7.2 Specifying an IR Observation

In the HST proposal system, the parameters for an IR observation available to General Observers are:

1. Configuration: always WFC3/IR.
2. Mode: always MULTIACCUM.
   
3. Aperture: must be specified. See Sections 7.4.4 and 7.4.5 for the dimensions, location, and reference point (target placement) of the full array and subarray apertures.
   
4. Spectral Element: must be specified for each exposure; see Section 7.5.
   
5. Optional Parameters: in the IR channel, SAMPSEQ specifies a predefined sequence of times at which the detector is read out, and NSAMP specifies the total number of readouts (and thus the total exposure time); see Section 7.7. SAMPSEQ and NSAMP are required for IR observations.
   
6. Special Requirements: see the Phase II Proposal Instructions for details of Special Requirements related to the timing of visits and for dithering and mosaicking. Also available is the exposure-level Special Requirement POS TARG for offsetting the target from the default reference point of the aperture (see Section 7.4.3 for a discussion of the IR channel coordinate system) and Appendix C for the POS TARGs associated with the WFC3/IR patterns.
   
7. Number of Iterations and Time per Exposure: in cases where two or more identical exposures are desired, the Number of Iterations may be set to 2 or more. In the IR channel, the Time per Exposure is fixed by the combination of SAMPSEQ and NSAMP (see item 5 above) and thus is not specified separately. Through various combinations of SAMPSEQ and NSAMP, it is possible to achieve exposure times ranging from 2.93 to 2756 s for readouts of the full detector array (Table 7.8); considerably shorter exposures are possible using subarrays (Table 7.9).