The Herschel Space Observatory is an ESA cornerstone mission that was be launched on 14 May 2009, alongside the Plank cosmic microwave background mission. Originally known as FIRST (Far InfraRed Submillimetre Telescope) its name was officially changed in the year 2000 in recognition of the 200th anniversary of the discovery of infrared radiation by William Herschel in 1800. Herschel covers the range from 55 to 672 microns (530-5000GHz) - a region that is effectively totally closed to ground-based astronomy - using a suite of three state-of-the-art instruments called PACS, SPIRE and HIFI.

Herschel is an observatory mission: that is, its time is distributed among the community instead of being used for a large-scale survey. It is also a consumables-limited mission - its useful life depends on the lifetime of the helium in the dewar that is used to cool the instruments and is expected to be in the range from 3.5 to 4 years. As an observatory mission its success thus depends on the quality of the science that the community carries out with it and how effectively the helium in its dewar is converted into science. The "helium into science" ratio will be the principal deciding factor in allocating time with the Herschel Space Observatory.

Many aspects of the Herschel Space Observatory are revolutionary. It is, thanks to its innovative design, the largest dedicated infrared telescope ever to be launched into space by a considerable margin. For the astronomer this converts into high sensitivity and a spatial resolution a factor of 6 better than any previous far-infrared telescope launched into space, making Herschel a pathfinder mission in the far-IR. In fact, Herschel is limited in sensitivity mainly by the confusion from the background of faint, unresolved sources. This makes Herschel a revolution for astronomy in a range of the far-IR that has hardly been exploited so far. Herschel observations will have a huge impact on astronomy and on our understanding of the universe.

This manual describes the observatory aspects of the mission: the spacecraft and its performance; the mission; the space environment in which the Herschel Space Observatory is operating (very different from previous missions such as IRAS, ISO and the HST); and use of Herschel - from how an observing proposal is received and treated, through to final archiving of the data. The aim is to give an overview of Herschel to the user, describing everything that a potential observer needs to know at a superficial level; where deeper knowledge is required afterwards, the observer should go to the specific documentation for each system or sub-system (e.g. the individual instrument manuals, the Data Processing user manual, etc.) The aim is that simply by reading this manual, or by using it for reference, someone who is planning to request time with Herschel has enough information to decide whether or not to proceed and to have a clear idea how to start.

When this manual was first written for the Guaranteed Time Key Programme Call back in November 2006, the launch of Herschel was still 30 months away and knowledge of how the spacecraft and instruments would behave in space was theoretical. Similarly, some important elements of the Science Ground Segment were still in development. As a result, this manual has undergone a deep revision and its contents have been re-thought and have hence changed considerably to reflect what is now a quite mature operational reality.