1.4	Special Considerations for Cycle 21

1.4.1 Current Instrument Status

1.4.2 Ongoing Calibration

1.4.3 New Capabilities

1.4.1 Current Instrument Status

WFC3 is operating as expected on HST, with its detectors at nominal setpoints of –83°C (UVIS) and –128°C (IR). There are no significant anomalies in its performance.

1.4.2 Ongoing Calibration

At the time of this writing, cycle 19 calibration observations have been nearly completed and cycle 20 calibration observations have begun. Descriptions of the calibration plan for each cycle are given in Appendix E:Reduction and Calibration of WFC3 Data. Much of the analysis of the calibration programs from past cycles has already been documented in WFC3 Instrument Science Reports published on the WFC3 Web site; documentation of the remaining analysis is on-going.

Proposers and users of WFC3 should monitor the Cycle 21 announcement Web site for the latest information:

http://www.stsci.edu/hst/proposing/docs/cycle21announce

Additional information will be released as part of WFC3 Space Telescope Analysis Newsletters (STANs) and other updates posted directly to the WFC3 Web site:

http://www.stsci.edu/hst/wfc3

Throughout this handbook, a special “Online” symbol indicates information that is especially likely to be updated on the WFC3 Web site.

1.4.3 New Capabilities

The major new capability of adding a flash at the end of a UVIS exposure (post-flash) was implemented for Cycle 20. Post-flash greatly increases the detection of faint sources in low background observations, where CTE losses would otherwise remove much or all of the flux from the sources. Most UVIS observers should consider using post-flash. It is useful for all UV, narrow band, and relatively short medium and broad band exposures where the detection of faint sources is required. It should replace Charge Injection in essentially all circumstances. (See Section 6.9.2) Those who are considering using Charge Injection rather than post-flash should consult STScI. See Section 5.4.11, Section 6.9, and the WFC3 UVIS CTE Web page for further information on CTE and post-flash.

The observing technique of spatial scanning was introduced for WFC3 in cycle 19. (See Section 6.11.3, Section 7.10.4, and Section 8.6.) This mode can be used to turn stars into well-defined streaks on the detector or to spread a stellar spectrum perpendicular to its dispersion. It is useful for:

•	Observations requiring high temporal sampling and/or time resolution

•	More efficient observation; higher S/N observations of exoplanet transits

•	High precision relative astrometric observations

•	Imaging and spectroscopy of brighter sources than previously possible

Information especially likely to be updated on the WFC3 Web site is indicated by this symbol.