CHAPTER 2:
Special
Considerations
for Cycle 17
In this chapter . . .
2.1 ACS Repair During SM4 / 6
2.2 Dithering in Cycle 17 / 6
Three-Gyro Guiding
We anticipate that all HST observations in Cycle 17 will be undertaken
in the Three-Gyro Science Mode, providing greater scheduling flexibility and greater sky coverage (at any epoch) than in Cycles 14, 15 and 16.
Proposers should plan their observations accordingly, using the Target
Availability Table in the Primer (Table 6.1) and the visibility tools on the
HST Phase I Proposal Roadmap Web site.
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6
2.1
Chapter 2: Special Considerations for Cycle 17
ACS Repair During SM4
After the failure of the side-2 electronics of ACS in January 2007, and
the previous anomaly in the CCD low voltage power supply (LVPS) of
side-1 in June 2006 the only functional channel of ACS has been the Solar
Blind Channel.
NASA Headquarters has approved a plan for the execution of the ACS
repair on SM4. The repair concept replaces the CCD electronic box (CEB)
in the Wide Field Channel. The new CEB will be powered by a
replacement LVPS that will be completely independent of the original unit,
which failed in June 2006. While the highest priority is restoring the WFC,
the ACS repair concept also provides a path for restoring the High
Resolution Channel by powering the original CEB of the HRC channel
with the new LVPS.
Most of the ground testing of the new electronics will be carried out
during the fall and winter 2007, after the publication of this document.
Some of the CCD performance may be impacted by the new electronics
and by the eighteen months of continuous radiation damage without
regular annealing cycles. These changes may include read noise, dark
current, hot pixel contamination and charge transfer efficiency.
Observer should consult the ACS Web site:
http://www.stsci.edu/hst/acs/
for the most up to date information on the predicted instrument
performance and available observation modes. The information on the Web
site will be updated regularly as our knowledge improves.
SM4
2.2
We will use this symbol to highlight trough the text all sections of
parameters that could be affected by the ACS repair. Readers are
encouraged to look for updates on the ACS Web site whenever they
encounter such symbol.
Dithering in Cycle 17
The ACS-WFPC2 Team has long recommended that observers dither
(or offset) their observations to remove hot pixels, cosmetic defects, and
cosmic rays in the combined data. Dithering allows for optimal sampling of
the point spread function and provides substantive improvements over the
use of CR-SPLITs, which do not remove hot pixels or other permanent
cosmetic defects (e.g., bad columns or the WFC interchip gap). Dithering
can be achieved in two ways: 1) By using exposure-to-exposure position
offsets specified explicitly by POS-TARGs, or 2) By using flexible
Dithering in Cycle 17
7
predefined dither patterns. Both POS-TARGs and dither patterns now
result in associations, and dither patterns can also be nested to achieve
various pixel sampling strategies for a wide variety of science goals.
Currently available dither patterns within APT and their recommended uses
are described on the ACS Dither Web page:
http://www.stsci.edu/hst/acs/proposing/dither
We strongly encourage observers to dither their ACS observations. This
is particularly important for cycle 17 given that we cannot predict the
impact of 18 months of continuous radiation damage without periodic
anneals. Observer who choose not to dither their ACS WFC and HRC
imaging exposures should provide sound scientific justification for not
doing so in the Phase I Proposal Description of Observations. The choice
of dither pattern is left to the discretion of the observer since it may depend
upon the science goals of the program. The ACS-WFPC2 Team is ready to
assist observers who need help in selecting the appropriate pattern for their
observations.
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Chapter 2: Special Considerations for Cycle 17