For a few specific cases, the FCS measurement can last significantly longer than the 32 sec. mentioned in Section 6.3.1. The FCS integration time is tied to the requested S/N on the source: the commanded FCS signal should be determined with a S/N equivalent to 3 times the source S/N with a maximum of 100. The FCS signal is derived by the source plus background flux given by the observer in the AOT, and the S/N is calculated by the AOT logic even if the observer enters an integration time.
Example: suppose one wants to detect 1 MJy/sr with a S/N of 10 on a background
of 1 MJy/sr. To achieve this an integration time of 32 sec is necessary. For
the FCS measurement, however, the logic requires a S/N of 30 for 2 MJy/sr
(= source + background flux). Thus the FCS integration time will become
approximately 
= 72 sec. Since PHT will adopt the next
higher power of 2 sec for the integration time, the FCS measurement will last
128 sec. To lower the FCS integration time one should either lower the value
of the source + background flux, or the S/N, or use the absolute photometry
AOTs (P05 or P25) where the FCS and the integration times are commanded
separately.
This type of overhead is typical for faint targets on low backgrounds such
as faint stars in the 2.5 - 7 
m range.
For the absolute photometry AOTs PHT05 and PHT25 the S/N requirement
for the FCS does not apply.