The Herschel Space Observatory is an ESA cornerstone mission that was launched on 14 May 2009, alongside the Planck 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 covered the range from about 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 was an observatory mission: that is, its time was distributed among the community instead of being used for a large-scale survey. It was also a consumables-limited mission - its useful life depended on the lifetime of the helium in the dewar that was used to cool the instruments and was expected to be in the range from 3.5 to 4 years from launch. The temperature in the cryostat started to rise at the end of Operational Day (OD) 1446 of the mission and the end of mission was declared early on OD-1447, just over 2 weeks short of completing 4 years in space; you can find information on the events that constituted the end of mission on the HSC web page (http://herschel.esac.esa.int/). If you have access to the ESA intranet (ESA users only), you should also look at the contents of the "Herschel - How it All Ended" video.

As an observatory mission its success thus depended on the quality of the science that the community carried out with it and how effectively the helium in its dewar was converted into science. The "helium into science" ratio was the principal deciding factor in allocating time with the Herschel Space Observatory.

Many aspects of the Herschel Space Observatory have been 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 has converted 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, over much of its wavelength range Herschel has been 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 had hardly been exploited before its launch. Herschel observations will have a huge impact on astronomy and on our understanding of the universe for many years to come.

This manual describes the observatory aspects of Herschel: the spacecraft and its performance; the mission; the space environment in which the Herschel Space Observatory has been operating (very different from previous missions such as IRAS, ISO and the HST); and use of Herschel - from how an observing proposal was 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 an observer or someone wanting to use Herschel data 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 work with Herschel data 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. At the time of this revision we are now well into post-Operations, six months after the end of helium and have characterised all aspects of Herschel's performance and operation as thoroughly as possible. As a result, this manual has undergone a further deep revision to reflect a mature post-Operations reality.

Similarly, when originally written, the emphasis in all Herschel documentation was very much on Uplink -- what was sent up to the satellite and how it would get to be sent up to the satellite -- rather than downlink -- the data received from the satellite and its processing. The contents of this manual still very much reflect an Uplink bias. For detailed information of the downlink aspects of the Herschel mission you should check the documentation for HIPE (the Herschel Interactive Processing Environment) at: http://herschel.esac.esa.int/hcss-doc-11.0/ (for HIPE 11), from early 2014, for HIPE 12 at: http://herschel.esac.esa.int/hcss-doc-12.0/ and, from early 2015, for HIPE 13 at http://herschel.esac.esa.int/hcss-doc-13.0/.

This manual is not intended to be a complete guide and reference book to all the intricacies of the Herschel mission. It is intended to cover the basics of Herschel for people who are new to the mission, or who want to find out a little more about Herschel without going into exhaustive technical detail. As such, a significant part of this manual is dedicated to mission history. Those who need deeper information are directed to the various instrument and software manuals and, to the Explanatory Supplement that will be published during post-Operations, of which a first draft is expected to be available in late 2014.