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.. _hstwcs_ui:
**************************************
HSTWCS Examples
**************************************
--------------------------
Create an HSTWCS Object
--------------------------
- Create an HSTWCS object using a pyfits HDUList and an extension number
``fobj = pyfits.open('some_file.fits')``
``w = wcsutil.HSTWCS(fobj, 3)``
- Create an HSTWCS object using a qualified file name.
``w = wcsutil.HSTWCS('j9irw4b1q_flt.fits[sci,1]')``
- Create an HSTWCS object using a file name and an extension number.
``w = wcsutil.HSTWCS('j9irw4b1q_flt.fits', ext=2)``
- Create an HSTWCS object from WCS with key 'O'.
``w = wcsutil.HSTWCS('j9irw4b1q_flt.fits', ext=2, wcskey='O')``
- Create a template HSTWCS object for a DEFAULT object.
``w = wcsutil.HSTWCS(instrument='DEFAULT')``
----------------------------------
Coordinate Transformation Examples
----------------------------------
All coordinate transformation functions accept input coordinates
as 2D numpy arrays or 2 sequences of X and Y coordinates.
``inpix = np.array([[1., 2.], [1,3], [1,4], [1,5]])``
or
``X = [1.,1.,1.,1.]``
``Y = np.array([2.,3.,4.,5.])``
In addition all transformation functions require an `origin` parameter
which specifies if the coordinates are 0 or 1 based. For example in FITS
and Fortran, coordinates start from 1, while in Python and C, the index
of the first image pixel is (0,0).
- Apply the entire detector to sky transformation at once:
``outpix=w1.all_pix2sky(inpix,1)``
``outpix=w1.all_pix2sky(X, Y,1)``
- The same transformation can be done in separate steps:
1. Apply the detector to image correction
``dpx = w.det2im(inpix,1)``
2. Aply the SIP polynomial distortion
``spx = w.sip_pix2foc(dpx, 1)``
3. Apply the non-polynomial distortion from the lookup table
``lutpx = w.p4_pix2foc(dpx,1)``
4. The undistorted coordinates are the sum of the input coordinates with
the deltas for the distortion corrections.
``fpix = dpx + (spx-dpx) +(lutpx-dpx)``
5. Finally the transformation from undistorted to world coordinates is done
by applying the linear WCS.
``wpix = w.wcs_pix2sky(fpix, 1)``
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