path: root/doc/source/headerlet_def.rst

===========================================================================
Definition of a Headerlet and Its Role in Updating WCS Information - DRAFT
===========================================================================

.. abstract::
   :author: Warren Hack, Nadezhda Dencheva
   :date: 22 Oct 2012

   
   A headerlet is a self-consistent representation of a single WCS solution for a single
   exposure complete with all distortion information. FITS is the data
   storage format currently supported. It has no observational data
   which makes it relatively small and light to distribute.
   It is, essentially, a mechanism for encapsulating WCS information
   which can later be used to update the WCS of a science file and allows
   improved astrometric solutions to be stored and passed around easily.
   The HST archive is expected to start accepting headerlets for HST data soon.
   However the implementaiton is not HST specific and FITS WCS standard, as well as
   all WCS conventions implemented in pywcs are supported.
   This report describes the format and contents of a headerlet
   along with the software implementation and methods for creating headerlets and using them
   to update the WCS of a science observation.

Introduction
============

The original motivation for this work was a WCS based replacement
of Multidrizzle, now released as Astrodrizzle, and specifically a
requirement for the availability and management of multiple WCS
sets, complete with distortion, representing different pointings,
within one science file. However, the concept of encapsulating
astrometric solutions is more general than that since each solution
may represent a different astrometric alignment, either with a catalog
or another image. Furthermore, computing accurate astrometric
solutions requires considerable effort and time so having a way to
distribute them, apply them to a science observation and switch
between different WCSs efficiently would facilitate many aspects of
data analysis.
Some of the immediate areas for the use of headerlets with HST data include
the HST archive, the HLA and other legacy projects which provide improved astrometry
of HST observations. The HST Archive, for example, provides access to all HST data, 
most of which can not be readily combined together due to errors in guide star astrometry 
imposing offsets between images taken using different pairs of guide stars.  
A lot of effort has gone into computing those offsets so that all the images taken 
at a particular pointing canbe combined successfully with astrometry matched to 
external astrometric catalogs.Unfortunately, there is no current mechanism for 
passing those updated solutions along to the community without providing entirely 
new copies of all the data.  

Source Image
============

Any science observation with a valid WCS described by the FITS standard or any of the
WCS conventions implemented in pywcs may serve as a source for creating a headerlet.

We describe as an example the type of WCS information of a typical HST ACS/WFC image as it
is distributed by the HST archive (OTFR) after being processed with the latest image 
calibrations, including applying the latest available distortion
models. The full description of the WCS is available in the 
:ref:`FITS Conventions Report <fits_conventions_tsr>` report by Hack et al.
The science header now contains the following set of keywords and extensions to fully 
describe the WCS with distortion:

* **Linear WCS keywords**: specifically, CRPIX, CRVAL, CTYPE, CD matrix keywords
* **SIP coefficients**: A_*_* and B_*_*, A_ORDER, B_ORDER, 
* ** The first order coefficients from the IDC table (needed by astrodrizle): OCX10, OCX11, OCY10, and OCY11 keywords
* **NPOL distortion**: if an NPOLFILE has been specified for the image, 
    CPDIS and DP record-value keywords to point to WCSDVARR extensions (FITS Distortion
    Paper convention)
* **Column correction**: if a D2IMFILE has been specified for use with the image, 
    the D2IMEXT, D2IMERR and AXISCORR keywords point to the D2IMARR extension
* **WCSDVARR extensions**: 2 extensions for each chip with lookup tables containing 
    the non-polynomial corrections, with each extension corresponding to an axis of 
    the image (X correction or Y correction)
* **D2IMARR extension**: an extension with a lookup table containing the 
    column-correction from the D2IMFILE.
 

Each science header will have its own set of these keywords and extensions that will
be kept together as part of the headerlet definition.  This avoids any ambiguity as
to what solution was used for any given WCS. 

An HST ACS/WFC exposure would end up with the following set of extensions:

    EXT#  FITSNAME      FILENAME              EXTVE DIMENS       BITPI OBJECT       

    0     j8hw27c4q_flt j8hw27c4q_flt.fits                       16                 
    1       IMAGE       SCI                   1     4096x2048    -32                
    2       IMAGE       ERR                   1     4096x2048    -32                
    3       IMAGE       DQ                    1     4096x2048    16                 
    4       IMAGE       SCI                   2     4096x2048    -32                
    5       IMAGE       ERR                   2     4096x2048    -32                
    6       IMAGE       DQ                    2     4096x2048    16                 
    7       IMAGE       D2IMARR               1     4096         -32                
    8       IMAGE       WCSDVARR              1     65x33        -32                
    9       IMAGE       WCSDVARR              2     65x33        -32                
    10      IMAGE       WCSDVARR              3     65x33        -32                
    11      IMAGE       WCSDVARR              4     65x33        -32                

These additional extensions add approximately 100kB to a typical ACS/WFC image 
making them a space efficient means of managing all the distortion and WCS information. 

