Each time the MSM is moved to select a new optical element or to tilt a grating, the resulting spectrum is projected onto the detector with a positional error (lack of repeatability) of ≤±
3 low-resolution (MAMA) pixels. Additionally, thermal effects can cause small drifts over multi-orbit observations. An internal calibration lamp observation (WAVECAL) will automatically be taken following each use of a new grating element or new tilt position and after ~1 orbit in any one setting in order to allow calibration of the zero point of the wavelength (dispersion) and spatial (perpendicular to dispersion) axes in the spectroscopic science data during post-observation data processing. These routine, automatically occurring, wavecal observations are expected to provide sufficient wavelength zero point accuracy for the vast majority of GO science. Only if your science requires particularly accurate tracking of the wavelength zero points do you need to insert additional wavecal observations in your exposure sequence (see also Chapter 11
At the conclusion of each ACCUM
exposure, the science data are read out from the detector in use and placed in STIS’ internal memory buffer, where they are stored until they can be transferred to the HST data recorder (and thereafter to the ground). This design makes for more efficient use of the instrument, as up to seven CCD or 1024 ×
1024 MAMA, or two 2048 ×
2048 MAMA (see Section Highres
) full-frame images can be stored in the internal buffer at any time. A frame can be transferred out of the internal buffer to the data recorder during subsequent exposures, as long as those exposures are longer than 3 minutes in duration.
STIS’ internal buffer stores the data in a 16 bit per pixel format. This structure imposes a maximum of 65,536 data numbers per pixel. For the MAMA detectors this maximum is equivalent to a limit on the total number of photons
per pixel which can be accumulated in a single exposure. For the CCD, the full well (and not the 16 bit buffer format) limits the photons per pixel which can be accumulated without saturating in a single exposure. See Chapter 7
and Chapter 11
for a detailed description of detectors and data taking with STIS.
STIS’ three detectors do not
operate in parallel with one another—only one detector can be used at any one time. Exposures with different STIS detectors can, however, be freely interleaved in an observing sequence, and there is no extra setup time or overhead in moving from one detector to another. The three detectors, sharing the bulk of their optical paths, also share a common field of view of the sky.
be used in parallel with any of the other science instruments on HST. Figure 3.2
shows the HST field of view after SM4. Dimensions in this figure are approximate; accurate aperture positions can be found on STScI’s Observatory webpage under “Pointing”.1
The STIS dispersion is along AXIS1
and the slits are parallel to AXIS2
. The policy for applying for parallel observing is described in the Call for Proposals. We provide suggestions for designing parallel observations with STIS in Section 12.9
. While the STIS CCD can always be used in parallel with another instrument, there are some restrictions on the use of the MAMA detectors in parallel, as described in Section 2.8