From: Saul Perlmutter (S_Perlmutter@lbl.gov)
Date: Wed Nov 10 2004 - 11:56:24 PST
attached mail follows:
Your observations taken for proposal 9727 may be degraded because the Fine Guidance Sensors could only acquire a single guide star at November 10, 2004 18:30. In this situation, the roll of the telescope is under GYRO control, which may allow a slow drift of the target on a circular arc centered on the single guidestar. The rate of the drift of the radiant of this circle is unknown, but typically is expected to be in the range of 1.0 to 1.5 milliarcsec/sec (possibly but very rarely as large as 5 milliarcsec/sec).
To calculate the approximate magnitude of the drift of the target on the detector, the distance of the target from the acquired guidestar is needed. The header of the observation log file jif.fits identifies the acquired guidestar (GSD_ID) and gives its right ascension (GSD_RA) and declination (GSD_DEC) in degrees. For example, for a target 10 arcmin from the guidestar, a drift of the guidestar-to-target radiant of 1 milliarcsec/sec during a 1000 second exposure would cause the target to move 0.0029 arcsec on the detector. The direction of the motion on the detector can be deduced from header keywords in the science data describing the position angle of the detector (e.g., PA_APER), in combination with the direction perpendicular to the radiant. In many cases, the drift will be a small fraction of a pixel or slitwidth, although in some cases an image exposure may appear smeared or the target may drift from the slit of a spectroscopic exposure.
Additional information about how single star guiding might affect ACS observations can be found at http://www.stsci.edu/hst/acs/faqs/guide_star.html
These observations were implemented as visit 92 in Science Mission Specification 043137B8. The exposure logsheet lines that defined the observations are listed in the following table:
Log
Sheet Exposure
Number ID Date Mode Target Name
--------------------------------------------------------------------------------
001 J8WQ92010 2004-11-10 17:01 UT ACCUM SUF02-065
002 N8WQK2010 2004-11-10 18:37 UT MULTI SUF02-065
--------------------------------------------------------------------------------
These observations do not qualify for an automatic repeat because some data were collected or your proposal was over 90% complete at the time of this problem. The intention of this report is to provide you with information on problems with the execution of your observations. If you have questions about what went wrong, you should get in touch with your contact scientist, if assigned, or send email to help@stsci.edu. If you find that these problems have compromised the scientific goals of your proposal and you believe that the observations should be repeated, you should submit a Hubble Observation Problem Report (HOPR) within three months of the date that the data were delivered. You can submit a HOPR via the Hubble Observation Problem Reporting Page (http://hst.stsci.edu/Post-observation/reporting_problems). If the observations are repeated, the existing data will become public.
- Automatic AlertPI Exception Report Generator
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