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General Principles  

A notable feature of the NDF data format is its extensibility, which is achieved by means of independent extensions[*] to the format, which can be defined and added to suit the needs of individual software authors. A key distinction between these extensions and the other contents of an NDF dataset is that the meaning and processing rules for data held in extensions are generally unknown to writers of format conversion utilities, whereas the standard components of an NDF have well-defined and universal meanings (see SUN/33).

This has important implications. It means, for instance, that it is relatively straightforward to write a general purpose utility to change (say) IRAF format into NDF format, so long as only standard NDF components need to be considered. However, if the receiving NDF application is equipped to handle data in its own NDF extension, then converting that additional data (i.e. extracting it from the IRAF file and putting it into the NDF extension) will require specialist knowledge, and so cannot be expected of a general purpose utility.

What is required is for conversion utilities to be extensible in the same way as the NDF datasets themselves. A standard utility could then be used to convert the bulk of the data, and a more specialised utility could simply add the extension information to the converted dataset.

As will be explained below, the NDF library supports this concept, but there still remains one problem. If, for example, you had written a software package and an associated utility that extracted specialist extension information from IRAF datasets, you would probably not want to repeat this work for every other possible data format that might come along in future - you would surely prefer to use the same specialist utility to access a whole range of foreign formats. This is where the NDF's FITS extension comes in.



next up previous
Next: The FITS Extension
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Starlink System Note 20
R.F.Warren-Smith & D.S.Berry
17th July 2000
E-mail:ussc@star.rl.ac.uk

Copyright © 2000 Council for the Central Laboratory of the Research Councils