Introduction

All STIS spectroscopy using slits (<3 arcseconds in size) and all coronographic observations will require an onboard STIS target-acquisition (ACQ) and possibly an acquisition/peakup exposure (ACQ/PEAK) to center the target in the scientific aperture. In this Chapter, we provide the basic information you need to choose an acquisition strategy for your program.

STIS target-acquisitions employ the CCD camera to image the target's field directly and onboard flight software processes the image to locate the position of the target. STIS acquisitions are very reliable, accurate (typically ±0.01 arcsecond for V < 21 point-sources), and quick (~6 minutes). For the narrow slits (0.1 arcsecond), an ACQ/PEAK is required, which is accurate to ~5% of the slit width used in the peakup, and takes typically ~6 minutes. For particularly faint targets (V > 21) or complex diffuse sources, overheads will be somewhat more and accuracies somewhat reduced (see details below).

For Phase I proposals, you do not need to determine the details of your acquisition, but need only to determine if an ACQ, and possibly an ACQ/PEAK, is required, include the necessary orbital time (which is normally dominated by the associated overheads) and assure yourself that your program can be accomplished.

For Phase II, you will need to work out the details of your acquisition procedure, and we provide two tools to assist in this task, as well as examples of different TA scenarios (see Examples). To determine the correct exposure time, we provide (via the STIS web page) a Target Acquisition Exposure Time Calculator (TA ETC). The input and output parameters in the TA ETC (as compared with the Imaging ETC) are specifically designed to facilitate exposure-time estimates for target-acquisition purposes. For example, the TA ETC input and output parameters take into account the following:

• The target acquisition can be done only with the CCD detector.
• The CHECKBOX (see below) size is always 3 x 3 pixels for a point-source, and n x n, where n is an odd number between 3 and 101, for diffuse sources.
• The CCDGAIN is always 4.
• The default S/N is 40 for the calculation of exposure time.

To determine the correct checkbox size for DIFFUSE targets, we provide a Target Acquisition Simulator (TAS), which implements the same algorithms as the flight software, and so should give results in good agreement with what will happen in orbit. The TAS takes as input an image, extracts a subarray centered on the coordinates provided, and searches for the brightest location by passing a checkbox over the subarray.

Below we describe acquisition and peakup exposures for spectroscopy. More details on acquisitions and target acquisition centering of targets behind the coronographic bars and wedges are described in Coronographic Spectroscopy, but examples are provided at the end of this chapter.

Initial Pointing

Following the initial guide-star acquisition for your visit, the target location in the aperture plane will be known to an accuracy of ~1-2 arcseconds. For scientific observations taken through spectroscopic slits which are less than 3 arcseconds in either dimension, and for imaging observations taken using one of the coronographic apertures, you will need to use an onboard STIS target acquisition and possibly an acquisition peakup exposure to center your target.

Figure 8.1 shows a decision flow for determining whether you require an acquisition or an acquisition and peakup to center your target.

Figure 8.1: Determining Acquisition Requirements

Acquisitions

STIS target-acquisition exposures (MODE=ACQ) 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. Acquisition exposures center your target in the slit or behind a coronographic bar to an accuracy (2) of ~0.01 arcsecond for a point-source, and 0.01 to 0.1 arcsecond for a diffuse object (larger targets have larger errors). A typical STIS point-source target acquisition takes ~6 minutes.

Peakups

An acquisition peakup exposure (MODE=ACQ/PEAKUP) must be taken following the target-acquisition exposure to refine the target acquisition centering of point or point-like sources in slits less than or equal to 0.1 arcsecond wide (or tall). Peakup exposures use a slit or coronographic aperture and are taken with the CCD as the detector and with either a mirror or a spectroscopic element in position on the grating wheel. Typical target acquisition centering accuracies following a peakup sequence are 0.05 times the dimension of the slit or bar. Typical STIS imaging point-source peakups take ~5-10 minutes; see Table 8.5 for the formulae needed to determine the duration of a peakup acquisition.

Figure 8.2 shows the complete decision tree for STIS target acquisitions.

Drift Rates

For most exposures, two guide stars will be used to support the observation. The drift rate of HST is less than 10 milliarcseconds per hour; thermal drifts internal to STIS at the slit plane are typically less. For some targets, however, it may not be possible to find a guide star pair to support the observation. In these cases, you must either use single guide stars, or (if possible) relax the scheduling requirements (e.g. expand the ORIENT range). If you need to use a single guide star, you should note that the pointing stability is degraded over the 2 guide star case, with a drift rate that is a function of on-target time. Table 8.1 gives the worst case object motion on a single guide star.

Table 8.1: Target Position Shift in Arcseconds over Time

	Seconds (x1000)						Orbits
	1	2	5	10	15	20	1	2	3	4
Drift Rate (milliarcsec)	0.005	0.01	0.03	0.06	0.08	0.10	0.03	0.06	0.10	0.13

For example, if a science observation in an 0.2 arcsecond slit is 3 orbits in duration, then the target would move to the edge of the aperture; a 2 orbit visit would leave the target halfway out. Thus, only single orbit visits should be done on a single guide star. However, for science in a 2 arcsecond slit, the motion over 4 orbits only takes the target 13% of the way out of the slit. Thus, if high photometric accuracy is not required, a single guide star should be sufficient for the larger slits.

Figure 8.2: Process of Defining a Target-Acquisition Scheme