HST Cycle 28 Primer: Scientific Instrument STIS

Overview of the Space Telescope Imaging Spectrograph (STIS)

Space Telescope Imaging Spectrograph (STIS)

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 × 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" × 25", a plate scale of approximately 0.025 arcsec/pixel, covering the wavelength range from 1150 Å to 1740 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 1570 Å and 3180 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" × 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. The latter is the recommended mode of operation and supports 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 values adopted for the 2020 ETC (Cycle 28) are 0.022, 0.027, and 0.032 e-/pixel/s for the bottom, middle, and top of the detector. 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.

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.

STIS has three newly-supported neutral density filtered slits for use with the first order and echelle observations. These three apertures, 31X0.05NDA, 31X0.05NDB, and 31X0.05NDC, provide attenuation factors of 6X, 14X, and 33X, respectively. See the STIS Instrument Handbook for more information on the basic properties of these apertures.

There is some evidence that in recent years the STIS instrument focus has changed (see STIS ISR 2017-01). Because the STIS corrector mechanism has not been moved since 1997, it is unlikely that the focus will be adjusted unless the performance suffers from additional degradation. The most obvious impact of these focus issues is a decrease in throughput when using apertures smaller than 0.2" in size. This will lower the S/N achieved by an observation of a given length and also reduces the accuracy of the absolute flux calibration. The exact throughput change for a given aperture will vary as the focus changes during the course of HST's normal orbital breathing. Analysis of recent echelle observations taken through the 0.2X0.06 and 0.2X0.09 apertures shows that the average throughput in recent years has been only about 80% of nominal, with occasional individual exposures with these apertures showing as much as a 40% throughput loss. Note that for these apertures, throughput variations of order 10% due to telescope breathing have always been commonly encountered throughout the lifetime of STIS. For the smallest aperture, the 0.1X0.03, average throughput in recent years has typically been only half of its nominal value. Since these throughput losses vary significantly from observation to observation, it is not possible to simply update the ETC throughputs, as the ETC must also warn against observations which are too bright or which may cause saturation, and must, therefore, adopt the highest throughput that might reasonably be encountered.

Focus offsets can also affect the relative flux calibration as a function of wavelength within a given observation. For modes covering a wide range of wavelengths, relative flux errors of 10% over the wavelength span of E140M and E230M observations done with the 0.2X0.06 aperture are now common. If combined with small aperture centering errors, these can sometimes increase to as much as 25%. Resolution changes caused by these focus variations appear to be small, although they are measurable in high S/N observations and may affect programs which require extremely high levels of observation-to-observation repeatability.

Users requiring a particular signal-to-noise value when using a small aperture should take these effects into account when estimating the amount of exposure time required. Users requiring absolute or relative flux calibration better than 10% are advised to use apertures of 0.2" wide or larger. For first order observations, the 52X0.5 or 52X2 apertures are recommended to achieve the best photometric results.

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 28 will be 0.0022 counts/pixel/sec. Updated information will be provided as it becomes available. For further details regarding STIS capabilities and for any late-breaking updates, see the STIS Instrument Handbook and the STIS website at STScI.


Next: HST Cycle 28 Primer: Additional Observing Modes

References

Dixon, W. V. D. 2011, "Effects of CTE Degradation on Cycle 18 Observations with the STIS CCD"

Related Links

STIS Instrument Handbook

STIS website

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