[fitsbits] Bintable proposals
Arnold Rots
arots at head-cfa.harvard.edu
Thu Nov 15 09:51:41 EST 2001
Bill Cotton wrote:
>
> Mark Calabretta writes:
> >
> > On Wed 2001/11/14 07:09:31 CDT, Bill Cotton wrote
> > in a message to: Mark Calabretta <Mark.Calabretta at atnf.CSIRO.AU>
> > and copied to: William Pence <pence at tetra.gsfc.nasa.gov>, FITSBITS <fitsbits at nr
> > ao.edu>
> >
> > > What you address is a fundamental FITS limitation rather than just
> > >a binary tables one. Each header is required to say what the length
> > >of its data unit is. This does present problems for data acquisition.
> >
> > Hi Bill,
> >
> > Am I correct in saying that this arises from the fact that Modcomps could
> > not do file expansion, i.e. the correct file size needed to be allocated
> > before writing could commence? [For those who've never heard of them,
> > Modcomps were/are an ancient type of computer mainly used for realtime
> > applications.]
>
> Nope, completely wrong. This has nothing to do with AIPS or
> Modcomps but rather with the nature of magnetic tapes. Once a record
> is written it cannot be changed. The intent of the FITS standard was
> to allow FITS readers to be able to skip over sections of the files
> that weren't wanted. This requires that the headers say how big the
> data section it.
Bill is absolutely right; this requirement is essential for one of FITS's most
useful features: to be able to put multiple extensions in a single file.
> ...
> -Bill
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Arnold H. Rots Chandra X-ray Science Center
Smithsonian Astrophysical Observatory tel: +1 617 496 7701
60 Garden Street, MS 81 fax: +1 617 495 7356
Cambridge, MA 02138 arots at head-cfa.harvard.edu
USA http://hea-www.harvard.edu/~arots/
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