Headerlet Definition
====================
A `headerlet` is a self-consistent, definition of a single WCS
including all distortion for all chips/detectors of a single exposure. 
This is different from alternate WCS defined in Greisen, E. W., and Calabretta (Paper I) 
in that by definition all alternate WCSs share the same distortion model while headerlets
may be based on different distortion models. A headerlet does not include alternate WCSs. 
It is stored as a multi-extension FITS file following the structure of the science file. 
The WCS information in the science header is saved in the header of an HDU with EXTNAME 'SIPWCS'. 
All other HDUs in the headerlet (containing distorion information) 
have the same EXTNAME as the science file.

SIPWCS - A New FITS Extension
-----------------------------

We introduce a new HDU with EXTNAME `SIPWCS`. It has no data and the header 
contains all the WCS-related keywords from the SCI 
header. As a minimum it contains the basic WCS keywords described in Paper 1.
If the science observation has a SIP distortion model, the SIP keywords are included 
in this extension. If the distortion of the science observation includes a non-polynomial
part, the keywords describing the extensions with the lookup tables
(EXTNAME=WCSDVARR) are also in this header. If there's a detector defect correction 
(row or column correction), the keywords describing the D2IMARR HDU are also in this 
header. In addition each SIPWCS header contains two keywords which point back to the HDU
of the original science file which was the source for it. These keywords are TG_ENAME and
TG_EVER and have the meaning of (extname, extver) for the science file.

The keywords in this extension are used by the software to overwrite the keywords
in the corresponding SCI header to update the WCS solution for each chip without any
computation. The new extension then serves not only as a record
of a single WCS solution derived for the image, but also the source of values for restoring
the SCI header WCS when desired.  

Headerlet File Structure
------------------------

This SIPWCS extension along with all WCSDVARR extensions and the D2IMARR extension if available 
fully describe the WCS of each chip.
The listing of the FITS extensions for a `headerlet` for a sample ACS/WFC exposure after writing
it out to a file is::

    EXT#  FITSNAME      FILENAME              EXTVE DIMENS       BITPI OBJECT       

    0     j8hw27c4q     j8hw27c4q_hdr.fits                       16
    1       IMAGE       SIPWCS                1                  8
    2       IMAGE       SIPWCS                2                  8
    3       IMAGE       WCSDVARR              1     65x33        -32                
    4       IMAGE       WCSDVARR              2     65x33        -32                
    5       IMAGE       WCSDVARR              3     65x33        -32                
    6       IMAGE       WCSDVARR              4     65x33        -32                
    7       IMAGE       D2IMARR               1     4096         -32                


.. note::

   A headerlet derived from a full-frame WFC3/UVIS image would only
   contain a PRIMARY header and two SIPWCS  extensions (one for each SCI extension)
   as WFC3/UVIS does not currently have non-polynomial distortion or any detector defect corrections.

The keywords used to populate the headerlet come from all the extensions of the updated
FITS file, as illustrated in the following figure.

.. figure:: images/Headerlet_figure_final.png
   :width: 95 %
   :alt: Relationship Between an ACS/WFC Image’s FITS File and a Headerlet
   :align: center

   This figure shows the keywords that are included in a headerlet, the extensions included 
   in a headerlet, and how a headerlet appears as a new extension when it gets appended to the
   original ACS/WFC file.


Headerlet Primary Header
-------------------------

The list below contains all keywords specific to the primary header of a headerlet with
the logic to determine their value. Note that all keywords will be present in the header
and 'required' and 'optional' below refers to their value.

 * `HDRNAME`  - (required) a unique name for the headerlet
                 - the value is given by the user as a parameter to `~stwcs.wcsutil.headerlet.create_headerlet` or `~stwcs.wcsutil.headerlet.write_headerlet`
                 - HDRNAME<wcskey> from the science file is used
                 - WCSNAME<wcskey> from the science file is used
                 - KeyError is raised
 * `DESTIM`   - (required) target image filename
                Used to determine if a headerlet can be applied to a science file.
                - the ROOTNAME keyword of the original science file
                - the name of the science file
 * `WCSNAME`  - (required) name for the WCS
                - the value is given by the user as a parameter to `~stwcs.wcsutil.headerlet.create_headerlet` or `~stwcs.wcsutil.headerlet.write_headerlet`
                - WCSNAME<wcskey> from the science file is used
                - the value of hdrname parameter is used
                - HDRNAME<wcskey> from the science file
                - KeyError is raised
 * `DISTNAME` - (optional) name of distortion model
                - The value of DISTNAME has the form <idctab rootname>-<npolfile rootname>-<d2imfile rootname>
                    and have a value of 'NONE' if no reference files are specified.
 * `SIPNAME`  - (optional) name of SIP model
                SIPNAME is constructed as <ROOTNAME>_<IDCTAB_rootname>, where
            
                ROOTNAME is the keyword from the science file header (or the file name)
                
