In this section, we provide a high-level summary of the basic design and operation of STIS, concentrating on the information most relevant to your HST observing proposal. Subsequent chapters provide more detailed information.
STIS uses three large-format (1024 x 1024 pixel) detectors (see Chapter 7 for more details), as follows:
STIS/CCD, with ~0.05 arcsecond square pixels, covering a nominal 52 x 52 arcsecond square field of view (FOV), operating from ~2000 to 10,300 Å.
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 1600 to 3100 Å.
STIS/FUV-MAMA, with ~0.024 arcsec pixels, and a nominal 25 x 25 arcsecond square FOV, operating in the far ultraviolet from 1150 to 1700 Å.
The CCD provides high quantum efficiency and good dynamic range in the near ultraviolet through near infrared. The CCD 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, but can be reduced to 20 sec for subarray readouts. CCD detectors are capable of high-dynamic-range observations. The dynamic range for a single exposure, ultimately is limited by the depth of the CCD full well (144,000 e-), which determines the total amount of charge (or counts) that can accumulate in any one pixel during any one exposure without causing saturation. For GAIN=1, it is further limited (to 33,000 e-) by saturation in the gain amplifier. Cosmic rays will affect all CCD exposures. CCD observations should be broken into multiple exposures (called CR-SPLITs) of no more than 1000 sec each, whenever possible, to allow removal of cosmic rays in post-observation data processing; During Phase II you can specify the CR-SPLIT optional parameter to do this (see Chapter 11).
The two MAMAs are photon-counting detectors which provide a two-dimensional ultraviolet 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 onboard 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 counts sec-1 pixel-1) and global (>285,000 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. We have therefore defined absolute local and global count-rate limits, which translate to a set of configuration dependent bright-object screening limits. Sources which violate the absolute count-rate limits in a given configuration cannot be observed in that configuration, as discussed under MAMA Bright-Object Limits.
Early concerns about the signal-to-noise attainable with the MAMAs have been alleviated by experience in orbit. Values of 50:1 per spectral resolution element in extracted spectra are routinely obtained for point sources with sufficient counting statistics when integrated over the extraction 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 special multiple slits with the echelles (see Chapter 12). Current information indicates that the flat fields are stable to ±1-2%, but the UV flat-field lamps have been used sparingly due to their limited lifetimes, so the long-term stability of the UV flats is not yet certain. See also Summary of Accuracies.
The STIS optical design includes corrective optics to compensate for HST's spherical aberration, a telescope focal-plane slit-wheel assembly, collimating optics, a grating-selection mechanism, fixed optics, and camera focal-plane detectors. An independent calibration-lamp assembly can illuminate the focal plane with a range of continuum and emission-line lamps. A simplified schematic showing major mechanisms and detectors, and a medium-resolution echelle mode light path is shown in Figure 3.1.
Figure 3.1: Simplified STIS Optical Design
The slit wheel contains apertures and slits for spectroscopic use and the clear, filtered and coronographic 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 so-called 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 which 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. As described in Routine Wavecals below, the grating wheel exhibits non-repeatability which is corrected for in post-observation data processing using contemporaneously obtained comparison-lamp exposures.
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, has been defined (see Chapter 4).
STIS has two independent calibration subsystems, the so-called HITM (Hole in the Mirror) system and the IM (Insert Mechanism) 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). Thus, when the HITM lamps are used light from the external sky still falls on the detector unless the STIS external shutter (not shown in Figure 3.1) is closed. 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 and all external light is blocked. Observers will be relieved to know that the ground system will automatically choose the right subsystem (see Basic Instrument Operations) and provide the necessary wavelength calibration exposures.
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