|Space Telescope Science Institute|
|WFC3 Instrument Handbook|
This section discusses the overheads involved in preparing the telescope for observations of a target. The next section describes the additional instrument-specific overheads involved in obtaining and storing the actual individual WFC3 exposures.At the beginning of a sequence of WFC3 exposures, the telescope must acquire guide stars. The time required for this initial guide-star acquisition is 6 minutes. If the observations extend into the following orbit(s) following Earth occultation, you must also include the overhead for guide-star re-acquisition (another 5 minutes at the beginning of each new orbit). The only exception to this re-acquisition overhead occurs when you are observing a target in the Continuous Viewing Zone (CVZ; see the HST Primer), in which case guide-star re-acquisitions are unnecessary.Following guide-star acquisition (or re-acquisition) and the first exposure during the orbit, it is likely that you will want to make a series of small telescope motions for the purpose of obtaining dithered observations. Each commanded movement of the telescope requires an amount of time ranging from 0.3 to 1.3 minutes, depending on the size of the motion. Furthermore, each time the telescope is moved more than 2 arcmin (e.g., for the purpose of constructing a mosaicked image), an additional guide-star acquisition will be necessary, requiring another 6 minutes.When using WFC3’s UVIS quad filters, it will often be necessary to repoint the telescope to place the target in the desired quadrant. This repositioning will require 1 minute. Due to the large field of view of WFC3/UVIS, offsets to different quadrants may require new guide stars to be acquired. Offsets between adjacent quadrants (e.g., A to B, or A to C) are small enough to maintain the same guide stars in many cases. However, offsets between diagonal quadrants (i.e., A to D, or B to C) are larger than 2 arcmin when using the “optimum” apertures, so the same guide stars cannot be used for diagonal quadrant offsets. For the “fixed” apertures, diagonal quadrant offsets are very close to 2 arcmin, so it is possible that the same guide stars may be used, depending on the placement of the particular guide stars. To determine whether quad filter observing sequences may require new guide star acquisitions, refer to Table 6.4 for quad filter positions, to Table 6.1 for aperture descriptions, and to the SIAF for aperture reference point definitions.Table 10.1 summarizes the observatory-level overheads involved in WFC3 observing.Table 10.1: Observatory Overhead Times.