The IRAS NDF Extension

Various IRAS90 applications store information within an ``NDF extension''. This is an HDS data structure which is ``tagged on'' to the main NDF structure, and is named ``IRAS''. It is used to store information which futher describes the data in the main NDF data array. Typical items are the waveband (12, 25, 60 or 100 $\mu$m), the instrument (survey array or CPC), etc. The contents of the extension depends on what sort of data is stored in the NDF and on what processing has been applied to the data. IRAS90 applications will read the information stored in the IRAS extension, use it, and write new information to it, as necessary.

The extension contains some components which are data structures rather than simple numerical or string values. Of these, the most important are:

CRDD_INFO

- Used to store information describing IRAS90 CRDD files (waveband, boresight pointing information, etc).

IMAGE_INFO

- Used to store information describing IRAS90 images (waveband, name of the application which created the image, etc)

ASTROMETRY

- Used to store information describing the position on the sky of each image pixel.

QUALITY_NAMES

- Used to store the quality names which are currently defined in an image or CRDD file.

The structure and content of the IRAS extension is described in detail in various additional IRAS90 documents, distributed with the package. These documents are intended for people who want to write their own IRAS90 applications, and are described in the ``IRAS90 Programmers Guide'', SUN/165.