[fitsbits] New DUMP FITS extension

[William Pence]

One of the proposed changes to the FITS Standard is a complete rewrite 
of the Appendix F (previously Appendix I) which lists the reserved FITS 
extension type names (i.e., the value of the XTENSION keyword). One of 
the registered extension types listed in that appendix is as follows:

  'DUMP    ' - Suggested extension name for storing a stream of
   binary data (such as a telemetry stream) in a FITS file.  This
   extension type was never implemented, but the FOREIGN extension
   type serves a similar purpose.

This definition may now need to be updated because I recently learned 
that the FITS data from Solar Optical Telescope on the Hinode satellite 
(http://solar-b.nao.ac.jp/index_e.shtml), launched in late 2006, 
contains a DUMP extension with the following minimal set of header keywords:

XTENSION= 'DUMP    '
BITPIX  =                8
NAXIS   =                1
NAXIS1  =              300
PCOUNT  =                0
GCOUNT  =                1
END

My understanding is that this extension contains the image header packet 
from the satellite telemetry.  The NAXIS1 value may vary, but otherwise 
all the DUMP extensions look the same.

The IAU FITS Working Group is charged with maintaining the list of all 
registered FITS extension types, and at least at some level, is 
responsible for defining how each type is supposed to be used.  A number 
of questions regarding the DUMP extension would need to be resolved 
before writing such a definition document:

1. Is this how the DUMP extension was originally envisioned to be 
implemented?  An alternate scheme, as used in the FOREIGN extension 
type, would be:

XTENSION= 'DUMP    '
BITPIX  =                8
NAXIS   =                0
PCOUNT  =              300
GCOUNT  =                1
END

Is one of these 2 forms preferable over the other?

2.  How does the DUMP extension as used in the Hinode data differ from 
an IMAGE or FOREIGN extension?  What are the fundamental differences, if 
any, between the DUMP and IMAGE (or FOREIGN) extensions?  What criteria 
should data providers use in deciding which extension type to use?

These questions are particularly relevant now because the new draft FITS 
Standard in section 3.4.1 states:  "New extension types should be 
created only when the organization of the information is such that it 
cannot be handled by one of the existing extension types."    (This 
requirement comes from the original "Generalized Extensions" FITS paper 
published in 1987).  Given that an IMAGE extension presumably could be 
used to store the same data, does the DUMP extension satisfy this 
requirement?

3.  Can the DUMP extension have a BITPIX value other than 8, and a NAXIS 
value other than 1?  Must PCOUNT always = 0 and GCOUNT always = 1?   If 
any of these keywords can have different values, how does this affect 
the interpretation of the data?

Presumably these questions, and more, should be answered in a short 
definition document that specifies exactly how the use DUMP extension 
should be used.   At the moment however, it is not clear to me that 
there is any consensus on the answer to these questions.

Bill Pence
-- 
____________________________________________________________________
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)