount of the charge or counts that can accumulate in any one pixel during any on exposure, without saturation) is limited by the CCD full well (roughly ~144,000 e¯), while for a single exposure taken with GAIN=1, the depth is limited by the gain amplifier saturation (~33,000 e¯). Cosmic rays affect all CCD exposures, and observers will generally want to split up their observations into a number of multiple exposures of less than 1,000 sec each. This allows cosmic ray removal in post-observation data processing. 

The MAMAs: The two MAMA detectors are photon counting detectors that provide a two-dimensional ultraviolet imaging capability. They can be operated either in ACCUM mode, to produce a time integrated image, or in TIME-TAG mode, to produce an event stream with high (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 performance and safety brightness limits. At high local (>50 counts/sec/pixel) and global (>285,000 counts/sec) 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. Specifically, charge is extracted from the micro-channel plate during UV observations, and over-illumination can cause decreased quantum efficiency (due to gain decline in the overexposed region) or catastrophic failure (high voltage arcing within the sealed tube due to excess gas generation from the plate). Thus, MAMA observations are subject to bright object checks.

Current information indicates that the pixel-to-pixel flat fields are stable at the 1% level, which is the signal-to-noise of the flats. Furthermore, these flats show no signs of DQE loss in regions where the detector has been heavily exposed.
A signal-to-noise of 50:1 per spectral resolution element is routinely obtained for extracted spectra of point sources when integrated over the observed aperture. Higher signal-to-noise values of 100-300 can be obtained by stepping the target along the slit in the first-order modes, or by use of FP-SPLIT slits with the echelles.

1.1.2 STIS Physical Configuration

The STIS optical design includes corrective optics to compensate for the spherical aberration of the HST, 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 milliarcsec 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 that is corrected in post-observation data processing using contemporaneously obtained comparison lamp exposures for wavelength calibrations.
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 range 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 Hole in the Mirror (HITM) 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). For wavecal data taken before 1998-Nov-9, light from the external sky fell on the detector when the HITM lamps were used, but for subsequent wavecal data, an external shutter is closed to block external sky light. The IM system contains flat fielding lamps (a tungsten lamp for CCD flats, a deuterium lamp for NUV-MAMA flats, and a Krypton lamp for FUV-MAMA flats) 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.