WFC3 Instrument Handbook for Cycle 24

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6.2	Specifying a UVIS Observation

Using the Astronomer’s Proposal Tool (APT), it is relatively simple to specify the parameters for a UVIS observation. Detailed documentation of the specifications can be found in the Phase II Proposal Instructions. The parameters available to General Observers for the UVIS channel are the following:

1.	Configuration: always WFC3/UVIS

2.	Mode: always ACCUM

3.	Aperture: must be specified; see Section 6.4.4 and Section 6.4.5 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 6.5

5.

Optional Parameters: the options include FLASH (to post-flash a specified number of electrons to reduce CTE losses; Section 6.9.2), CR-SPLIT (to split an exposure into a specified number of subexposures for cosmic ray rejection - but note that CR rejection can also be accomplished by combining dithered exposures; Section 6.11.2), BIN (to perform on-chip binning by a specified number - but beware that this increases the area affected by bad pixels and cosmic rays; Section 6.4.4), BLADE (to make short exposures with less vibration by selecting shutter blade A; requires approval - see Section 6.11.4), and INJECT (to implement charge injection to reduce CTE losses - but the use of this option is not generally permitted for science observations; see Section 6.9.2).

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 6.4.3 for a discussion of the UVIS channel coordinate systems, Section 6.4.5 for reference points, and Appendix C for the POS TARGs associated with the WFC3 patterns).

7.	Number of Iterations and Time per Exposure: the exposure time for the UVIS channel must be an integer multiple of 0.1 s, ranging from 0.5 to 3600 s, except that 0.6 s is not allowed; see Section 6.7.1 for details).