###Exposure Time Calculator###

     * What overhead do I incur if I change ACS detectors during an
     observing sequence?
     When proposing for HST time in Phase I, each user
     is asked to estimate the number of orbits needed for
     his/her observations accounting not only for the exposure times,
     but also for overhead times, e.g. readout time, positioning of
     filter wheel, or other.
     Within an orbit, there are additonal overhead times if
     switching from HRC to SBC or switching from SBC to HRC, which
     are 17 and 14 minutes respectively.
     For a complete discussion of overheads, please read Chapter 9
     in the ACS Instrument Handbook.