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            && exists $FITS_headers->{SUBHEADERS}->[0]->{$keyword}) {
    my @subs = @{$FITS_headers->{SUBHEADERS}};
    for my $s (@subs) {
      if (exists $s->{$keyword} && defined $s->{$keyword}) {
        push(@values, $s->{$keyword});
      }
    }
  } elsif (exists $FITS_headers->{I1}
           && exists $FITS_headers->{I1}->{$keyword}) {
    # need to find out how many In we have
    my $i = 1;
    while (exists $FITS_headers->{"I$i"}) {
      push(@values, $FITS_headers->{"I$i"}->{$keyword});
      $i++;
    }
  }

  return (wantarray ? @values : $values[0] );
}

=item B<via_subheader_undef_check>

Version of via_subheader that removes undefined values from the list before
returning the answer. Useful for SCUBA-2 where the first dark may not include
the TCS information.

Same interface as via_subheader.

=cut

sub via_subheader_undef_check {
  my $self = shift;
  my @values = $self->via_subheader( @_ );

  # completely filter out undefs
  @values = grep { defined $_ } @values;
  return (wantarray ? @values : $values[0] );
}

=back

=head1 PROTECTED IMPORTS

Not all translation methods warrant a full blown inheritance.  For
cases where one or two translation routines should be imported
(e.g. reading DATE-OBS FITS standard headers without importing the
additional FITS methods) a special import routine can be used when
using the class.

  use Astro::FITS::HdrTrans::FITS qw/ ROTATION /;

This will load the from_ROTATION and to_ROTATION methods into
the namespace.

=cut

sub import {
  my $class = shift;

  # this is where we are going to install the methods
  my $callpkg = caller();

  # Prepend the from_ and to_ prefixes
  for my $key (@_) {
    # The key can be fully specified with from_ and to_ already
    # In that case we do not want to loop over from_ and to_
    my @directions = qw/ from_ to_ /;
    if ($key =~ /^from_/ || $key =~ /^to_/) {
      @directions = ( '' );     # empty prefix
    }

    for my $dir (@directions) {
      my $method = $dir . $key;
      #print "Importing method $method\n";
      no strict 'refs';

      if (!defined *{"$class\::$method"}) {
        croak "Method $method is not available for export from class $class";
      }

      # assign it
      *{"$callpkg\::$method"} = \&{"$class\::$method"};
    }
  }

}

=head1 WRITING A TRANSLATION CLASS

In order to create a translation class for a new instrument it is
first necessary to work out the different types of translations that
are required; whether they are unit mappings (a simple change of
keyword but no change in value), constant mappings (a constant is
returned independently of the FITS header), mappings that already
exist in another class or complex mappings that have to be explicitly
coded.

All translation classes must ultimately inherit from
C<Astro::FITS::HdrTrans::Base>.

The first step in creation of a class is to handle the "can this class
translate the supplied headers" query that will be requested from
the C<Astro::FITS::HdrTrans> package. If the instrument name is present
in the standard "INSTRUME" FITS header then this can be achieved simply
by writing a C<this_instrument> method in the subclass that will return
the name of the instrument that can be translated. If a more complex
decision is required it will be necessary to subclass the C<can_translate>
method. This takes the headers that are to be translated (either in FITS
or generic form since the method is queried for either direction) and
returns a boolean indicating whether the class can be used.

Once the class can declare it's translation instrument the next
step is to write the actual translation methods themselves. If any
unit- or constant-mappings are required they can be setup by defining
the %UNIT_MAP and %CONST_MAP (the names are unimportant) hashes
and calling the base class automated method constructor:

  __PACKAGE__->_generate_lookup_methods( \%CONST_MAP, \%UNIT_MAP );

If your translations are very similar to an existing set of translations
you can inherit from that class instead of C<Astro::FITS::HdrTrans::Base>.