Once the telescope acquires its guide stars, your target will be within 0.2-0.3 arcsec
of the aperture center when using GSC-II. This is significantly better than the 1-2 arcsec accuracy prior to Cycle 15 when GSC-I was being used. For science observations taken through apertures less than three arcsec in either dimension, and for observations involving the coronagraphic bars, a target acquisition exposure should be taken to center the target in your chosen science aperture. Furthermore, if either dimension of the aperture is ≤
0.1 arcsec, the acquisition exposure should be followed by one or more peakup exposures to refine the target centering of point or point-like sources. The nominal accuracy of STIS point source (V < 21 mag) target acquisitions is 0.01 arcsec, with a peak-up accuracy of 5% of the slit width used. Acquisition exposures always use the CCD, one of the filtered or unfiltered apertures for CCD imaging, and a mirror as the optical element in the grating wheel (as opposed to a dispersive element). Peakup exposures use a science slit or coronagraphic aperture, the CCD, and either a mirror or a spectroscopic element in the MSM.
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 an uncertainty (lack of repeatability) of roughly ±
3 pixels. In addition, thermal effects cause the spectrum to drift slowly with time (typical drifts are 0.1 pixels per orbit, with extreme cases of forced large temperature swings as high as 0.35 pixels per orbit). An internal calibration lamp observation (wavecal
) is automatically taken, following each use of a new grating element or new scan position (grating tilt) and every 40 minutes thereafter, in order to allow calibration of the zero point of the wavelength (dispersion) and spatial (cross-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 large 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.
At the conclusion of each exposure, the science data are read out from the detector
in use and placed in the STIS internal buffer memory, 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 four MAMA full frame images can be stored in the internal buffer at any time. The frames can be transferred out of the internal buffer to the data recorder during subsequent exposures, as long as those exposures are longer than three minutes.
The STIS internal buffer stores the data in a 16-bit per pixel format. This format
imposes a maximum of 65,536 data numbers per pixel. For the MAMA detectors, this number is equivalent to a limit on the total number of photons
per pixel that can be accumulated in a single exposure. For a single exposure with the CCD, the gain amplifier saturation level (33,000 e-
) limits the total counts per pixel that can be sustained at GAIN=1
, while the CCD full well (144,000 e-
or 36,000 DN) limits the total counts per pixel that can be sustained at GAIN=4
The three STIS detectors do not operate in parallel; only one detector can be used at
one time. Exposures with different STIS detectors can, however, be freely interleaved in an observing sequence, which incurs the overheads associated with changes in grating and aperture. The three detectors, sharing the bulk of their optical paths, also share a common field of view of the sky. While the STIS CCD can always
be used in parallel with any of the other science instruments on the HST
, there are restrictions in the parallel use of the MAMA detectors.