WFC3 STAN Issue 32, July 2020

1. Updated CALSPEC and Schedule for Implementation of Flux Calibration Changes for HST Instruments

Motivated by improvements in stellar atmosphere models, in March 2020 the CALSPEC models for the primary spectrophotometric standard stars were updated, and, in addition, a re-examination of Vega’s spectral flux resulted in a ~0.9% (grey) increase in its absolute flux.  The net result of the March 2020 changes is an increase in flux of up to 3% in the UV at wavelengths shorter than 0.15 microns, 2% increase between 0.15 and 0.4 microns, and an increase of 2% to 4% increase longward of 8 microns. From 0.4 to 8 microns, the change is <1.5%.  Details are described in Bohlin, Hubeny and Rauch 2020, AJ, 160, 21. This update is designated as CALSPECv11.

CALSPEC is the database of the composite spectra of stars used as flux standards for HST. Stellar atmosphere models are available for a large fraction of the stars that are recommended as standards. Spectral energy distributions (SEDs) are based on observations made with one or more space instruments.  CALSPECv11 updated the standard star models and the SEDs based on STIS, WFC3 and/or NICMOS spectroscopy (i.e. identified as _stis_, _stisnic_, _stiswfcnic_ ).  FOS and IUE spectra have not been updated.  CALSPECv11 files are available in the current CALSPEC directory. All versions of CALSPEC models and SEDs are available in the full database at https://archive.stsci.edu/hlsps/reference-atlases/cdbs/calspec/.  Further CALSPEC information is available here.

Each of the HST Instrument Teams will be updating flux calibrations and zeropoints over the next year, along with the concomitant throughput, sensitivity tables and photometry tables, as indicated below.

The ACS Team is preparing updated, CALSPECv11-consistent, absolute flux calibration to be released in September 2020 for most of its filter complement.  Solar Blind Channel filters with  revised zeropoints will include F115LP, F122M, F125LP, F140LP, F150LP, and F165LP.  High Resolution Channel filters with revised zeropoints will include F220W, F250W, F330W, and F344N.  Combined-use Wide Field Channel and High Resolution Channel filters with revised zeropoints will include F435W, F475W, F502N, F555W, F550M, F606W, F625W, F658N, F660N, F775W, F814W, F892N, and F850LP.

The WFC3 team, to keep the flux calibration of its two channels consistent, will update the UVIS and IR channel absolute flux calibrations and zeropoints to CALSPECv11 simultaneously, and will provide time-dependent UVIS zeropoints. This work is expected to be completed by fall 2020.

The COS team will use the new CALSPEC 11 models to recalibrate its photometric throughput tables (FLUXTABs) and time dependent sensitivity tables (TDSTABs). The update will include regenerating the FLUXTABs and TDSTABs for both the FUV detector at all lifetime positions and the NUV detector. The COS team will be carrying out this work during 2021.

The STIS team will coordinate efforts to deliver imaging zeropoint updates in line with ACS and WFC3 using the new CALSPEC v11 models.  Updates will be made to the imaging photometric conversion tables (PHOTTABs) and image photometry keyword table (IMPHTTAB) by Winter 2020/2021.  In the coming year, work will be carried out to derive new sensitivity curves for the spectroscopic modes where changes to the CALSPEC models exceed relative or absolute flux accuracy requirements.  Updates for the most impacted modes will be prioritized with new spectroscopic PHOTTAB deliveries in Summer 2021.

For any questions concerning the flux calibrations, please contact the HST helpdesk (https://stsci.service-now.com/hst).

Acronyms:

ACS: Advanced Camera for Surveys

COS: Cosmic Origins Spectrograph

FOS: Faint Object Spectrograph (retired HST instrument)

IUE: International Ultraviolet Explorer

NICMOS: Near Infrared Camera and Multi object Spectrograph (inactive HST instrument)

STIS: Space Telescope Imaging Spectrograph

WFC3: Wide Field Camera 3

SED: spectral energy distribution

2. Updates to Suggested Observation Practices: CTE-Mitigation and Supplemental UVIS Dither Patterns

Two ISRs (ISR 2020-07, ISR 2020-08) and a White Paper (CTE White Paper 2020)were recently published to provide updated suggestions for UVIS observation planning.  These suggestions help users ensure better CTE mitigation and a sub-pixel dithering.

We have developed a new model for WFC3/UVIS's CTE losses and will be releasing it in the pipeline this fall as a pixel-based correction. Details of the new model and how it differs from the previous model are provided in the new ISR 2020-08. Further, we are now recommending that the target background level for full-frame images be increased from 12 e-/pix to at least 20 e-/pix to ensure that CTE losses remain at a manageable level. A high-level summary of these updates is available in a White Paper published on the WFC3 page.  Both the White Paper and the ISR provide more detailed recommendations for GOs to use in planning future observations.  Also, Cycle 28 GOs should feel free to contact their contact scientist (CS) with any additional questions or concerns they may have regarding these updated suggestions.  

Lastly, ISR 2020-07 evaluates dithering strategies used by WFC3/UVIS observations in the archive and finds that users would benefit from some additional patterns for small dithers.  Small dithers are used for improved sub-pixel sampling and defect mitigation.  The study constructs coherent dither patterns for datasets that have between N=2 and N=9 similar exposures.  The study finds that a set of 9 dithers is able to sample the scene finely enough to recover all the structure in a WFC3/UVIS image.  For smaller numbers of dithers (N=2,3,4) we find the DITHER-LINE and DITHER-BOX convenience patterns in APT do an excellent job.  For dither patterns with more steps, GOs should consider the ISR 2020-07 patterns provided in tables of POS-TARGs which can be manually implemented in APT. 

3. New Documentation

ISR 2020-07: Supplemental Dither Patterns for WFC3/UVIS.  J. Anderson.

ISR 2020-08: Strategies for Mitigation of CTE Losses in WFC3/UVIS. J. Anderson.

2020 CTE White Paper:  CURRENT CTE-MITIGATION RECOMMENDATIONS. J. Anderson. 

The complete WFC3 ISR archive is available on this page.

Need help? stsci.service-now.com/hst

To unsubscribe from the WFC3_NEWS list, please send a blank email to WFC3_NEWS-signoff-request@MAILLIST.STSCI.EDU

The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.