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[fitsbits] Proposed Changes to the FITS Standard



 
 
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  #21  
Old August 19th 07, 04:13 PM posted to sci.astro.fits
Steve Allen
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Default [fitsbits] Proposed Changes to the FITS Standard

On Sat 2007-08-18T16:30:24 -0400, Craig Markwardt hath writ:
Another proposed change, the case of the EXTEND keyword being made
optional, will also impose a software-change burden. Software which
previously relied on that keyword will now be required to check for
the presence of extensions in a different way.


Isn't this horse already pretty far out of sight of the barn, too?

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  #22  
Old August 20th 07, 06:00 AM posted to sci.astro.fits
Craig Markwardt
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Default [fitsbits] Proposed Changes to the FITS Standard


Steve Allen writes:
On Sat 2007-08-18T16:30:24 -0400, Craig Markwardt hath writ:
Another proposed change, the case of the EXTEND keyword being made
optional, will also impose a software-change burden. Software which
previously relied on that keyword will now be required to check for
the presence of extensions in a different way.


Isn't this horse already pretty far out of sight of the barn, too?


I'm not sure which is the horse and which is the barn.

Has EXTEND ever been optional for files with extensions?

Craig
  #23  
Old August 20th 07, 08:57 PM posted to sci.astro.fits
Steve Allen
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Default [fitsbits] Proposed Changes to the FITS Standard

On Mon 2007-08-20T01:00:32 -0400, Craig Markwardt hath writ:
Has EXTEND ever been optional for files with extensions?


Has EXTEND ever been relevant for FITS files at all?

I would like to know of any applications which actually use it.

If I am not mistaken the CFITSIO toolkit always sets EXTEND to true.

In my experience there are two types of reading applications:
Apps which don't know what they would do with an extension, don't even
look for them, and don't care whether EXTEND is set or not.
Apps which know that they are expecting extension(s), don't care
whether EXTEND is set or not, and may fail with an unhelpful message
if they do not find the particular extension(s) they seek.

So what applications are there, other than fv, which do not a priori
know what they are expecting when they read a FITS file?
In particular, are there any which, when they find extensions, actually
take different actions based on what they discover?

These are the sorts of questions which prompted the FITS MIME effort,
and the sorts of questions which we realized we could not address there.

--
Steve Allen WGS-84 (GPS)
UCO/Lick Observatory Natural Sciences II, Room 165 Lat +36.99855
University of California Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m
  #24  
Old August 20th 07, 09:29 PM posted to sci.astro.fits
Phil Hodge
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Posts: 5
Default [fitsbits] Proposed Changes to the FITS Standard

Steve Allen wrote:

So what applications are there, other than fv, which do not a priori
know what they are expecting when they read a FITS file?
In particular, are there any which, when they find extensions, actually
take different actions based on what they discover?



The catfits task in the STSDAS package under IRAF prints a "table of
contents" for a FITS file. If the EXTEND keyword is absent, catfits
does not look for extensions. I won't defend the EXTEND keyword; I'm
just answering your question by giving an example. If an application
expects data in one or more extensions, there isn't much point in the
application first checking the EXTEND keyword.

Phil
  #25  
Old August 20th 07, 10:17 PM posted to sci.astro.fits
William Thompson
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Posts: 10
Default [fitsbits] Proposed Changes to the FITS Standard

The SECCHI instrument on the STEREO mission produces some files with table
extensions, and other files which don't. The value of the EXTEND keyword is set
to either T or F to account for this.


Steve Allen wrote:
On Mon 2007-08-20T01:00:32 -0400, Craig Markwardt hath writ:

Has EXTEND ever been optional for files with extensions?



Has EXTEND ever been relevant for FITS files at all?

I would like to know of any applications which actually use it.

If I am not mistaken the CFITSIO toolkit always sets EXTEND to true.

In my experience there are two types of reading applications:
Apps which don't know what they would do with an extension, don't even
look for them, and don't care whether EXTEND is set or not.
Apps which know that they are expecting extension(s), don't care
whether EXTEND is set or not, and may fail with an unhelpful message
if they do not find the particular extension(s) they seek.

So what applications are there, other than fv, which do not a priori
know what they are expecting when they read a FITS file?
In particular, are there any which, when they find extensions, actually
take different actions based on what they discover?

These are the sorts of questions which prompted the FITS MIME effort,
and the sorts of questions which we realized we could not address there.

--
Steve Allen WGS-84 (GPS)
UCO/Lick Observatory Natural Sciences II, Room 165 Lat +36.99855
University of California Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m
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Code 671
Greenbelt, MD 20771
USA

301-286-2040

  #26  
Old August 21st 07, 10:08 AM posted to sci.astro.fits
Preben Grosbol
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Posts: 11
Default [fitsbits] Proposed Changes to the FITS Standard

On Monday 20 August 2007 07:00, Craig Markwardt wrote:
Has EXTEND ever been optional for files with extensions?


