1.5 Special Considerations for Cycle 33

1.5.1 Current Instrument Status

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

1.5.2 Ongoing Calibration

At the time of this writing, Cycle 32 calibration observations have begun, and the majority of Cycle 31 calibration observations have concluded. Descriptions of the calibration plan for each cycle are given in Appendix E and on the WFC3 Calibration webpage.

Much of the analysis of the calibration programs from past cycles has already been documented in WFC3 Instrument Science Reports published on the WFC3 website; documentation of the remaining analysis is on-going.

Additional information will be released as part of WFC3 Space Telescope Analysis Newsletters (STANs) and other updates posted directly to the WFC3 website. To sign up for the STAN mailing list, please send a blank email to WFC3_NEWS-subscribe-request@MAILLIST.STSCI.EDU.

1.5.3 Special Capabilities

UVIS SHUTTER BLADE: The capability of commanding the UVIS shutter to use exclusively one of the two sides of the shutter blade in short exposures, resulting in less vibration and PSF smearing, was implemented early in cycle 21. This option will be made available to the observer when it is critical to the scientific success of a program (see Section 6.10.4). For short exposures, observers should avoid 0.7-s exposure times if possible, as the shutter timing jitter is significantly increased compared to 0.5-s or 1.0-s exposures (WFC3 ISR 2023-04).

UVIS POST-FLASH: The capability of adding a flash at the end of a UVIS exposure (post-flash) was implemented in Cycle 20, and APT began to issue diagnostic reports on exposures with inadequate or excessive flash in Cycle 23. 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 wide-band exposures, reducing CTE losses to a level where the pixel-based CTE correction in the calibration pipeline functions best. The use of Charge Injection to mitigate CTE losses is not recommended (see Section 6.9.2). See Section 5.4.11, Section 6.9, and the WFC3 UVIS CTE webpage for further information on CTE and post-flash.

SPATIAL SCANS: The observing technique of spatial scanning was introduced for WFC3 in Cycle 19. (See Sections 6.11.3, 7.10.4, and 8.6.) Spatial scan mode was added to the ETC for UVIS and IR imaging and IR spectroscopy for cycle 24. 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;
  
• High precision relative astrometric observations;
  
• Imaging and spectroscopy of sources brighter than possible via stare mode.
  

HST entered reduced gyro mode in June 2024. This change limits spatial scan speeds to 5 arcseconds, a notable reduction from 3-gyro mode. For more details, please see the HST Reduced Gyro Mode Primer.