[fitsbits] Bintable proposals

[Bill Cotton]

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.

 > 
 > As I understand it the history goes something like this: the genesis of
 > FITS is linked to AIPS; AIPS was intended to run on Modcomps; thus early
 > versions of AIPS did not have file expansion; therefore FITS didn't assume
 > it.  This limitation has never been relaxed.
 > 

   Again, completely wrong.  See above.  Anyway, you're thinking about
IBMs - not Modcomps.  AIPS never ran on OS/360 but FITS tapes could
still be read on it.

-Bill