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WFC3 STAN - Issue 17, March 2014

WFC3 Space Telescope Analysis Newsletter - Issue 17, March 2014

New information about WFC3 in the "New in the Last 45 Days" and "Late Breaking News" sections.

This and previous issues of the STAN.

Contents:

Cycle 22 Phase I Deadline Reminder and Updates to APT

M. Bourque, K. Peterson

Solicitation of Hubble Space Telescope Cycle 22 Phase I proposals is currently open.  Cycle 22 runs from October 1st, 2014 to September 30th, 2015.  Users who wish to submit Cycle 22 Phase I proposals must submit through the Astronomers Proposal Tool (APT) before the April 11th, 2014 8:00 PM EDT deadline.  For more information, please visit the Cycle 22 Announcement page, the HST Call for Proposals for Cycle 22, the STScI Phase I Roadmap, and the HST Primer.  Observers needing assistance are encouraged to contact the STScI Helpdesk by sending an email to help@stsci.edu.


Additionally, users planning to submit Phase I proposals are required to install the latest version of APT.  The Cycle 22 release of APT contains the following updates/changes:

  1. Easier Investigator lookup - APT uses a new search method for looking up Investigators in the STScI database. These changes should make it easier to find people even if you do not know the exact spelling, and to find people with multi-word last names.
  2. Easier Investigator update - Two new buttons have been added to APT: One to take you to the web page for updating an address and another for adding a new Investigator. You will see these buttons after searching for an Investigator in the STScI database.
  3. Change in format of the PDF summary - To help the reviewers focus on the science (and not the Investigators), the format of the PDF file APT creates for the TAC has been modified. For more information please see the explanation in the Late Breaking News.
     

For more information, please visit the APT website

Updates to the WFC3 Instrument Handbook for Cycle 22

L. Dressel

We have made the standard updates to the Instrument Handbook, including the addition of references (throughout the Handbook) to the WFC3 Instrument Science Reports that were published in the past year and a summary of the WFC3 calibration plan for the upcoming cycle (Appendix E).  These updates are part of the Version 6.0 release in January, 2014.  Additional updates include:  
  • An introduction to the new BLADE=A option for very short WFC3/UVIS exposures, which can be used (with permission) to avoid vibration-driven PSF smearing (Sections 1.5.3, 6.2, 6.10.4)

  • A new section on persistence as an instrumental effect on the WFC3/IR detector (Section 5.7.9) and updated guidance to observers in predicting and minimizing persistence in WFC3/IR exposures (Section 7.9.4, Appendix D.2)

  • A revised discussion of sky background in WFC3 exposures, including the measurement of zodiacal light as a function of a target’s ecliptic latitude and angular distance from the sun using WFC3/IR exposures (Section 9.7) and investigation of an airglow line that can significantly increase the background level in some of the WFC3/IR filters and grisms (Section 7.9.5)

  • Changes in the WFC3/UVIS dark current (Table 5.1, Section 5.4.8) and the number of hot pixels (Section 5.4.9) which result from measuring dark exposures made with post-flash to reduce CTE losses (Section 6.9.2), since fewer electrons appear to trail out of hot pixels into adjacent pixels

  • Reasons for preferring the lower chip (2) and the lower left quadrant (C) for exposures requiring less than the full field of view of the WFC3 detector (Section 6.4.4)

  • Illustration of a diagram that can be generated in APT (Figure 8.9) to visualize the start and end of a spatial scan, an observing technique discussed in Sections 6.11.3, 7.10.4, and 8.6.

  • Revised examples of using the ETC to derive exposure times and signal-to-noise for UVIS exposures (Section 9.9). 

Elevated Background Levels in the F105W, F110W Filters and IR Grisms

G. Brammer, N. Pirzkal, P. McCullough, J. MacKenty

Diagnosis:
Variations in the background within an orbit timescale can be seen in some of the WFC3/IR broad-band filters and grisms. The calwf3 pipeline assumes constant count rates throughout an exposure in order to flag cosmic rays and bad pixels; some of the observed background variations are strong enough so that a large fraction of the entire detector is flagged as cosmic rays by calwf3, resulting in significantly non-gaussian noise of the FLT pipeline products.

The dark “IR Blobs” seen in WFC3/IR images and the imaging flat-fields can be used to estimate the spectrum of the source that causes the variable background levels.  In grism observations taken while the telescope is in the Earth shadow, the blobs create a smooth depression in the grism background corresponding to the relatively smooth zodiacal continuum spectrum.  During times of elevated background levels, however, the spectrum becomes point-like, suggesting a strong emission line component.  Extracting a negative spectrum of the IR blobs, we find that the line is seen at the same wavelength in both the G102 and G141 grisms at 10,830 Å, which we identify as coming from metastable helium atoms in the upper atmosphere and which has been previously observed from the ground during twilight (Shefov 1961; Bishop & Link 1999).

We find that the 10,830 Å line contributes to the IR background whenever the telescope itself is out of the Earth shadow.  Its strength increases with decreasing target-to-limb angle, i.e., a longer path length through illuminated atmosphere, and it contributes an additional 0.5-1 e-/s to the IR background even at large limb angles with the telescope looking closer to zenith.  For the worst case of day-side observations that graze the limb, the observed background can be as much as 5-6 times higher than the nominal zodiacal light for individual reads.  


Mitigation:
Observers planning background-limited observations in the Y-band at 1 µm should consider using the F098M filter in place of F105W, as the former is not sensitive to the 10,830 Å feature.  

For archival exposures with strongly variable backgrounds that result in many pixels flagged as cosmic rays and/or non-gaussian noise in the FLT images, one can rerun calwf3 setting CRCORR=OMIT in the raw files to turn off the linear-ramp fitting and cosmic ray rejection.  This technique loses some dynamic range for saturated sources whose flux would normally be recovered from the unsaturated part of the ramp.  Furthermore, the FLT products will contain all of the incident cosmic rays, though these can be effectively identified given a sufficient number of dithered exposures with software such as AstroDrizzle.

Further information on the Helium emission line background can be found in WFC3 ISR 2014-03 (Brammer et al. 2014).


References:
Bishop, J. & Link, R., 1999, J. Geophys. Res., 104, 17157
Shefov, N. N., 1963, Planet. Space Sci., 10, 73 

Update (04/08/14): In section 4.3 of ISR-2014-03 (Brammer et al. 2014), we provide additional information for users considering trading the F105W filter for the F098M filter, which is not sensitive to the He 10,830 Å line.

New Documentation 

ISR 2014-01 Flux Calibration Monitoring: WFC3/IR G102 and G141 Grisms - J. Lee, N. Pirzkal, B. Hilbert

The complete WFC3 ISR archive is at/hst/wfc3/documents/ISRs/
Need help? /hst/wfc3/help.html

 


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