Header(3) User Contributed Perl Documentation Header(3)NAMEAstro::FITS::Header - Object Orientated interface to FITS HDUs
SYNOPSIS
$header = new Astro::FITS::Header( Cards => \@array );
DESCRIPTION
Stores information about a FITS header block in an object. Takes an
hash with an array reference as an arguement. The array should contain
a list of FITS header cards as input.
REVISION
$Id: Header.pm,v 1.26 2003/10/23 03:28:28 bradc Exp $
METHODS
Constructor
new Create a new instance from an array of FITS header cards.
$item = new Astro::FITS::Header( Cards => \@header );
returns a reference to a Header object. If you pass in no cards,
you get the (required) first SIMPLE card for free.
Accessor Methods
tieRetRef
Indicates whether the tied object should return multiple values as
a single string joined by newline characters (false) or it should
return a reference to an array containing all the values.
Only affects the tied interface.
tie %keywords, "Astro::FITS::Header", $header, tiereturnsref => 1;
$ref = $keywords{COMMENT};
Defaults to returning a single string in all cases (for backwards
compatibility)
subhdrs
Set or return the subheaders for a Header object
$header->subhdrs(@hdrs);
@hdrs = $header->subhdrs;
item
Returns a FITS::Header:Item object referenced by index, "undef" if
it does not exist.
$item = $header->item($index);
keyword
Returns keyword referenced by index, "undef" if it does not exist.
$keyword = $header->keyword($index);
itembyname
Returns an array of Header::Items for the requested keyword if
called in list context, or the first matching Header::Item if
called in scalar context. Returns "undef" if the keyword does not
exist. The keyword may be a regular expression created with the
"qr" operator.
@items = $header->itembyname($keyword);
$item = $header->itembyname($keyword);
index
Returns an array of indices for the requested keyword if called in
list context, or an empty array if it does not exist. The keyword
may be a regular expression created with the "qr" operator.
@index = $header->index($keyword);
If called in scalar context it returns the first item in the array,
or "undef" if the keyword does not exist.
$index = $header->index($keyword);
value
Returns an array of values for the requested keyword if called in
list context, or an empty array if it does not exist. The keyword
may be a regular expression created with the "qr" operator.
@value = $header->value($keyword);
If called in scalar context it returns the first item in the array,
or "undef" if the keyword does not exist.
comment
Returns an array of comments for the requested keyword if called in
list context, or an empty array if it does not exist. The keyword
may be a regular expression created with the "qr" operator.
@comment = $header->comment($keyword);
If called in scalar context it returns the first item in the array,
or "undef" if the keyword does not exist.
$comment = $header->comment($keyword);
insert
Inserts a FITS header card object at position $index
$header->insert($index, $item);
the object $item is not copied, multiple inserts of the same object
mean that future modifications to the one instance of the inserted
object will modify all inserted copies.
replace
Replace FITS header card at index $index with card $item
$card = $header->replace($index, $item);
returns the replaced card.
remove
Removes a FITS header card object at position $index
$card = $header->remove($index);
returns the removed card.
replacebyname
Replace FITS header cards with keyword $keyword with card $item
$card = $header->replacebyname($keyword, $item);
returns the replaced card. The keyword may be a regular expression
created with the "qr" operator.
removebyname
Removes a FITS header card object by name
@card = $header->removebyname($keyword);
returns the removed cards. The keyword may be a regular expression
created with the "qr" operator.
splice
Implements a standard splice operation for FITS headers
@cards = $header->splice($offset [,$length [, @list]]);
$last_card = $header->splice($offset [,$length [, @list]]);
Removes the FITS header cards from the header designated by $offset
and $length, and replaces them with @list (if specified) which must
be an array of FITS::Header::Item objects. Returns the cards
removed. If offset is negative, counts from the end of the FITS
header.
cards
Return the object contents as an array of FITS cards.
@array = $header->cards;
allitems
Returns the header as an array of FITS::Header:Item objects.
@items = $header->allitems();
General Methods
configure
Configures the object, takes an array of FITS header cards or an
array of Astro::FITS::Header::Item objects as input. If you feed
in nothing at all, it uses a default array containing just the
SIMPLE card required at the top of all FITS files.
$header->configure( Cards => \@array );
$header->configure( Items => \@array );
Does nothing if the array is not supplied.
freeze
Method to return a blessed reference to the object so that we can
store ths object on disk using Data::Dumper module.
Operator Overloading
These operators are overloaded:
"" When the object is used in a string context the FITS header block
is returned as a single string.
Private methods
These methods are for internal use only.
_rebuild_lookup
Private function used to rebuild the lookup table after modifying
the header block, its easier to do it this way than go through and
add one to the indices of all header cards following the modifed
card.
TIED INTERFACE
The "FITS::Header" object can also be tied to a hash:
use Astro::FITS::Header;
$header = new Astro::FITS::Header( Cards => \@array );
tie %hash, "Astro::FITS::Header", $header
$value = $hash{$keyword};
$hash{$keyword} = $value;
print "keyword $keyword is present" if exists $hash{$keyword};
foreach my $key (keys %hash) {
print "$key = $hash{$key}\n";
}
Basic hash translation
Header value type is determined on-the-fly by parsing of the input
values. Anything that parses as a number or a logical is converted to
that before being put in a card (but see below).
