Efficient use of the WFPC2 requires an understanding of the overhead times of the instrument. In this section, the various causes of overhead are presented in a manner that should allow the user to make a fairly accurate prediction of the cost in time of a given sequence of exposures. This information is provided for completeness and background. Guidelines in the Phase I proposal instructions and RPS2 should be followed to develop Phase I and II proposals, respectively. (See also "Choosing Exposure Times" on page 152.)
In summary, it is not possible to schedule exposures in different filters less than 3 minutes apart: commands to the WFPC2 are processed at spacecraft "major frame" intervals of one minute. A filter wheel may be returned to its "clear" position and another filter selected in one minute. An exposure takes a minimum of one minute, and a readout of the CCDs takes one or two minutes depending on the exposure time. Hence a simple exposure requires a minimum of 3 minutes.
- Telescope alignments. A telescope alignment is, in practice, a set of images uninterrupted by a POS_TARG target position change or the end of orbit. The start of an alignment requires 1 minute overhead in order to synchronize timing with a major frame (all commands to the instrument take place on major frames which last 1 minute). The end of alignment uses one minute for tape recorder overhead. If scans are being performed, another minute of overhead is required and, if images are requested in real-time, another 2 minutes must be added to the alignment end. There are additional overheads at the start of each target visibility period associated with guide star acquisition (9 minutes), or reacquisition (6 minutes).
- Filter changes. A filter change requires at least 1 minute, the use of 2 filters requires 2 minutes of overhead. Furthermore, since the filter history is lost across telescope alignments, at least one minute is spent on selecting the filter at an alignment start, regardless of the filter in place before the alignment.
- CCD clearing. Clearing the CCD is done before every exposure and requires 14 seconds. This time is part of the first major frame of the exposure. Therefore, time taken for a given exposure (excluding all other overheads) is the exposure time plus 14 seconds rounded up to the next integral minute. For example, all legal exposure times up to 40 seconds inclusive cost one minute.
- CCD readout. The readout time for an exposure is one minute. An additional minute is required for exposures 180 sec. or longer, taken with CLOCKS=NO. This extra minute can be saved by using CLOCKS=YES, but this is not generally recommended (see section 2.6, "Serial Clocks", on page 27). If the exposure is CR-SPLIT, the readout overheads (calculated with the split exposure times) are of course doubled. There is normally no overhead time advantage in reading out a subset of the CCDs. The exception is if the WFPC2 readout occurs in parallel with the operation of a second instrument, when at least 2 minutes is required to read all 4 CCDs.
- Spatial scans/Dithering. Spatial scans are specially designed sequences of images taken with telescope pointing changes periodically placed between successive images. The pointings in a spatial scan must either be equally spaced points on a single line, or a grid of points formed by the intersection of two sets of equally spaced parallel (but not necessarily mutually perpendicular) lines. Scans avoid the much larger alignment overheads associated with the use of POS-TARG special requirements. Dithering is the use of small spatial displacements to allow better removal of chip defects and/or the reconstruction of sub-pixel resolution. During Phase II the user will be given access to "canned" dithering routines which will allow him/her to avoid many of the tricky details involved in planning spatial scans. The overheads in these canned routines is the same as that of a user-planned spatial scan.
- Spatial scan vs. sequence. The overhead of a spatial scan is similar to that of a sequence of images taken in one alignment; however, at least one minute of overhead is required for each change in pointing. Furthermore, an extra minute of overhead is incurred at the end of the scan and typically about 1 minute of overhead is used at the beginning of the scan positioning the first image.
Table 2.4: Instrument Overheads. The first and last exposures of an
alignment contain extra overheads.