The duration limits for all AORs are currently 18 hours for any Herschel observation as observatory maintenance and the need for a daily uplink means that longer AORs are not practical (for stare observations with a fixed pointing, the limit is 13 hours). If the estimated duration (not including the slew overhead) of your AOR is longer than this time, the software will return a message telling you to make the observation shorter. In general, AORs longer than about 9 hours (half an operational day) may be more difficult to schedule.

If requesting an AOR with a duration close to the maximum permitted, you should be aware of the danger that if the way observations are carried out by the instruments is further optimised, AORs that are "just possible" based on the best time at AO, may become impossible later if the overheads or integration times for a particular instrument or configuration have to be increased.

	Warning
	The 18h limit also applies to concatenated observations. Concatenations that exceed 18h total time cannot be scheduled.