Per-card comment fields can be accessed using the tied interface by
specifying a key name of "key_COMMENT". This works because in general
"_COMMENT" is too long to be confused with a normal key name.
$comment = $hdr{CRPIX1_COMMENT};
will return the comment associated with CRPIX1 header item. The comment
can be modified in the same way:
$hdr{CRPIX1_COMMENT} = "An axis";
You can also modify the comment by slash-delimiting it when setting the
associated keyword:
$hdr{CRPIX1} = "34 / Set this field manually";
If you want an actual slash character in your string field you must
escape it with a backslash. (If you're in double quotes you have to
use a double backslash):
$hdr{SLASHSTR} = 'foo\/bar / field contains "foo/bar"';
Keywords are CaSE-inNSEnSiTIvE, unlike normal hash keywords. All
keywords are translated to upper case internally, per the FITS
standard.
Aside from the SIMPLE and END keywords, which are automagically placed
at the beginning and end of the header respectively, keywords are
included in the header in the order received. This gives you a modicum
of control over card order, but if you actually care what order they're
in, you probably don't want the tied interface.
Comment cards
Comment cards are a special case because they have no normal value and
their comment field is treated as the hash value. The keywords
"COMMENT" and "HISTORY" are magic and refer to comment cards; nearly
all other keywords create normal valued cards. (see "SIMPLE and END
cards", below).
Multi-card values
Multiline string values are broken up, one card per line in the string.
Extra-long string values are handled gracefully: they get split among
multiple cards, with a backslash at the end of each card image.
They're transparently reassembled when you access the data, so that
there is a strong analogy between multiline string values and multiple
cards.
In general, appending to hash entries that look like strings does what
you think it should. In particular, comment cards have a newline
appended automatically on FETCH, so that
$hash{HISTORY} .= "Added multi-line string support";
adds a new HISTORY comment card, while
$hash{TELESCOP} .= " dome B";
only modifies an existing TELESCOP card.
You can make multi-line values by feeding in newline-delimited strings,
or by assigning from an array ref. If you ask for a tag that has a
multiline value it's always expanded to a multiline string, even if you
fed in an array ref to start with. That's by design: multiline string
expansion often acts as though you are getting just the first value
back out, because perl string-to-number conversion stops at the first
newline. So:
$hash{CDELT1} = [3,4,5];
print $hash{CDELT1} + 99,"\n$hash{CDELT1}";
prints "102\n3\n4\n5", and then
$hash{CDELT1}++;
print $hash{CDELT1};
prints "4".
In short, most of the time you get what you want. But you can always
fall back on the non-tied interface by calling methods like so:
((tied $hash)->method())
If you prefer to have multi-valued items automagically become array
refs, then you can get that behavior using the "tiereturnsref" method:
tie %keywords, "Astro::FITS::Header", $header, tiereturnsref => 1;
When tiereturnsref is true, multi-valued items will be returned via a
reference to an array (ties do not respect calling context). Note that
if this is configured you will have to test each return value to see
whether it is returning a real value or a reference to an array if you
are not sure whether there will be more than one card with a duplicate
name.
Type forcing
Because perl uses behind-the-scenes typing, there is an ambiguity
between strings and numeric and/or logical values: sometimes you want
to create a STRING card whose value could parse as a number or as a
logical value, and perl kindly parses it into a number for you. To
force string evaluation, feed in a trivial array ref:
$hash{NUMSTR} = 123; # generates an INT card containing 123.
$hash{NUMSTR} = "123"; # generates an INT card containing 123.
$hash{NUMSTR} = ["123"]; # generates a STRING card containing "123".
$hash{NUMSTR} = [123]; # generates a STRING card containing "123".
$hash{ALPHA} = "T"; # generates a LOGICAL card containing T.
$hash{ALPHA} = ["T"]; # generates a STRING card containing "T".
Calls to keys() or each() will, by default, return the keywords in the
order n which they appear in the header.
When the key refers to a subheader entry, a hash reference is returned.
If a hash reference is stored in a value it is converted to a
"Astro::FITS::Header" object.
SIMPLE and END cards
No FITS interface would becomplete without special cases.
When you assign to SIMPLE or END, the tied interface ensures that they
are first or last, respectively, in the deck -- as the FITS standard
requires. Other cards are inserted in between the first and last
elements, in the order that you define them.
The SIMPLE card is forced to FITS LOGICAL (boolean) type. The FITS
standard forbids you from setting it to F, but you can if you want --
we're not the FITS police.
The END card is forced to a null type, so any value you assign to it
will fall on the floor. If present in the deck, the END keyword always
contains the value " ", which is both more-or-less invisible when
printed and also true -- so you can test the return value to see if an
END card is present.
SIMPLE and END come pre-defined from the constructor. If for some
nefarious reason you want to remove them you must explicitly do so with
"delete" or the appropriate method call from the object interface.
COPYRIGHT
Copyright (C) 2001-2002 Particle Physics and Astronomy Research Council
and portions Copyright (C) 2002 Southwest Research Institute. All
Rights Reserved.
This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
AUTHORS
Alasdair Allan <aa@astro.ex.ac.uk>, Tim Jenness
<t.jenness@jach.hawaii.edu>, Craig DeForest
<deforest@boulder.swri.edu>, Jim Lewis <jrl@ast.cam.ac.uk>
perl v5.10.0 2003-10-23 Header(3)