|
|
| .. _astropy-wcs: |
|
|
| *************************************** |
| World Coordinate System (`astropy.wcs`) |
| *************************************** |
|
|
| .. _wcslib: http://www.atnf.csiro.au/people/mcalabre/WCS/wcslib/index.html |
| .. _distortion paper: http://www.atnf.csiro.au/people/mcalabre/WCS/dcs_20040422.pdf |
| .. _SIP: http://irsa.ipac.caltech.edu/data/SPITZER/docs/files/spitzer/shupeADASS.pdf |
|
|
| Introduction |
| ============ |
|
|
| `astropy.wcs` contains utilities for managing World Coordinate System |
| (WCS) transformations in FITS files. These transformations map the |
| pixel locations in an image to their real-world units, such as their |
| position on the sky sphere. These transformations can work both |
| forward (from pixel to sky) and backward (from sky to pixel). |
|
|
| It performs three separate classes of WCS transformations: |
|
|
| - Core WCS, as defined in the `FITS WCS standard`_, based on Mark |
| Calabretta's `wcslib`_. (Also includes ``TPV`` and ``TPD`` |
| distortion, but not ``SIP``). |
|
|
| - Simple Imaging Polynomial (`SIP`_) convention. (See :doc:`note about SIP in headers <note_sip>`.) |
|
|
| - table lookup distortions as defined in the FITS WCS `distortion |
| paper`_. |
|
|
| Each of these transformations can be used independently or together in |
| a standard pipeline. |
|
|
| Getting Started |
| =============== |
|
|
| The basic workflow is as follows: |
|
|
| 1. ``from astropy import wcs`` |
|
|
| 2. Call the `~astropy.wcs.WCS` constructor with an |
| `astropy.io.fits` `~astropy.io.fits.Header` and/or |
| `~astropy.io.fits.HDUList` object. |
|
|
| 3. Optionally, if the FITS file uses any deprecated or |
| non-standard features, you may need to call one of the |
| `~astropy.wcs.wcs.WCS.fix` methods on the object. |
|
|
| 4. Use one of the following transformation methods: |
|
|
| - From pixels to world coordinates: |
|
|
| - `~astropy.wcs.wcs.WCS.all_pix2world`: Perform all three |
| transformations in series (core WCS, SIP and table lookup |
| distortions) from pixel to world coordinates. Use this one |
| if you're not sure which to use. |
|
|
| - `~astropy.wcs.wcs.WCS.wcs_pix2world`: Perform just the core |
| WCS transformation from pixel to world coordinates. |
|
|
| - From world to pixel coordinates: |
|
|
| - `~astropy.wcs.wcs.WCS.all_world2pix`: Perform all three |
| transformations (core WCS, SIP and table lookup |
| distortions) from world to pixel coordinates, using an |
| iterative method if necessary. |
|
|
| - `~astropy.wcs.wcs.WCS.wcs_world2pix`: Perform just the core |
| WCS transformation from world to pixel coordinates. |
|
|
| - Performing `SIP`_ transformations only: |
|
|
| - `~astropy.wcs.wcs.WCS.sip_pix2foc`: Convert from pixel to |
| focal plane coordinates using the `SIP`_ polynomial |
| coefficients. |
|
|
| - `~astropy.wcs.wcs.WCS.sip_foc2pix`: Convert from focal |
| plane to pixel coordinates using the `SIP`_ polynomial |
| coefficients. |
|
|
| - Performing `distortion paper`_ transformations only: |
|
|
| - `~astropy.wcs.wcs.WCS.p4_pix2foc`: Convert from pixel to |
| focal plane coordinates using the table lookup distortion |
| method described in the FITS WCS `distortion paper`_. |
|
|
| - `~astropy.wcs.wcs.WCS.det2im`: Convert from detector |
| coordinates to image coordinates. Commonly used for narrow |
| column correction. |
|
|
| For example, to convert pixel coordinates from a two dimensional image to world coordinates:: |
| |
| >>> from astropy.wcs import WCS |
| >>> w = WCS('image.fits') |
| >>> lon, lat = w.all_pix2world(30, 40, 0) |
| >>> print(lon, lat) |
| 31.0 41.0 |
|
|
|
|
| Using `astropy.wcs` |
| =================== |
|
|
| Loading WCS information from a FITS file |
| ---------------------------------------- |
|
|
| This example loads a FITS file (supplied on the commandline) and uses |
| the WCS cards in its primary header to transform. |
|
|
| .. literalinclude:: examples/from_file.py |
| :language: python |
|
|
| Building a WCS structure programmatically |
| ----------------------------------------- |
|
|
| This example, rather than starting from a FITS header, sets WCS values |
| programmatically, uses those settings to transform some points, and then |
| saves those settings to a new FITS header. |
|
|
| .. literalinclude:: examples/programmatic.py |
| :language: python |
|
|
| .. note:: |
| The members of the WCS object correspond roughly to the key/value |
| pairs in the FITS header. However, they are adjusted and |
| normalized in a number of ways that make performing the WCS |
| transformation easier. Therefore, they can not be relied upon to |
| get the original values in the header. To build up a FITS header |
| directly and specifically, use `astropy.io.fits.Header` directly. |
|
|
| .. _wcslint: |
|
|
| Validating the WCS keywords in a FITS file |
| ------------------------------------------ |
|
|
| Astropy includes a commandline tool, ``wcslint`` to check the WCS |
| keywords in a FITS file:: |
| |
| > wcslint invalid.fits |
| HDU 1: |