                IDCTAB_rootname is the rootname of the idctab file
                
                so for example, SIPNAME for a science file j94f05bgq_flt.fits and an idctab file
                postsm4_idc.fits is j94f05bgq_postsm4

                If the SIP coefficients are present in the header but IDCTAB is m issing or invalid,
                then SIPNAME is set to UNKNOWN. If there's no polynomial model, SIPNAME is set to 
                NOMODEL.
 * `NPOLFILE` - (optional) name of npol reference file
                
                NPOLFILE keyword from science file primary header
                
                UNKNOWN if NPOLFILE keyword is missing or invalid but data extensions exist
                
                or NOMODEL 
                
 * `IDCTAB`   - (optional)
                
                IDCTAB keyword from science file primary header or N/A
                
 * `D2IMFILE` - (optional)
 
                D2IMFILE keyword from science file primary header or N/A
                
 * `AUTHOR`   - (optional) name of person who created the headerlet
 * `DESCRIP`  - (optional) short description of the headerlet solution
 * `NMATCH`   - (optional) number of sources used in the new solution fit, if updated from the Archive’s default WCS
 * `CATALOG`  - (optional) areference frame used to define the astrometric solution
 * `UPWCSVER` - (optional) version of STWCS used to create the WCS of the original image
 * `PYWCSVER` - (optional) version of PyWCS used to create the WCS of the original image


These keywords are used to determine whether a headerlet can be applied to a
given exposure or not. Some of the keywords provide more
information about the solution itself, how it was derived, and by whom.

Working With Headerlets
=======================

Headerlets are implemented in a python module `~stwcs.wcsutil.headerlet` which uses PyWCS for 
WCS management and PyFITS for FITS file handling.

Headerlet Software Functionality
--------------------------------


    #. Create a headerlet (on disk or in memory) from a specific WCS of a science observation. 
       This can be the Primary or an alternate WCS.
    #. Apply a WCS from a headerlet to the Primary WCS of a science observation (and 
       optionally save the original WCS as an alternate WCs or a different headerlet).
    #. Copy a WCS from a headerlet as an alternate WCS.
    #. Attach a headerlet to a science file.
    #. Archive a WCS of a science file as a headerlet attached to the file.
    #. Delte a headerlet attached to a science file.
    #. Print a summary of all headerlets attached to a science file.
    
An optional GUI interface is available through teal and includes functions for writing a headerlet,
applying a headerlet, etc. A full listing of all functions with GUI interface is available 
after `stwcs.wcsutil` is imported.

Headerlet API And Examples
--------------------------

`Note: For an up-to-date API always consult the current the SSB documentation pages.`

The headerlet API as of the time of writing this report is documented in :ref:`Appendix1`.

This section describes the current draft API for working with `headerlets` as 
implemented in the `stwcs.wcsutil.headerlet` module. First, there's a potentially 
confusing point that should be cleared up:  A `headerlet`, as implemented, is simply 
a FITS file containing multiple extensions that contain all the parameters necessary 
to reproduce the WCS solution in the science image it was created from.
When a `headerlet` is applied to an image, a copy of the original `headerlet` file is 
appended to the image's HDU list as a special extension HDU called a `Headerlet HDU`.  
A `Headerlet HDU` consists of a simple header describing the `headerlet`, and has as its data
the `headerlet` file itself, (which may be compressed).  A `Headerlet HDU` has an 'XTENSION' 
value of 'HDRLET'.  Though PyFits can handle such a non-standard extension type sensibly, 
this hasn't been tested with other common FITS readers yet.  If it becomes
necessary, `Headerlet HDUs` could be implemented using a standard extension type like 'IMAGE'.

To create a `headerlet` from an image, a `createHeaderlet()` function is provided::

    >>> from stwcs.wcsutil import headerlet
    >>> hdrlet = headerlet.createHeaderlet('j94f05bgq_flt.fits', 'VERSION1')
    >>> type(hdrlet)
    <class 'stwcs.wcsutil.headerlet.Headerlet'>
    >>> hdrlet.info()
    Filename: (No file associated with this HDUList)
    No.    Name         Type      Cards   Dimensions   Format
    0    PRIMARY     PrimaryHDU      12  ()            
    1    SIPWCS      ImageHDU       111  ()            
    2    SIPWCS      ImageHDU       110  ()            
    3    WCSDVARR    ImageHDU        15  (65, 33)      float32
    4    WCSDVARR    ImageHDU        15  (65, 33)      float32
    5    WCSDVARR    ImageHDU        15  (65, 33)      float32
    6    WCSDVARR    ImageHDU        15  (65, 33)      float32
    7    D2IMARR     ImageHDU        12  (4096,)       float32

As you can see, the `Headerlet` object is similar to a normal pyfits `HDUList` object.  `createHeaderlet()` can be given either the path
to a file, or an already open `HDUList` as its first argument.

