6.2 Specifying a UVIS Observation

«Previous Next»

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 add a specified number of electrons per pixel to the image 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 dithering provides improved calibrated science products over CR-SPLIT (see Appendix C), BIN (to perform on-chip binning by a specified number - but beware that this mode 1) increases the area affected by bad pixels and cosmic rays and 2) is not advantageous with post-flash; Section 6.4.4), BLADE (to minimize vibration effects in short <5 sec exposures by selecting shutter blade A; requires approval - see Section 6.10.4), 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). Note that the Exposure Time Calculator provides the best estimate of background level for FLASH calculations, not APT.
  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 dither 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 (Section 6.7.1) and 0.7 s is slightly less favorable than 0.5 s or > 0.8 s (WFC3 ISR 2023-04).
«Previous Next»