Location of FOS Spectra: Cycle 4



Anuradha Koratkar, Charles (Tony) Keyes, Sherie Holfeltz

Space Telescope Science Institute



Instrument Science Report CAL/FOS--133

January, 1995







Abstract



The optimal locations of the FOS spectra were determined for all
detector/grating combinations using calibration maps obtained during
April - October 1994 (proposal ID 5530).  New optimal locations of the
spectra determined during the analysis have been updated in the Project
Data Base (PDB) for each aperture/detector/grating combination and
became effective as of 4 Nov. 1994.

The analysis shows that there is a spatial drift with time in the
positions of the spectra for all gratings observed with the FOS/BLUE
detector while the locations of the spectra for all gratings and the
FOS/RED detector are scattered. The amount of scatter in the mean
y-base location has increased since on-board GIM (geo-magnetically
induced motion) correction started and routine DEPERM (clearing of
the ambient magnetic field in the detector) was turned off. A test has
been scheduled to check the effect of DEPERM on the scatter in the
YBASES.

The uncertainties in the locations of the spectra have affected both
ACQ/BIN pointing accuracy and FOS photometric accuracy. The size of
these uncertainties with ACQ/BIN has forced us to follow ACQ/BIN with a
time-consuming ACQ/PEAK to improve the target centering for science
with all the small (smaller than 1.0) apertures. The photometric
quality of the data, especially for FOS/RED and the 1.0 aperture, are
compromised due to the large fluctuations in the location of the
spectrum. Further, this is not a simple matter of losing light, but the
effect is also wavelength dependent. On average, assuming that the
locations of the spectra are known to an accuracy of only 20 YBASE
units, for the large apertures (greater than or equal to 1.0) we can
lose 3% of the photometry for point sources and can lose up to 20% for
extended objects!