What do you do with a `Headerlet` object?  Its main purpose is to apply its WCS solution to another file.  This can be done using the
`Headerlet.apply()` method::

    >>> hdrlet.apply('some_other_image.fits')

Or you can use the `applyHeaderlet()` convenience function.  It takes an existing `headerlet` file path or object as its first argument;
the rest of its arguments are the same as `Headerlet.apply()`.  As with `createHeaderlet()` both of these can take a file path or opened
`HDUList` objects as arguments.

When a `headerlet` is applied to an image, an additional `headerlet` containing that image's original WCS solution is automatically created,
and is appended to the file's HDU list as a `Headerlet HDU`.  However, this behavior can be disabled by setting the `createheaderlet` keyword
argument to `False` in either `Headerlet.apply()` or `applyHeaderlet()`.

When opening a file that contains `Headerlet HDU` extensions, it will normally look like this in PyFits::

    >>> import pyfits
    >>> hdul = pyfits.open('94f05bgq_flt_with_hlet.fits')
    >>> hdul.info()
    Filename: j94f05bgq_flt_with_hlet.fits
    No.    Name         Type      Cards   Dimensions   Format
    0    PRIMARY     PrimaryHDU     248  ()            int16
    1    SCI         ImageHDU       286  (4096, 2048)  float32
    2    ERR         ImageHDU        76  (4096, 2048)  float32
    3    DQ          ImageHDU        66  (4096, 2048)  int16
    4    SCI         ImageHDU       282  (4096, 2048)  float32
    5    ERR         ImageHDU        74  (4096, 2048)  float32
    6    DQ          ImageHDU        66  (4096, 2048)  int16
    7    WCSCORR     BinTableHDU     56  10R x 23C     [40A, I, 1A, D, D, D, D, D, D, D, D, 24A, 24A, D, D, D, D, D, D, D, D, J, 40A]
    8    WCSDVARR    ImageHDU        15  (65, 33)      float32
    9    WCSDVARR    ImageHDU        15  (65, 33)      float32
    10   WCSDVARR    ImageHDU        15  (65, 33)      float32
    11   WCSDVARR    ImageHDU        15  (65, 33)      float32
    12   D2IMARR     ImageHDU        12  (4096,)       float32
    13   HDRLET  NonstandardExtHDU   13
    14   HDRLET  NonstandardExtHDU   13

The names of the `headerlet` extensions are both HDRLET, but its type shows up as `NonstandardExtHDU`.  Their headers can be read, and while
their data can be read you'd have to know what to do with it (the data is actually either a tar file or a gzipped tar file containing the
`headerlet` file).  However, if you have `stwcs.wcsutil.headerlet` imported, PyFits will recognize these extensions as `Headerlet HDUs`::

    >>> import stwcs.wcsutil.headerlet
    >>> # Note that it's necessary to reopen the file
    >>> hdul = pyfits.open('j94f05bgq_flt_with_hlet.fits')
    >>> hdul.info()
    Filename: j94f05bgq_flt_with_hlet.fits
    No.    Name         Type      Cards   Dimensions   Format
    0    PRIMARY     PrimaryHDU     248  ()            int16
    1    SCI         ImageHDU       286  (4096, 2048)  float32
    2    ERR         ImageHDU        76  (4096, 2048)  float32
    3    DQ          ImageHDU        66  (4096, 2048)  int16
    4    SCI         ImageHDU       282  (4096, 2048)  float32
    5    ERR         ImageHDU        74  (4096, 2048)  float32
    6    DQ          ImageHDU        66  (4096, 2048)  int16
    7    WCSCORR     BinTableHDU     56  10R x 23C     [40A, I, 1A, D, D, D, D, D, D, D, D, 24A, 24A, D, D, D, D, D, D, D, D, J, 40A]
    8    WCSDVARR    ImageHDU        15  (65, 33)      float32
    9    WCSDVARR    ImageHDU        15  (65, 33)      float32
    10   WCSDVARR    ImageHDU        15  (65, 33)      float32
    11   WCSDVARR    ImageHDU        15  (65, 33)      float32
    12   D2IMARR     ImageHDU        12  (4096,)       float32
    13   HDRLET      HeaderletHDU    13
    14   HDRLET      HeaderletHDU    13
    >>> print hdul['HDRLET', 1].header.ascard
    XTENSION= 'HDRLET  '           / Headerlet extension                            
    BITPIX  =                    8 / array data type                                
    NAXIS   =                    1 / number of array dimensions                     
    NAXIS1  =               102400 / Axis length                                    
    PCOUNT  =                    0 / number of parameters                           
    GCOUNT  =                    1 / number of groups                               
    EXTNAME = 'HDRLET  '           / name of the headerlet extension                
    HDRNAME = 'j94f05bgq_orig'     / Headerlet name                                 
    DATE    = '2011-04-13T12:14:42' / Date FITS file was generated                  
    SIPNAME = 'IDC_qbu1641sj'      / SIP distortion model name                      
    NPOLFILE= '/grp/hst/acs/lucas/new-npl/qbu16424j_npl.fits' / Non-polynomial correction
    D2IMFILE= '/grp/hst/acs/lucas/new-npl/wfc_ref68col_d2i.fits' / Column correction
    COMPRESS=                    F / Uses gzip compression 

