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LSST Data Management Base Package
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approximateWcs.py
Go to the documentation of this file.
2# LSST Data Management System
3# Copyright 2008-2017 LSST Corporation.
4#
5# This product includes software developed by the
6# LSST Project (http://www.lsst.org/).
7#
8# This program is free software: you can redistribute it and/or modify
9# it under the terms of the GNU General Public License as published by
10# the Free Software Foundation, either version 3 of the License, or
11# (at your option) any later version.
12#
13# This program is distributed in the hope that it will be useful,
14# but WITHOUT ANY WARRANTY; without even the implied warranty of
15# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
16# GNU General Public License for more details.
17#
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19# the GNU General Public License along with this program. If not,
20# see <http://www.lsstcorp.org/LegalNotices/>.
21#
22
23__all__ = ["approximateWcs"]
24
25import numpy as np
26
27import lsst.geom
28import lsst.afw.table as afwTable
29import lsst.afw.geom as afwGeom
30from lsst.meas.base import SingleFrameMeasurementTask
31from lsst.meas.astrom.sip import makeCreateWcsWithSip
32from lsst.afw.geom.utils import assertWcsAlmostEqualOverBBox
33
34
36 """A fake unit test case class that will enable us to call
37 assertWcsAlmostEqualOverBBox from the method approximateWcs"""
38
39 def fail(self, msgStr):
40 raise UserWarning("WCS fitting failed " + msgStr)
41
42
43def approximateWcs(wcs, bbox, order=3, nx=20, ny=20, iterations=3,
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
Tag types used to declare specialized field types.
Definition misc.h:31
A FunctorKey used to get or set celestial coordinates from a pair of lsst::geom::Angle keys.
Definition aggregates.h:292
Custom catalog class for record/table subclasses that are guaranteed to have an ID,...
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:521