[fitsbits] Bintable proposals

[Arnold Rots]

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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