HST Call for Proposals and HST Primer for Cycle 24

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A.5 WFC3 Examples

A.5.1

Example 1: IR, 1 orbit, 2 filters

The WFC3 IR channel is used to obtain images of a target in filters F110W and F160W. Assume that the ETC has shown that exposure times of 10 minutes for F110W and 20 minutes for F160W are adequate for the science goals. These exposure times can be achieved with the up-the-ramp MULTIACCUM sequences SPARS50 (11.7 min) and SPARS100 (23.4 min), respectively. Also assume an orbital visibility¹ of 54 minutes. A summary of the orbit calculations is shown in Table A.16

Table A.16:

Orbit Calculation for WFC3 Example 1

Action	Time (minutes)	Explanation
Guide-star acquisition	6.0	Needed at start of observation of new target
IR overheads for 2 exposures	2 в 1.0 = 2.0	Includes filter changes, camera set-ups, and readouts
Science exposure in F110W	11.7
Science exposure in F160W	23.4
Total time used	43.1

The total time used in the orbit shows that the target can be imaged in the selected filters within one orbit. Furthermore, the first exposure can be dumped from the buffer during the second exposure. The approximately nine minutes of unused time could be used for an additional exposure, during which the second exposure would be dumped.

A.5.2

Example 2: UVIS, Dithering, 2 Orbits, 1 Filter

This example illustrates the orbit calculation for a UVIS observation with a WFC3 UVIS box dithering pattern, which implements imaging at four pointings. The goal of the observation is to obtain a dithered image of a field such that it bridges the ~1 arcsec inter-chip gap between the UVIS CCDs when the images are combined.

For the purposes of this example, the following conditions are adopted: (1) the exposure time necessary to reach the desired signal-to-noise ratio is 80 minutes. (2) The orbital visibility for each orbit is 58 minutes. (3) Assume that cosmic ray removal will be done using the drizzlepac package; therefore the exposure will not be broken into sub-exposures. (4) One 20 minute exposure is obtained at each pointing in a four-point dither, taken over two orbits.

The orbit calculation for this visit is shown in Table A.17. Note that for a two arcsec offset maneuver, the three dither offsets will take 0.5 minutes each.

Table A.17:

Orbit Calculation for WFC3 Example 2

Action	Time (minutes)	Explanation
Orbit 1
Guide-star acquisition	6.0	Needed at start of observation of new target
UVIS overhead for first exposure	2.6	Includes filter change, camera set-up, and readout
UVIS overhead for second exposure	2.1	Includes readout
Spacecraft maneuver	0.5	To offset from first to second dither pointing
Two science exposures	2 в 20 = 40.0	Exposures at the first two pointings in the dither pattern
Total time used in orbit 1	51.2
Orbit 2
Guide star reacquisition	5.0	Needed at start of new orbit to observe same target
UVIS overheads for 3rd and 4th exposures	2 в 2.1 = 4.2	Includes readouts
Spacecraft maneuvers	2 в 0.5 = 1.0	To offset to the 3rd and 4th dither pointings
Two science exposures	2 в 20 = 40.0	Exposures at the final two pointings in the dither pattern
Total time used in orbit 2	50.2

No overhead is incurred to dump the exposures because they are all longer than 339 seconds. Thus the desired exposures can be accomplished within the two orbits, and there are about seven to eight minutes of unused orbital visibility per orbit that could be used to increase the exposure times.

1

Information about orbital visibility as a function of declination is available in Chapter 6.