`HeaderletHDU` objects are similar to other HDU objects in PyFits.  However, they have a special `.headerlet` attribute that returns
the actual `headerlet` contained in the HDU data as a `Headerlet` object::

    >>> hdrlet = hdul['HDERLET', 1].headerlet
    >>> hdrlet.info()
    Filename: (No file associated with this HDUList)
    No.    Name         Type      Cards   Dimensions   Format
    0    PRIMARY     PrimaryHDU      12  ()            uint8
    1    SIPWCS      ImageHDU       111  ()            uint8
    2    SIPWCS      ImageHDU       110  ()            uint8
    3    WCSDVARR    ImageHDU        15  (65, 33)      float32
    4    WCSDVARR    ImageHDU        15  (65, 33)      float32
    5    WCSDVARR    ImageHDU        15  (65, 33)      float32
    6    WCSDVARR    ImageHDU        15  (65, 33)      float32
    7    D2IMARR     ImageHDU        12  (4096,)       float32

This is useful if you want to view the contents of the `headerlets` attached to a file.

.. _Appendix1:

Appendix 1: Headerlet API
=========================

* :ref:`apply_headerlet_as_alternate`
* :ref:`apply_headerlet_as_primary`
* :ref:`archive_as_headerlet`
* :ref:`attach_headerlet`
* :ref:`create_headerlet`
* :ref:`delete_headerlet`
* :ref:`extract_headerlet`
* :ref:`print_summary`
* :ref:`restore_all_with_distname`
* :ref:`restore_from_headerlet`
* :ref:`write_headerlet`

.. _apply_headerlet_as_alternate:

apply_headerlet_as_alternate
----------------------------

::

    def apply_headerlet_as_alternate(filename, hdrlet, attach=True, wcskey=None,
                                    wcsname=None, logging=False, logmode='w'):
        """
        Apply headerlet to a science observation as an alternate WCS
    
        Parameters
        ----------
        filename: string
                 File name of science observation whose WCS solution will be updated
        hdrlet: string
                 Headerlet file
        attach: boolean
              flag indicating if the headerlet should be attached as a
              HeaderletHDU to fobj. If True checks that HDRNAME is unique
              in the fobj and stops if not.
        wcskey: string
              Key value (A-Z, except O) for this alternate WCS
              If None, the next available key will be used
        wcsname: string
              Name to be assigned to this alternate WCS
              WCSNAME is a required keyword in a Headerlet but this allows the
              user to change it as desired.
        logging: boolean
              enable file logging
        logmode: 'a' or 'w'
        """

.. _apply_headerlet_as_primary:

apply_headerlet_as_primary
--------------------------

::

    def apply_headerlet_as_primary(filename, hdrlet, attach=True, archive=True,
                                    force=False, logging=False, logmode='a'):
        """
        Apply headerlet 'hdrfile' to a science observation 'destfile' as the primary WCS
    
        Parameters
        ----------
        filename: string
                 File name of science observation whose WCS solution will be updated
        hdrlet: string
                 Headerlet file
        attach: boolean
                True (default): append headerlet to FITS file as a new extension.
        archive: boolean
                True (default): before updating, create a headerlet with the
                WCS old solution.
        force: boolean
                If True, this will cause the headerlet to replace the current PRIMARY
                WCS even if it has a different distortion model. [Default: False]
        logging: boolean
                enable file logging
        logmode: 'w' or 'a'
                 log file open mode
        """

.. _archive_as_headerlet:

archive_as_headerlet
--------------------

::

    def archive_as_headerlet(filename, hdrname, sciext='SCI',
                            wcsname=None, wcskey=None, destim=None,
                            sipname=None, npolfile=None, d2imfile=None,
                            author=None, descrip=None, history=None,
                            nmatch=None, catalog=None,
                            logging=False, logmode='w'):
        """
        Save a WCS as a headerlet extension and write it out to a file.
    
        This function will create a headerlet, attach it as an extension to the
        science image (if it has not already been archived) then, optionally,
        write out the headerlet to a separate headerlet file.
    
        Either wcsname or wcskey must be provided, if both are given, they must match a valid WCS
        Updates wcscorr if necessary.
    
