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, 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.
The 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 the high spatial resolution of the HST
. The 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 place science targets on slits and coronagraphic bars.
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.
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 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-
). This 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 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 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 μ
sec) 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
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.
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 (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.
The 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 was 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.