At the time when extensions were introduced, most readers would
not even think of looking for them. Thus, it was important that users
could look on their primary header and see (and then complain) that
there were extension data available in the file which were not detected
by the local reader.

Now that extensions are common and most readers would check, I see
little problem in making EXTEND optional. The ESO readers, I know of,
all would scan the full file no matter what.

Preben
  #27  
Old August 21st 07, 11:07 AM posted to sci.astro.fits
Maren Purves
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Posts: 17
Default [fitsbits] Proposed Changes to the FITS Standard

Hate to say this, but we're starting to have a generation
problem. I got into FITS at the time of binary arrays,
not at the start (so I'm in the 'middle; generation). But
at this time we have people asking about how a FITS file
is different from an image and people asking about FITS
files with and without extensionsi - do we still have
FITS files without extensions?

It being "once FITS always FITS" I object to making the
EXTEND keyword optional.

Aloha,
Maren

On Tue, 21 Aug 2007, Preben Grosbol wrote:

On Monday 20 August 2007 07:00, Craig Markwardt wrote:
Has EXTEND ever been optional for files with extensions?


At the time when extensions were introduced, most readers would
not even think of looking for them. Thus, it was important that users
could look on their primary header and see (and then complain) that
there were extension data available in the file which were not detected
by the local reader.

Now that extensions are common and most readers would check, I see
little problem in making EXTEND optional. The ESO readers, I know of,
all would scan the full file no matter what.

Preben
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  #28  
Old August 21st 07, 11:41 AM posted to sci.astro.fits
LC's NoSpam Newsreading account[_2_]
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Posts: 23
Default [fitsbits] Proposed Changes to the FITS Standard

On Fri, 17 Aug 2007, Steve Allen wrote:

I would like to see the IAUFWG establish a registry (in the sense of
the IANA) wherein all the documented FITS conventions have unique
names, and I would like to see a series of keywords which can be
placed in the PHDU to assert that "this FITS file employs these named
conventions".


I sort of endorse this statement (not really familiar with the
requirements of a IANA registry, and we should consider feasibility
w.r.t. current IAUFWG convention registry) ... but yes, there should be
some univocal recipe (based on one or more keywords, sort of glorified
"magic" file ... something like this was discussed when talking about
the establishment of the current registry) to assert that a given FITS
extension employs these named conventions (I do not see why this shall
be in the PHDU and apply to the entire FITS file, as some conventions
may apply only to some extensions).

Lucio Chiappetti

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  #29  
Old August 21st 07, 12:20 PM posted to sci.astro.fits
LC's NoSpam Newsreading account[_2_]
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Posts: 23
Default [fitsbits] Proposed Changes to the FITS Standard

On Tue, 21 Aug 2007, Maren Purves wrote:

Hate to say this, but we're starting to have a generation
problem. I got into FITS at the time of binary arrays,
not at the start (so I'm in the 'middle; generation).


Hmm ... if nearly contemporary to FITS (I graduated 1 month before
FITS's birthday :-)) but of course wasn't in the Deciding Bodies that
early (or as early as generalized extensions were introduced).

do we still have FITS files without extensions?


I guess there will be plenty, those which the programmer-type people
will ask "how different with JPEG". Seriously, there is no need to use
an extension for a plain image or even a datacube.

It being "once FITS always FITS" I object to making the
EXTEND keyword optional.


The "fault" about EXTEND was in its original definition. The keyword
EXTEND=T was present "if the FITS file MAY contain extensions" and
it being equal to T did "not imply extensions be present". As such it
resembles a bit the present versioning/no-versioning discussion. But it
was/is pretty useless to tell whether the file really contains
extensions. So the safest way taken by FITS readers was to scan
anyhow after the PHDU for named XTENSION kwds.

I doubt there are many readers around which won't do that is EXTEND is
absent.

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  #30  
Old August 21st 07, 12:44 PM posted to sci.astro.fits
Thierry Forveille
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Posts: 14
Default [fitsbits] Proposed Changes to the FITS Standard

Maren Purves a écrit :
- do we still have
FITS files without extensions?

Yes we do :-) For instruments that naturally generate measurements
on a regular 2D grid (such as simple optical/IR imagers with a
single CCD/IR sensor) the pre-extensions standard works perfectly
and they typicall use it. Several (mostly older) CFHT instruments
do that, and I expect most optical observatories to be in a
similar situation. If extensions ever were made compulsory those
instruments could of course put their data in an IMAGE extension,
but there is little point in doing that.

It being "once FITS always FITS" I object to making the
EXTEND keyword optional.

I don't particularly object to the EXTEND change (which doesn't
invalidate any old FITS file, just gives a (very) small added
flexibility to new ones, so doesn't break the "once FITS always
FITS rule"), but don't see its point either. Of course it saves
80 bytes in every FITS file, but I suppose there must be some
stronger reason :-)
 




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