Any HST data taken with a paired aperture prior to January 1, 1995 may need manual editing of certain .d0 header fields prior to recalibration with calfos.
Several programs used both the upper and lower portions of a paired aperture with Exposure Logsheet (or RPS2) entries of STEP-PATT=STAR-SKY, OBJ-OBJ, or OBJ-SKY in order to sample different portions of extended objects in alternating 10-second dwells throughout an exposure. The diagnosis of this problem is slightly complicated, but in the following, please realize that whenever "chopping" between apertures was invoked, the data recorded through each aperture segment were always recorded separately so that correct calibration of the data from each aperture segment is possible after the manual header update.
Prior to January 1, 1995 important keywords in the headers of all FOS paired aperture exposures for STEP-PATT not equal to SINGLE were constructed on the assumption that SKY was sampled in one aperture segment. This occurred even if STEP-PATT=OBJ-OBJ had been specified in the logsheet. Additionally, prior to February 1, 1994 the only way to obtain spectra with both segments of the paired apertures in this alternating fashion was to specify the potentially misleading STEP-PATT=STAR-SKY. Naturally, calfos calibrates these exposures as if sky subtraction is intended and the final .c1 file contains the difference between nominal default star and sky apertures.
As a general rule, no sky subtraction observations were intended by any observer in the FOS science program after SV ended (January 1, 1992). On a very few occasions OBJ-SKY observations were obtained through implementation error when only OBJ (i.e., STEP-PATT=SINGLE) was intended. Particularly for extended targets, you should broadly assume that any SKY observation, whether explicitly requested in the logsheet STEP-PATT or not, may contain additional spectra from nearby regions of an extended source and should be recalibrated only after header update.The simplest approach to correcting this problem is to change the YSTEPn keywords (i.e., YSTEP1, YSTEP2, etc) from "SKY" to "OBJ." Values of "NUL" or "BCK" should not be changed. Modern versions of calfos (revised after January 1, 1995) will process these updated data correctly and produce separate calibrated spectra for the two aperture segments.
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Last updated: 01/14/98 14:47:13