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How are observations affected when only a single guidestar is acquired? In some cases, observations are made using only a single guidestar instead of the usual two. Either the PI consents to this in consultation with the PC when two suitable guidestars cannot be found, or one FGS fails to acquire its guidestar during the guidestar acquisition/reacquisition. 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 for any particular observation, 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, you will need to find the distance of the target from the acquired guidestar. 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. |
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