OPTICAL PATH

Light from the HST main mirror is first corrected and then brought to a focus at the slit wheel. The choice of slit, grating wheel element, and tip/tilt angle (specified by designating a desired central wavelength) completely defines the optical configuration. From the grating wheel, the light travels to the appropriate detector either directly or via a fold flat and camera mirror, or via an echelle grating and camera mirror. Shutter and blocker mechanisms are configured as required to keep light out of the instrument during slit and grating wheel motions, to start and stop CCD exposures, and to control stray light cross talk between the various optical paths.

ENTRANCE APERTURES - SLIT WHEEL

One of 65 slits or apertures can be selected. These consist of clear slits of assorted widths and lengths for long-slit and echelle spectroscopy, ND-filtered slits for calibration and bright-object spectroscopy, filtered and unfiltered apertures for imaging. Alternate light paths can also illuminate the slit plane with the output of various line and continuum lamps for wavelength calibration, flat-fielding, and target acquisition.

There also are camera apertures of 50 x 50 and 25 x 25 arcsec. Some have occulting bars incorporated. The telescope can be positioned to place bright stars behind the occulting bars to allow viewing and observation of faint objects in the Field of View (FOV). In addition, there is a special occulting mask or coronagraph - a finger in the aperture that can be positioned over a bright star to allow examination of any faint material nearby. In effect, it simulates a total eclipse of a nearby star. This mode is particularly useful to search for faint companion stars or planetary disks around stars.

MODE SELECT MECHANISM (MSM)

The MSM is a rotating wheel having 21 optical elements: 16 first-order gratings (including six order-sorting gratings used in the echelle modes), an objective prism and four mirrors. Its axis is a shaft with two inclined outer sleeves, one sleeve fitting inside the other. The sleeves are constructed so that rotation of one sleeve rotates a wheel to orient the appropriate optic into the beam. Rotation of the second sleeve changes the inclination of the wheel axis or the tilt of the optic to select the wavelength range and point the dispersed beam to the corresponding detector. One of the mirrors can be selected to take an image of an object.

For first-order spectra, a first-order grating is selected for the wavelength and dispersion. The beam then is pointed to a camera mirror, which focuses the spectrum onto the detector, or goes directly to the detector itself.

For an echelle spectrum, an order-sorting grating that directs the light to one of the four fixed echelle gratings is selected, and the dispersed echellogram is focused via a camera mirror onto the appropriate detector. The detectors are housed at the rear of the bench, so they can easily dissipate heat through an outer panel. The optical bench is thermally controlled. An onboard computer controls the detectors and mechanisms.

OBSERVATION MODES

The observing modes of STIS can be divided into four spectral bands.

Band 1 (115-170 nm) is covered by a photon counting Multi-Anode Microchannel Plate Array (MAMA) detector (below left), designated the far-ultraviolet (FUV) MAMA, with an opaque CsI photocathode deposited directly on the front of its single curved-channel microchannel plate (C-plate).

Band 2 (165-310 nm) is covered by a similar detector, the near-ultraviolet (NUV) MAMA, utilizing a semitransparent CsTe photocathode on the inside of the detector window. This detector provides additional coverage down to 115 nm in imaging and prism modes, and it serves as a backup to the FUV MAMA.

Bands 3 and 4 (305-555 and 550-1000 nm, respectively) are covered by a backside-thinned, UV-enhanced, multi-pinned-phase, 1024x1024 pixel Charge-Coupled Detector (CCD) (below right). The CCD covers bands 3 and 4 as its prime operating range and provides backup to the NUV MAMA in the 180-305 nm range. The CCD is also used for target acquisition.

IMAGING OPERATIONAL MODES

STIS can be used to acquire an image of an object in UV or visible light. To do this, an open aperture is selected and a mirror placed in the beam by the MSM. The instrument has nine filters that can be selected. The cameras for the CCD and the MAMAs have different magnification factors. The FOV is 25 x 25 arcsec for the MAMAs and 50 x 50 arcsec for the CCD.