=head2 guess_instrument - Guess the instrument a FITS file came from

=for usage

   $im = rfits('foo.fits');
   $inst = guess_instrument( $im );

=for ref

If only everyone used the INSTRUME keyword in FITS files, then this 
routine would be unnecessary.  But many files exist that do NOT use the 
INSTRUME tag.  C<guess_instrument> overcomes that by making educated guesses
if INSTRUME is missing.  Add clauses as necessary.  

AUTHOR

Craig DeForest, cerca 2002

=cut

sub guess_instrument {
  my($hdr) = shift;
  
  return undef unless ref $hdr eq 'HASH';
  
  return $hdr->{INSTRUME} if defined $hdr->{INSTRUME};
  
  # Michelson Doppler Imager often puts a "DPC_STR" in the output
  return 'MDI' if( defined($hdr->{DPC_STR}) );
  return 'MDI' if( $hdr->{ORIGIN} =~ m/SOI SDS/s);

  # All out of guesses.
  return undef;
}