History in IRAS90

 There is an option for IRAS90 to maintain a history of the processing applied to each NDF. This history information is automatically created by each application, and is stored within the NDF in a ``HISTORY'' component. It may be displayed, removed or added to manually using the HISTORY application.

History information can become quite voluminous, causing the NDFs to occupy correspondingly larger amounts of disk space. For this reason, the default behaviour is for IRAS90 applications not to store any history information. This can be over-ridden using the HISTORY parameters associated with many of the IRAS90 applications.

HISTORY parameters are used to specify if applications should store history in their output NDFs or not. If HISTORY has a true value then history information is stored in the output NDF. If it has a false value then no history is added to the output and any existing history is deleted. This ensures that if an NDF does contain history, then the history is complete. As has often been said, an incomplete history is more dangerous than no history at all.

It is not necessary to repeatedly assign values to HISTORY parameters for each successive application. Once a value has been assigned to a HISTORY parameter, all subsequent applications use the same value until a new value is assigned to a HISTORY parameter. This is achieved by associating the HISTORY parameter of each application with the ``global'' parameter IRAS90_HISTORY.