2.7 Overhead Times

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 given in the Phase I and Phase II Proposal Instructions should be followed to develop Phase I and II proposals, respectively. (See also Section 6.6.)

1. Telescope alignments. A telescope alignment is, in practice, a set of images uninterrupted by 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 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 (currently 6 minutes), or reacquisition (currently 5 minutes).
2. 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.
3. CCD clearing. Clearing the CCD is done before every exposure and requires 16 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 16 seconds rounded up to the next integral minute. For example, all legal exposure times up to 40 seconds inclusive cost one minute.
4. 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). 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.
5. Dithering. 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 spatial offsets will require additional overheads, which must be included. POS-TARG special requirements can also be used to generate offsets.
6. During early proposal Cycles "spatial scans" were used to effect series of offsets, but they are no longer available, and have been replaced with the "dither" commands. The overhead of a spatial scan was similar to that of a sequence of images taken in one alignment; however, at least one minute of overhead was required for each change in pointing. Furthermore, an extra minute of overhead was incurred at the end of the scan, and typically about 1 minute of overhead was used at the beginning of the scan positioning the first image.

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 typical sequence of events is:

• Return filter wheel to "clear" position, select new filter (1 min).
• Expose image (minimum 1 min).
• Readout CCDs (1 or 2 min depending on exposure time and CLOCKS). Hence a simple exposure requires a minimum of 3 minutes.

  Table 2.4: Instrument Overheads. The first and last exposures of an alignment contain extra overheads.

  Overhead Type	Time (min.)	Overhead
  First exposure	1	Major frame uncertainty, clock synchronization
  First exposure	1	To put in initial filter
  Per image:	1	Per filter change
  Per image:	int(t)+1	t=(shutter-open time in seconds +16 seconds)/60
  Per image:	1	If CLOCKS=NO (default) and exposure 180 sec
  Per image:	1	Readout
  Per image:	1	If image done in parallel with another instrument
  Last exposure	1	Data recorder overhead
  Last exposure	2	If data requested down in real-time
  Last exposure	1	If a scan was done