4.5 Space Telescope Imaging Spectrograph (STIS)

Summary of STIS changes after SM4

The Space Telescope Imaging Spectrograph (STIS) was installed aboard HST in February 1997. This scientific instrument provides ultraviolet and optical spectroscopy and imaging through three channels. STIS can be used to obtain spatially resolved, long-slit (or slitless) spectroscopy over the 1150 Å to 10,300 Å wavelength range with spectral resolving power from R ~ 500 to 17,500. It can also be used to perform echelle spectroscopy over the 1150 Å - 3100 Å wavelength range at spectral resolving powers of R ~ 30,000 to 114,000, covering broad spectral ranges of Δλ ~ 800 Å and 200 Å, respectively. STIS can also be used for optical and solar-blind ultraviolet imaging. Three detectors, each with a 1024 x 1024 pixel format, support spectroscopy and imaging as follows:

•	The Far Ultraviolet Channel (STIS/FUV-MAMA) uses a solar-blind, CsI, Multi-Anode Microchannel detector Array (MAMA), with a field of view of 25" x 25", a plate scale of 0.025 arcsec/pixel, covering the wavelength range from 1150 Å to 1700 Å.

•	The Near Ultraviolet Channel (STIS/NUV-MAMA) uses a Cs2Te MAMA detector with the same field of view and plate scale, and covers wavelengths between 1600 Å and 3100 Å.

•	The STIS/CCD detector, a thinned and backside-illuminated SITe CCD with a coating optimized for the near-ultraviolet, covers the range from 1650 Å to 11,000 Å. This channel’s field of view is 52" x 52" and the CCD has a plate scale of 0.05 arcsec/pixel.

The MAMA detectors can be used in ACCUM or TIME-TAG modes, with the latter supporting time-resolutions down to 125 microseconds. The STIS/CCD can be cycled in ~20 seconds when using small subarrays. The CCD and the MAMAs also provide coronagraphic spectroscopy in the visible and ultraviolet. Coronagraphic CCD imaging is also available. Each of the STIS detectors can also be used for imaging observations, however, only limited filter choices are available.

Summary of STIS changes after SM4

STIS was successfully repaired during Servicing Mission 4. Capabilities are very similar to those prior to the 2004 failure. Overall sensitivities have declined by only a few percent.

However, the STIS CCD has continued to accumulate radiation damage. The mean STIS CCD dark current is estimated to be 0.0165 e-/pixel/s during Cycle 21. Charge transfer efficiency (CTE) declines can also have a strong effect on faint sources observed with the STIS CCD. Users should remember that the STIS ETC does not correct for CTE losses. They can determine the loss due to CTE using the formula described in the STIS Instrument Handbook at:

http://www.stsci.edu/hst/stis/software/analyzing/scripts/
cteloss_descrip.html

CTE effects also produce extended “tails” on hot pixels and cosmic rays that degrade images and spectra, creating an additional source of noise. See STIS ISR 2011-02 for a description of how this can affect STIS CCD data. Use of the E1 aperture positions which place STIS spectra closer to the CCD readout edge will substantially mitigate CTE effects and is strongly recommended for faint targets.

After its initial recovery following Servicing Mission 4, the NUV-MAMA showed a much larger dark current than had been previously seen; however, this excess is slowly declining. For planning purposes users should assume that the STIS/NUV-MAMA dark current during Cycle 21 will be 0.0018 counts/pixel/sec. Updated information will be provided as it becomes available. For further details regarding STIS capabilities after Servicing Mission 4, and for any late breaking updates, see the Cycle 21 STIS Instrument Handbook and the STIS Web site at STScI.