###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.