WFC3 Space Telescope Analysis Newsletter - Issue 23, June 2016
Late-breaking news about WFC3 is posted here.
Previous issues of the WFC3 STAN are archived here.
- Cycle 24 Phase II Deadline Reminder and Updates to APT
- Changes to ETC Calculations
- Improvements to Zodiacal Light Computations in the ETC
- Planning "Drift and Shift" Observations
- New Documentation
Observers who are awarded time on HST for Cycle 24 should note that the submission deadline for Phase II proposals is July 21, 2016. Information regarding Phase II development and submission will be posted here. Cycle 24 phase II proposals must be submitted through the latest version of the Astronomers Proposal Tool (APT 24.2). Updates included in this version of APT will be described here.
Users should be aware that the updated WFC3/UVIS inverse sensitivity values (PHOTFLAM) may result in ETC calculations that are 3% to 7% brighter than in previous versions. New values of the inverse sensitivity for full-frame UVIS filters were recently calculated using the chip-dependent throughputs (WFC3 ISR 2016-01). Previous calculations of PHOTFLAM using AstroDrizzle suffered a 3% to 7% loss in signal due to a subtle effect in the drizzling which resulted in improper flagging of some pixels as cosmic rays. The new inverse sensitivity values are correspondingly brighter, and Synphot files were accordingly updated (WFC3 ISR 2016-07). Therefore, since the ETC uses synphot files to compute exposure times and signal to noise, users may notice that ETC returns slightly different values. A complete description of the changes will be published in WFC3 ISR 2016-03.
ETC 24.2 will be released in June and made available here. Improvements have been made which affect the computation of zodiacal light from an input of RA, Dec, and helioecliptic longitude or date. A new table of zodiacal light as a function of helioecliptic longitude and ecliptic latitude enables the computation of zodiacal light closer to the sun, going slightly within the 50-degree lower limit to the angle between the target and the Sun. The new table uses a finer grid for more accurate interpolation near that limit. An error that affected the computation of the helioecliptic coordinates (and therefore of the zodiacal light) for some target positions has also been corrected.
Ivelina Momcheva, Pieter van Dokkum, Gabriel Brammer, John MacKenty
The “drift and shift” (DASH) method of observations, introduced in Cycle 24 (Momcheva et al. 2016), allows users to carry out wide-shallow observations with the WFC3/IR camera on the Hubble Space Telescope (HST) by guiding on gyros alone. Using the first four orbits of a pilot Cycle 23 program (GO-14114; PI: van Dokkum), Momcheva et al. (2016) showed that both the telescope behavior and the data quality of DASH observations are as expected. Here we provide supporting information to users planning Phase II proposals with this observing method.
The DASH observing strategy needs to balance between the amount of data that can be stored in memory during the exposure, the drift rate, and the sampling frequency. It also depends on the target position in the sky, as that determines the orbital visibility period.For GO-14114, we only considered the COSMOS field (Dec ~ 10 deg). Fields at larger declination have longer visibility windows. We selected a read-out mode that matched the drift rate. With SPARS25 mode (25-second reads), the drift is only about one quarter of a WFC3/IR pixel between consecutive non-destructive reads (see Figure 5 of Momcheva et al. 2016). The maximum number of pointings that fit within an orbit with this readout mode is eight, four with NSAMP=11 and four with NSAMP=12, with total exposure times of 253 and 278 seconds, respectively (see Table 1 of Momcheva et al. 2016). By following this observing strategy, programs can cover 1 square degree in 100 orbits to a depth of H(F160W) = 25.0. While this is the most efficient observing strategy (fraction of orbital visibility used for on-target exposure is 66%) and can be used as a starting point, other variations are possible, depending on the scientific needs of the program. Users are encouraged to adapt the observing strategy to their own science goals.
No modifications to the APT are needed to plan these observations. When using APT to plan DASH observations, we recommend the following procedure:
- On the Proposal Information page, select the “Program Coordinator Use Only” box. This option allows users to manually specify the Pointing Control System (PCS) mode.
- Each tile of the mosaic is defined as an exposure within a visit. The first target within a visit is a Fixed Target. Subsequent targets can be specified either as Fixed Target or as POS TARG offsets relative to the first target (when the observations are carried out, the telescope will step from one pointing to the next, without returning to the starting position). The two methods of specifying the positions of the exposures are equivalent. The POS TARG warning that the offset is larger than the instrument field of view can be ignored.
- The guiding mode is set by “Exp PCS Mode”. This option is in the Form Editor Exposure section “Implementation Requirements” (only visible if PC Use Only is checked). Set this option to “Fine” for the first exposure and “Gyro” for subsequent exposures. The same can be done in the lowest table panel (“Exp PCS Mode” is the ninth field from the right). A guide-star acquisition is required at the beginning of each orbit.
Observers should carefully consider the offset pattern to minimize the length of moves. Offsets up to 1.0 degree are possible with a small angle maneuver (SAM); however, the repositioning overheads increase with the offset size. A table of repositioning overheads for large SAMs is provided below (for SAMs smaller than 2 arcmin, see Chapter 6.4.4 of the HST Primer).
|Offset [arcmin]||Small Angle Maneuver Overhead|
For additional details, users can download and examine the GO-14114 Phase II APT file. Contact Scientists and Program Coordinators will be available to assist users in the preparation of the Phase II proposals.
- ISR 2016-04 UVIS 2.0: Chip-Dependent Flats – J. Mack, T. Dahlen, E. Sabbi, A. S. Bowers
- ISR 2016-09 WFC3 Cycle 22 Proposal 14007: UVIS Gain – C. Martlin, H. Gunning
- ISR 2016-08 WFC3/UVIS Dark Calibration: Monitoring Results and Improvements to Dark Reference Files – M. Bourque, S. Baggett
- ISR 2016-07 Updated WFC3/UVIS Chip-Dependent SYNPHOT/PYSYNPHOT Files – S.E. Deustua
- ISR 2016-10 WFC3/UVIS EPER CTE Cycles Aug 2009–Apr 2016 – H. Khandrika, S. Baggett, A. Bowers
The archive of all WFC3 Instrument Science Reports (ISRs) is here.
The latest version (4.0) of the WFC3 Data Handbook is here.
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