[iaufwg] convention or standard

[William Pence]

Preben Grosbol wrote:
>           The transport
>      of foreign bit-streams is an issue and should be considered.  The 'DUMP'
>      extension was reserved partly for such things (e.g. raw data acquisition
>      dumps) but never specified.  There are aspects in the 'FOREIGN' extension
>      which should be reconsidered if one would try to move it towards a
>      standard (with another name) such as handling of privileges and
>      description of the format of the bit-stream.

The FOREIGN extension type and the proposed DUMP and COMPRESS
extension types all serve the same basic purpose: they are a container for 
an opaque stream of bytes.  The only parameter that FITS needs to know is 
the total number of bytes.  In the FOREIGN case, the PCOUNT keyword is used 
to record the size of the data stream, but in the proposed COMPRESS case 
(and probably in the DUMP case as well) the data size is given by the NAXIS1 
keyword (with NAXIS = 1).

Instead of inventing a new extension type for each new case that comes along 
(e.g., someone might create a new 'TELEMETRY' extension type to record the 
raw stream of spacecraft telemetry bytes), it would be better to adopt a 
single standard way to pack a stream of bytes into a FITS file.

The simplest solution with the least impact on existing software and users 
would be to use the IMAGE extension for this purpose, with NAXIS = 1 and 
NAXIS1 equal to the size of the byte stream.  Other keywords can be added to 
the header to provide any other meta data about the byte stream.

It might be argued that this is a misuse of the IMAGE extension since the 
data are not an image in the usual sense of the word, but my own feeling is 
that the advantages of reusing IMAGE for this purpose outweigh this 
philosophical objection.

If it were decided that IMAGE is not suitable for this purpose, then perhaps 
we should consider designing a new standard extension type just for storing 
a stream of bytes, so other groups would be less tempted to invent their own 
new type of extension for each individual case.

Bill
-- 
____________________________________________________________________
Dr. William Pence                       pence at milkyway.gsfc.nasa.gov
NASA/GSFC Code 662       HEASARC        +1-301-286-4599 (voice)
Greenbelt MD 20771                      +1-301-286-1684 (fax)