        Parameters
        ----------
        filename: string or HDUList
               Either a filename or PyFITS HDUList object for the input science file
                An input filename (str) will be expanded as necessary to interpret
                any environmental variables included in the filename.
        hdrname: string
            Unique name for this headerlet, stored as HDRNAME keyword
        sciext: string
            name (EXTNAME) of extension that contains WCS to be saved
        wcsname: string
            name of WCS to be archived, if " ": stop
        wcskey: one of A...Z or " " or "PRIMARY"
            if " " or "PRIMARY" - archive the primary WCS
        destim: string
            DESTIM keyword
            if  NOne, use ROOTNAME or science file name
        sipname: string or None (default)
                 Name of unique file where the polynomial distortion coefficients were
                 read from. If None, the behavior is:
                 The code looks for a keyword 'SIPNAME' in the science header
                 If not found, for HST it defaults to 'IDCTAB'
                 If there is no SIP model the value is 'NOMODEL'
                 If there is a SIP model but no SIPNAME, it is set to 'UNKNOWN'
        npolfile: string or None (default)
                 Name of a unique file where the non-polynomial distortion was stored.
                 If None:
                 The code looks for 'NPOLFILE' in science header.
                 If 'NPOLFILE' was not found and there is no npol model, it is set to 'NOMODEL'
                 If npol model exists, it is set to 'UNKNOWN'
        d2imfile: string
                 Name of a unique file where the detector to image correction was
                 stored. If None:
                 The code looks for 'D2IMFILE' in the science header.
                 If 'D2IMFILE' is not found and there is no d2im correction,
                 it is set to 'NOMODEL'
                 If d2im correction exists, but 'D2IMFILE' is missing from science
                 header, it is set to 'UNKNOWN'
        author: string
                Name of user who created the headerlet, added as 'AUTHOR' keyword
                to headerlet PRIMARY header
        descrip: string
                Short description of the solution provided by the headerlet
                This description will be added as the single 'DESCRIP' keyword
                to the headerlet PRIMARY header
        history: filename, string or list of strings
                Long (possibly multi-line) description of the solution provided
                by the headerlet. These comments will be added as 'HISTORY' cards
                to the headerlet PRIMARY header
                If filename is specified, it will format and attach all text from
                that file as the history.
        logging: boolean
                enable file folling
        logmode: 'w' or 'a'
                 log file open mode
        """

.. _attach_headerlet:

attach_headerlet
----------------

::

        def attach_headerlet(filename, hdrlet, logging=False, logmode='a'):
            """
            Attach Headerlet as an HeaderletHDU to a science file
        
            Parameters
            ----------
            filename: string, HDUList
                    science file to which the headerlet should be applied
            hdrlet: string or Headerlet object
                    string representing a headerlet file
            logging: boolean
                    enable file logging
            logmode: 'a' or 'w'
            """

.. _create_headerlet:

create_headerlet
----------------

::

    def create_headerlet(filename, sciext='SCI', hdrname=None, destim=None,
                        wcskey=" ", wcsname=None,
                        sipname=None, npolfile=None, d2imfile=None,
                        author=None, descrip=None, history=None,
                        nmatch=None, catalog=None,
                        logging=False, logmode='w'):
        """
        Create a headerlet from a WCS in a science file
        If both wcskey and wcsname are given they should match, if not
        raise an Exception
        
        Parameters
        ----------
        filename: string or HDUList
               Either a filename or PyFITS HDUList object for the input science file
                An input filename (str) will be expanded as necessary to interpret
                any environmental variables included in the filename.
        sciext: string or python list (default: 'SCI')
               Extension in which the science data with the linear WCS is. 
               The headerlet will be created from these extensions.
               If string - a valid EXTNAME is expected
               If int - specifies an extension with a valid WCS, such as 0 for a
               simple FITS file
               If list - a list of FITS extension numbers or strings representing
               extension tuples, e.g. ('SCI, 1') is expected.
        hdrname: string
               value of HDRNAME keyword
               Takes the value from the HDRNAME<wcskey> keyword, if not available from WCSNAME<wcskey>
               It stops if neither is found in the science file and a value is not provided
        destim: string or None
                name of file this headerlet can be applied to
                if None, use ROOTNAME keyword
        wcskey: char (A...Z) or " " or "PRIMARY" or None
                a char representing an alternate WCS to be used for the headerlet
                if " ", use the primary (default)
                if None use wcsname
        wcsname: string or None
                if wcskey is None use wcsname specified here to choose an alternate WCS for the headerlet
        sipname: string or None (default)
                 Name of unique file where the polynomial distortion coefficients were
                 read from. If None, the behavior is:
                 The code looks for a keyword 'SIPNAME' in the science header
                 If not found, for HST it defaults to 'IDCTAB'
                 If there is no SIP model the value is 'NOMODEL'
                 If there is a SIP model but no SIPNAME, it is set to 'UNKNOWN'
        npolfile: string or None (default)
                 Name of a unique file where the non-polynomial distortion was stored.
                 If None:
                 The code looks for 'NPOLFILE' in science header.
                 If 'NPOLFILE' was not found and there is no npol model, it is set to 'NOMODEL'
                 If npol model exists, it is set to 'UNKNOWN'
        d2imfile: string
                 Name of a unique file where the detector to image correction was
                 stored. If None:
                 The code looks for 'D2IMFILE' in the science header.
                 If 'D2IMFILE' is not found and there is no d2im correction,
                 it is set to 'NOMODEL'
                 If d2im correction exists, but 'D2IMFILE' is missing from science
                 header, it is set to 'UNKNOWN'
        author: string
                Name of user who created the headerlet, added as 'AUTHOR' keyword
                to headerlet PRIMARY header
        descrip: string
                Short description of the solution provided by the headerlet
                This description will be added as the single 'DESCRIP' keyword
                to the headerlet PRIMARY header
        history: filename, string or list of strings
                Long (possibly multi-line) description of the solution provided
                by the headerlet. These comments will be added as 'HISTORY' cards
                to the headerlet PRIMARY header
                If filename is specified, it will format and attach all text from
                that file as the history.
        nmatch: int (optional)
                Number of sources used in the new solution fit
        catalog: string (optional)
                Astrometric catalog used for headerlet solution
        logging: boolean
                 enable file logging
        logmode: 'w' or 'a'
                 log file open mode
    