| WCS key ' ': |
| - RADECSYS= 'ICRS ' / Astrometric system |
| RADECSYS is non-standard, use RADESYSa. |
| - The WCS transformation has more axes (2) than the image it is |
| associated with (0) |
| - 'celfix' made the change 'PV1_5 : Unrecognized coordinate |
| transformation parameter'. |
|
|
| HDU 2: |
| WCS key ' ': |
| - The WCS transformation has more axes (3) than the image it is |
| associated with (0) |
| - 'celfix' made the change 'In CUNIT2 : Mismatched units type |
| 'length': have 'Hz', want 'm''. |
| - 'unitfix' made the change 'Changed units: 'HZ ' -> 'Hz''. |
|
|
| Bounds checking |
| --------------- |
|
|
| Bounds checking is enabled by default, and any computed world |
| coordinates outside of [-180°, 180°] for longitude and [-90°, 90°] in |
| latitude are marked as invalid. To disable this behavior, use |
| `astropy.wcs.Wcsprm.bounds_check`. |
|
|
| Supported projections |
| ===================== |
|
|
| As `astropy.wcs` is based on `wcslib`_, it supports the standard |
| projections defined in the `FITS WCS standard`_. These projection |
| codes are specified in the second part of the ``CTYPEn`` keywords |
| (accessible through `Wcsprm.ctype <astropy.wcs.Wcsprm.ctype>`), for |
| example, ``RA---TAN-SIP``. The supported projection codes are: |
|
|
| - ``AZP``: zenithal/azimuthal perspective |
| - ``SZP``: slant zenithal perspective |
| - ``TAN``: gnomonic |
| - ``STG``: stereographic |
| - ``SIN``: orthographic/synthesis |
| - ``ARC``: zenithal/azimuthal equidistant |
| - ``ZPN``: zenithal/azimuthal polynomial |
| - ``ZEA``: zenithal/azimuthal equal area |
| - ``AIR``: Airy's projection |
| - ``CYP``: cylindrical perspective |
| - ``CEA``: cylindrical equal area |
| - ``CAR``: plate carrée |
| - ``MER``: Mercator's projection |
| - ``COP``: conic perspective |
| - ``COE``: conic equal area |
| - ``COD``: conic equidistant |
| - ``COO``: conic orthomorphic |
| - ``SFL``: Sanson-Flamsteed ("global sinusoid") |
| - ``PAR``: parabolic |
| - ``MOL``: Mollweide's projection |
| - ``AIT``: Hammer-Aitoff |
| - ``BON``: Bonne's projection |
| - ``PCO``: polyconic |
| - ``TSC``: tangential spherical cube |
| - ``CSC``: COBE quadrilateralized spherical cube |
| - ``QSC``: quadrilateralized spherical cube |
| - ``HPX``: HEALPix |
| - ``XPH``: HEALPix polar, aka "butterfly" |
|
|
| And, if built with wcslib 5.0 or later, the following polynomial |
| distortions are supported: |
|
|
| - ``TPV``: Polynomial distortion |
| - ``TUV``: Polynomial distortion |
|
|
| .. note:: |
|
|
| Though wcslib 5.4 and later handles ``SIP`` polynomial distortion, |
| for backward compatibility, ``SIP`` is handled by astropy itself |
| and methods exist to handle it specially. |
|
|
| Subsetting and Pixel Scales |
| =========================== |
|
|
| WCS objects can be broken apart into their constituent axes using the |
| `~astropy.wcs.WCS.sub` function. There is also a `~astropy.wcs.WCS.celestial` |
| convenience function that will return a WCS object with only the celestial axes |
| included. |
|
|
| The pixel scales of a celestial image or the pixel dimensions of a non-celestial |
| image can be extracted with the utility functions |
| `~astropy.wcs.utils.proj_plane_pixel_scales` and |
| `~astropy.wcs.utils.non_celestial_pixel_scales`. Likewise, celestial pixel |
| area can be extracted with the utility function |
| `~astropy.wcs.utils.proj_plane_pixel_area`. |
|
|
| Matplotlib plots with correct WCS projection |
| ============================================ |
|
|
| The :ref:`WCSAxes <wcsaxes>` framework, previously a standalone package, allows |
| the :class:`~astropy.wcs.WCS` to be used to define projections in Matplotlib. |
| More information on using WCSAxes can be found :ref:`here <wcsaxes>`. |
|
|
| .. plot:: |
| :context: reset |
| :include-source: |
| :align: center |
|
|
| from matplotlib import pyplot as plt |
| from astropy.io import fits |
| from astropy.wcs import WCS |
| from astropy.utils.data import get_pkg_data_filename |
|
|
| filename = get_pkg_data_filename('tutorials/FITS-images/HorseHead.fits') |
|
|
| hdu = fits.open(filename)[0] |
| wcs = WCS(hdu.header) |
|
|
| fig = plt.figure() |
| fig.add_subplot(111, projection=wcs) |
| plt.imshow(hdu.data, origin='lower', cmap=plt.cm.viridis) |
| plt.xlabel('RA') |
| plt.ylabel('Dec') |
|
|
| Other information |
| ================= |
|
|
| .. toctree:: |
| :maxdepth: 1 |
|
|
| relax |
| history |
| wcsapi |
|
|
| .. note that if this section gets too long, it should be moved to a separate |
| doc page - see the top of performance.inc.rst for the instructions on how to do |
| that |
| .. include:: performance.inc.rst |
|
|
| See Also |
| ======== |
|
|
| - `wcslib`_ |
|
|
| Reference/API |
| ============= |
|
|
| .. automodapi:: astropy.wcs |
| :inherited-members: |
|
|
| .. automodapi:: astropy.wcs.utils |
|
|
| Acknowledgments and Licenses |
| ============================ |
|
|
| `wcslib`_ is licenced under the `GNU Lesser General Public License |
| <http://www.gnu.org/licenses/lgpl.html>`_. |
|
|
