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[fitsbits] New DUMP FITS extension



 
 
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Old August 16th 07, 08:51 PM posted to sci.astro.fits
William Pence
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Default [fitsbits] New DUMP FITS extension

Doug,

The Hinode satellite project is now publicly distributing FITS data
files with this DUMP extension. They apparently saw the DUMP extension
name in the list of proposed extensions in the appendix to the FITS
standard and decided to use that extension name for their telemetry
data. Even though there is no specification of what the DUMP extension
keywords should look like, they assumed the keywords would essentially
be the same as in an image extension with BITPIX = 8 and NAXIS = 1.

The question is what do we (the FITS community in general, and the
IAUFWG in particular) do now? Should we draft a definition document
describing the DUMP extension (as is currently being done for the
FOREIGN extension type)? Do we want to go further and endorse the DUMP
extension for use in other data sets distributed by other projects? Or,
would we rather discourage the use of the DUMP (or any other new
extension type) if the data can easily be contained in an existing
extension type? Personally, I would rather not see a proliferation of
new extension types that are structurally identical to the existing
IMAGE extension.

Bill

Doug Tody wrote:
Hi Bill -

I'm not sure I understand: this DUMP extension was proposed, but has never
been specified or implemented? If this is the case, it doesn't actually
exist, and we should be able to ignore it. (Also it is functionally
redundant with the FOREIGN extension).

- Doug


On Thu, 16 Aug 2007, William Pence wrote:

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.


 




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