3.3 Basic Instrument Operations

Target Acquisitions and Peakups

Once the telescope acquires its guide stars, your target will normally be within ~1–2 arcseconds of the aperture center. For science observations taken through slits which are smaller than 3 arcseconds in either dimension, and for use of the coronagraphic bars, you will need to specify a target acquisition exposure to center the target in the science aperture. The nominal accuracy of STIS point source (V < 21) target acquisitions is 0.01 arcsecond. If either dimension of the aperture is less than or equal to 0.1 arcsecond, the acquisition exposure should be followed by one or more peakup exposures to refine the centering of point or point-like sources. Peakup accuracy is 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. Peakup exposures use a science slit and the CCD, with either a mirror or a spectroscopic element in the grating wheel. Target acquisitions and acquisition peakups are described in detail in Chapter 8.

Routine Wavecals

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).

Data Storage and Transfer

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 7 CCD or 1024 × 1024 MAMA, or two 2048 × 2048 MAMA (see Section 11.1 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.

Parallel Operations

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.

STIS can 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.

Figure 3.2: Post-SM4 HST Field of View.

3.3.1 Typical STIS Observing Sequence

In the optical, STIS is often used to observe extremely faint objects, so long observations are common. The combination of high spatial resolution, spectral resolution, and low read noise from the CCD will encourage the taking of multiple (~1000 minute) exposures to allow cosmic ray rejection. Observations with the MAMA detectors do not suffer from cosmic rays or read noise, but long integration times will often be needed to obtain sufficient signal-to-noise in the photon-starved UV.

A typical STIS observing sequence is expected to consist of an initial target acquisition and possibly an acquisition peakup to center the target in a slit, followed by a series of long (~10–40 minute) exposures with a single optical element at a given wavelength setting. It may also include a series of multiple long exposures taken with different gratings or with a single grating at a number of tilts. Observers will generally not take their own wavecal exposures; routine automatic wavecals will allow wavelength and spatial zero points to be determined in post-observation data processing, requiring no input from the user. Observations at ≥7000 Å should be accompanied by fringe-flat exposures as discussed in Chapter 11.

1 Pointing webpage: http://www.stsci.edu/hst/observing