        Returns
        -------
        Headerlet object
        """

.. _delete_headerlet:

delete_headerlet
----------------

::

        def delete_headerlet(filename, hdrname=None, hdrext=None, distname=None,
                             logging=False, logmode='w'):
            """
            Deletes HeaderletHDU(s) from a science file
        
            Notes
            -----
            One of hdrname, hdrext or distname should be given.
            If hdrname is given - delete a HeaderletHDU with a name HDRNAME from fobj.
            If hdrext is given - delete HeaderletHDU in extension.
            If distname is given - deletes all HeaderletHDUs with a specific distortion model from fobj.
            Updates wcscorr
        
            Parameters
            ----------
            filename: string or HDUList
                   Either a filename or PyFITS HDUList object for the input science file
                    An input filename (str) will be expanded as necessary to interpret
                    any environmental variables included in the filename.
            hdrname: string or None
                HeaderletHDU primary header keyword HDRNAME
            hdrext: int, tuple or None
                HeaderletHDU FITS extension number
                tuple has the form ('HDRLET', 1)
            distname: string or None
                distortion model as specified in the DISTNAME keyword
            logging: boolean
                     enable file logging
            logmode: 'a' or 'w'
            """
        
.. _extract_headerlet:

extract_headerlet
-----------------

::

        def extract_headerlet(filename, output, extnum=None, hdrname=None,
                              clobber=False, logging=True):
            """
            Finds a headerlet extension in a science file and writes it out as
            a headerlet FITS file.
        
            If both hdrname and extnum are given they should match, if not
            raise an Exception
        
            Parameters
            ----------
            filename: string or HDUList or Python list
                This specifies the name(s) of science file(s) from which headerlets
                will be extracted.
        
                String input formats supported include use of wild-cards, IRAF-style
                '@'-files (given as '@<filename>') and comma-separated list of names.
                An input filename (str) will be expanded as necessary to interpret
                any environmental variables included in the filename.
                If a list of filenames has been specified, it will extract a
                headerlet from the same extnum from all filenames.
            output: string
                   Filename or just rootname of output headerlet FITS file
                   If string does not contain '.fits', it will create a filename with
                   '_hlet.fits' suffix
            extnum: int
                   Extension number which contains the headerlet to be written out
            hdrname: string
                   Unique name for headerlet, stored as the HDRNAME keyword
                   It stops if a value is not provided and no extnum has been specified
            clobber: bool
                If output file already exists, this parameter specifies whether or not
                to overwrite that file [Default: False]
            logging: boolean
                     enable logging to a file
        
            """

.. _print_summary:
 
print_summary
-------------
 
 ::
 
        def print_summary(summary_cols, summary_dict, pad=2, maxwidth=None, idcol=None,
                           output=None, clobber=True, quiet=False ):
           """
           Print out summary dictionary to STDOUT, and possibly an output file
       
           """
    
.. _restore_all_with_distname:

restore_all_with_distname
-------------------------

::

    def restore_all_with_distname(filename, distname, primary, archive=True,
                                  sciext='SCI', logging=False, logmode='w'):
        """
        Restores all HeaderletHDUs with a given distortion model as alternate WCSs and a primary
    
        Parameters
        --------------
        filename: string or HDUList
               Either a filename or PyFITS HDUList object for the input science file
                An input filename (str) will be expanded as necessary to interpret
                any environmental variables included in the filename.
        distname: string
            distortion model as represented by a DISTNAME keyword
        primary: int or string or None
            HeaderletHDU to be restored as primary
            if int - a fits extension
            if string - HDRNAME
            if None - use first HeaderletHDU
        archive: boolean (default True)
            flag indicating if HeaderletHDUs should be created from the
            primary and alternate WCSs in fname before restoring all matching
            headerlet extensions
        logging: boolean
             enable file logging
        logmode: 'a' or 'w'
        """
        
