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8. WHAT IS THE TYPICAL PHOTOMETRIC PRECISION FOR AN INDIVIDUAL OBSERVATION?
The individual observations of the eight standard stars provide statistics of the photometric precision of the FOS. Table 4 summarizes the broadband repeatability, while Figure 6 illustrates the repeatability graphically for a 5Å wavelength bin size. Individual observations of the four WD standards are ratioed to the WD models in the UV, where the only confusing spectral feature in the denominator spectrum is L-a. Except for the high S/N and for the target acquisition precision of 0.026" in each axis, these observations should be typical of any GO observation of an isolated point source in the larger FOS apertures. There are some 3% features of up to 500Å wide in the top three panels of Figure 6a that may be attributable to changes in the photocathode or to small shifts of the spectrum on the detector cathode. An amazingly precise short term repeatability for observations within ~1-2 days, even with intervening separate target acquisitions, is illustrated by the ratios in the top two panels of both Figure 6a and 6b. The first two observations of G191B2B include the data that determine the flat field correction and are at a typical S/N of greater than 200 per 5Å bin, while the remainder of the data have S/N greater than about 100.
Therefore, the narrow features that are bigger than ~3% must be caused by flat field misalignment or time variability, except for L-a (1216A) in all stars and CIV (1550A) and MgII (2800A) in G191B2B. On the red side, dips at 1930, 2310, and 2580 that grow systematically with time from the top to the bottom of Figure 6b are due to the same changing flat field phenomenon that was present in cycles 1-3. The data exists for defining the correction to the changing flat fields and should be implemented in the pipeline by the end of August 1995. The occasional red side blips of up to ~3% at 2505Å are probably caused by misalignment of the data with the flats.