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LSST Data Management Base Package
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lsst.meas.astrom.approximateWcs Namespace Reference

Classes

class  _MockTestCase
 

Functions

 approximateWcs (wcs, bbox, order=3, nx=20, ny=20, iterations=3, skyTolerance=0.001 *lsst.geom.arcseconds, pixelTolerance=0.02, useTanWcs=False)
 

Function Documentation

◆ approximateWcs()

lsst.meas.astrom.approximateWcs.approximateWcs ( wcs,
bbox,
order = 3,
nx = 20,
ny = 20,
iterations = 3,
skyTolerance = 0.001*lsst.geom.arcseconds,
pixelTolerance = 0.02,
useTanWcs = False )
Approximate an existing WCS as a TAN-SIP WCS

The fit is performed by evaluating the WCS at a uniform grid of points
within a bounding box.

Parameters
----------
wcs : `lsst.afw.geom.SkyWcs`
    wcs to approximate
bbox : `lsst.geom.Box2I`
    the region over which the WCS will be fit
order : `int`
    order of SIP fit
nx : `int`
    number of grid points along x
ny : `int`
    number of grid points along y
iterations : `int`
    number of times to iterate over fitting
skyTolerance : `lsst.geom.Angle`
    maximum allowed difference in world coordinates between
    input wcs and approximate wcs (default is 0.001 arcsec)
pixelTolerance : `float`
    maximum allowed difference in pixel coordinates between
    input wcs and approximate wcs (default is 0.02 pixels)
useTanWcs : `bool`
    send a TAN version of wcs to the fitter? It is documented to require that,
    but I don't think the fitter actually cares

Returns
-------
fitWcs : `lsst.afw.geom.SkyWcs`
    the fit TAN-SIP WCS

Definition at line 43 of file approximateWcs.py.

44 skyTolerance=0.001*lsst.geom.arcseconds, pixelTolerance=0.02, useTanWcs=False):
45 """Approximate an existing WCS as a TAN-SIP WCS
46
47 The fit is performed by evaluating the WCS at a uniform grid of points
48 within a bounding box.
49
50 Parameters
51 ----------
52 wcs : `lsst.afw.geom.SkyWcs`
53 wcs to approximate
54 bbox : `lsst.geom.Box2I`
55 the region over which the WCS will be fit
56 order : `int`
57 order of SIP fit
58 nx : `int`
59 number of grid points along x
60 ny : `int`
61 number of grid points along y
62 iterations : `int`
63 number of times to iterate over fitting
64 skyTolerance : `lsst.geom.Angle`
65 maximum allowed difference in world coordinates between
66 input wcs and approximate wcs (default is 0.001 arcsec)
67 pixelTolerance : `float`
68 maximum allowed difference in pixel coordinates between
69 input wcs and approximate wcs (default is 0.02 pixels)
70 useTanWcs : `bool`
71 send a TAN version of wcs to the fitter? It is documented to require that,
72 but I don't think the fitter actually cares
73
74 Returns
75 -------
76 fitWcs : `lsst.afw.geom.SkyWcs`
77 the fit TAN-SIP WCS
78 """
79 if useTanWcs:
80 crpix = wcs.getPixelOrigin()
81 crval = wcs.getSkyOrigin()
82 cdMatrix = wcs.getCdMatrix(crpix)
83 tanWcs = afwGeom.makeSkyWcs(crpix=crpix, crval=crval, cdMatrix=cdMatrix)
84 else:
85 tanWcs = wcs
86
87 # create a matchList consisting of a grid of points covering the bbox
88 refSchema = afwTable.SimpleTable.makeMinimalSchema()
89 refCoordKey = afwTable.CoordKey(refSchema["coord"])
90 refCat = afwTable.SimpleCatalog(refSchema)
91
92 sourceSchema = afwTable.SourceTable.makeMinimalSchema()
93 SingleFrameMeasurementTask(schema=sourceSchema) # expand the schema
94 sourceCentroidKey = afwTable.Point2DKey(sourceSchema["slot_Centroid"])
95
96 sourceCat = afwTable.SourceCatalog(sourceSchema)
97
98 matchList = []
99
100 bboxd = lsst.geom.Box2D(bbox)
101 for x in np.linspace(bboxd.getMinX(), bboxd.getMaxX(), nx):
102 for y in np.linspace(bboxd.getMinY(), bboxd.getMaxY(), ny):
103 pixelPos = lsst.geom.Point2D(x, y)
104 skyCoord = wcs.pixelToSky(pixelPos)
105
106 refObj = refCat.addNew()
107 refObj.set(refCoordKey, skyCoord)
108
109 source = sourceCat.addNew()
110 source.set(sourceCentroidKey, pixelPos)
111
112 matchList.append(afwTable.ReferenceMatch(refObj, source, 0.0))
113
114 # The TAN-SIP fitter is fitting x and y separately, so we have to iterate to make it converge
115 for indx in range(iterations):
116 sipObject = makeCreateWcsWithSip(matchList, tanWcs, order, bbox)
117 tanWcs = sipObject.getNewWcs()
118 fitWcs = sipObject.getNewWcs()
119
120 mockTest = _MockTestCase()
121 assertWcsAlmostEqualOverBBox(mockTest, wcs, fitWcs, bbox, maxDiffSky=skyTolerance,
122 maxDiffPix=pixelTolerance)
123
124 return fitWcs
A FunctorKey used to get or set celestial coordinates from a pair of lsst::geom::Angle keys.
Definition aggregates.h:292
A floating-point coordinate rectangle geometry.
Definition Box.h:413
std::shared_ptr< SkyWcs > makeSkyWcs(daf::base::PropertySet &metadata, bool strip=false)
Construct a SkyWcs from FITS keywords.
Definition SkyWcs.cc:543