.. _restore_from_headerlet:

restore_from_headerlet
----------------------

::

    def restore_from_headerlet(filename, hdrname=None, hdrext=None, archive=True,
                               force=False, logging=False, logmode='w'):
        """
        Restores a headerlet as a primary WCS
    
        Parameters
        ----------
        filename: string or HDUList
               Either a filename or PyFITS HDUList object for the input science file
                An input filename (str) will be expanded as necessary to interpret
                any environmental variables included in the filename.
        hdrname: string
            HDRNAME keyword of HeaderletHDU
        hdrext: int or tuple
            Headerlet extension number of tuple ('HDRLET',2)
        archive: boolean (default: True)
            When the distortion model in the headerlet is the same as the distortion model of
            the science file, this flag indicates if the primary WCS should be saved as an alternate
            nd a headerlet extension.
            When the distortion models do not match this flag indicates if the current primary and
            alternate WCSs should be archived as headerlet extensions and alternate WCS.
        force: boolean (default:False)
            When the distortion models of the headerlet and the primary do not match, and archive
            is False, this flag forces an update of the primary.
        logging: boolean
               enable file logging
        logmode: 'a' or 'w'
        """

.. _write_headerlet:

write_headerlet
---------------

::

    def write_headerlet(filename, hdrname, output=None, sciext='SCI',
                            wcsname=None, wcskey=None, destim=None,
                            sipname=None, npolfile=None, d2imfile=None,
                            author=None, descrip=None, history=None,
                            nmatch=None, catalog=None,
                            attach=True, clobber=False, logging=False):
    
        """
        Save a WCS as a headerlet FITS file.
    
        This function will create a headerlet, write out the headerlet to a
        separate headerlet file, then, optionally, attach it as an extension
        to the science image (if it has not already been archived)
    
        Either wcsname or wcskey must be provided; if both are given, they must
        match a valid WCS.
    
        Updates wcscorr if necessary.
    
        Parameters
        ----------
        filename: string or HDUList or Python list
            This specifies the name(s) of science file(s) from which headerlets
            will be created and written out.
            String input formats supported include use of wild-cards, IRAF-style
            '@'-files (given as '@<filename>') and comma-separated list of names.
            An input filename (str) will be expanded as necessary to interpret
            any environmental variables included in the filename.
        hdrname: string
            Unique name for this headerlet, stored as HDRNAME keyword
        output: string or None
            Filename or just rootname of output headerlet FITS file
            If string does not contain '.fits', it will create a filename
            starting with the science filename and ending with '_hlet.fits'.
            If None, a default filename based on the input filename will be
            generated for the headerlet FITS filename
        sciext: string
            name (EXTNAME) of extension that contains WCS to be saved
        wcsname: string
            name of WCS to be archived, if " ": stop
        wcskey: one of A...Z or " " or "PRIMARY"
            if " " or "PRIMARY" - archive the primary WCS
        destim: string
            DESTIM keyword
            if  NOne, use ROOTNAME or science file name
        sipname: string or None (default)
             Name of unique file where the polynomial distortion coefficients were
             read from. If None, the behavior is:
             The code looks for a keyword 'SIPNAME' in the science header
             If not found, for HST it defaults to 'IDCTAB'
             If there is no SIP model the value is 'NOMODEL'
             If there is a SIP model but no SIPNAME, it is set to 'UNKNOWN'
        npolfile: string or None (default)
             Name of a unique file where the non-polynomial distortion was stored.
             If None:
             The code looks for 'NPOLFILE' in science header.
             If 'NPOLFILE' was not found and there is no npol model, it is set to 'NOMODEL'
             If npol model exists, it is set to 'UNKNOWN'
        d2imfile: string
             Name of a unique file where the detector to image correction was
             stored. If None:
             The code looks for 'D2IMFILE' in the science header.
             If 'D2IMFILE' is not found and there is no d2im correction,
             it is set to 'NOMODEL'
             If d2im correction exists, but 'D2IMFILE' is missing from science
             header, it is set to 'UNKNOWN'
        author: string
            Name of user who created the headerlet, added as 'AUTHOR' keyword
            to headerlet PRIMARY header
        descrip: string
            Short description of the solution provided by the headerlet
            This description will be added as the single 'DESCRIP' keyword
            to the headerlet PRIMARY header
        history: filename, string or list of strings
            Long (possibly multi-line) description of the solution provided
            by the headerlet. These comments will be added as 'HISTORY' cards
            to the headerlet PRIMARY header
            If filename is specified, it will format and attach all text from
            that file as the history.
        attach: bool
            Specify whether or not to attach this headerlet as a new extension
            It will verify that no other headerlet extension has been created with
            the same 'hdrname' value.
        clobber: bool
            If output file already exists, this parameter specifies whether or not
            to overwrite that file [Default: False]
        logging: boolean
             enable file logging
        """
    
References
----------

.. _FITSConventions: http://mediawiki.stsci.edu/mediawiki/index.php/Telescopedia:FITSDistortionConventions