It is first necessary to define the set of specialist NDF extensions that should be recognised. This is normally done via the environment variable NDF_XTN, as follows:
setenv NDF_XTN CCDPACK,IRAS90
This is simply a comma-separated list of extension names conforming to the naming conventions described in SUN/33. It applies to all foreign format datasets, unless overridden. The order in which extensions occur in this list determines the order in which they will be imported. They will be exported in the reverse order.
On occasion, it may be necessary to use a different list of NDF
extensions for a particular foreign format. Most commonly, this
involves simply using an empty list for formats that do not require
any extension handling (data compression of ordinary NDF data files
would be an example - see §). To specify a
separate extension list for a particular foreign format, an
environment variable is used whose name is constructed by prefixing
`NDF_XTN_' to the format name (in upper case). For example:
setenv NDF_XTN_COMPRESSED ''
would over-ride the normal extension list with an empty one so that no extension handling would occur when COMPRESSED format data is accessed.