The FES can be used to estimate a visual magnitude of stars and other objects with centrally-condensed brightness distributions. This magnitude may be used to modify exposure time estimates for a variable target. The most recent FES calibration is that performed by Pérez (1991), who found:
And by using the same Color correction determined for the previous reference point (Imhoff and Wasatonic 1986), namely,
CTS(corr) = CTS(obs)[1 + 0.022 x (-2.653 - focus step)]
COLOR = -0.271087 (B-V) - 0.063880 (B-V)² + 0.137764 (B-V)³
= -2.5 log ( CTS(corr) / [1 - 1.2 x 10^(-4) CTS(corr)^0.781] ) + COLOR + K
CTS(obs) = counts OUT (of the aperture) recorded on the script,
K = 10.995 ± 0.078 for underlap mode,
K = 16.350 ± 0.063 for overlap mode,
and CTS(obs) = CTS(obs)/4 for slow track.
The term in the denominator is the dead-time correction and is especially important for stars with large FES counts (>5000). The FES tracking mode is recorded on the script. Underlap mode is used for stars brighter than 4.5 magnitude. Slow track (which integrates four times longer than fast track) is required for stars fainter than magnitude 11.5. No FES photometric calibration has as yet been officially adopted by Three-Agency agreement. The raw FES counts of a target, measured at the reference point before the exposure is started, are written on the script by the TO and (after April 1986) automatically recorded by the ground system in the event round-robin portion of the science header.
Should greater accuracy in derived FES magnitudes be desired, you are advised to make differential measurements using a nearby star with a color and magnitude similar to that of your target, or by using a field star as described by Rahe et al. (1980). See also Holm and Rice (1981). The "FES scattered light anomaly" experienced after January 22, 1991 contributes an additional complication to obtaining accurate FES magnitudes.