Avoid writing garbage EQUINOX to FITS.

For ICRS coordinates, EQUINOX is not required. Previously, we wrote a nonsense number, but this confused some FITS readers (notably DS9 7.4). Now, we write nothing.
lsst · Feb 26, 2016 · 81462b9 · 81462b9
1 parent ce64d60
commit 81462b9
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Showing 3 changed files with 25 additions and 2 deletions.
diff --git a/src/formatters/TanWcsFormatter.cc b/src/formatters/TanWcsFormatter.cc
@@ -185,7 +185,11 @@ afwForm::TanWcsFormatter::generatePropertySet(afwImg::TanWcs const& wcs) {
     }
 
     wcsProps->add("NAXIS", wcs._wcsInfo[0].naxis, "number of data axes");
-    wcsProps->add("EQUINOX", wcs._wcsInfo[0].equinox, "Equinox of coordinates");
+    // EQUINOX is "not relevant" (FITS definition, version 3.0, page 30) when
+    // dealing with ICRS, and may confuse readers. Don't write it.
+    if (strncmp(wcs._wcsInfo[0].radesys, "ICRS", 4) != 0) {
+        wcsProps->add("EQUINOX", wcs._wcsInfo[0].equinox, "Equinox of coordinates");
+    }
     wcsProps->add("RADESYS", std::string(wcs._wcsInfo[0].radesys), "Coordinate system for equinox");
     wcsProps->add("CRPIX1", wcs._wcsInfo[0].crpix[0], "WCS Coordinate reference pixel");
     wcsProps->add("CRPIX2", wcs._wcsInfo[0].crpix[1], "WCS Coordinate reference pixel");

diff --git a/src/formatters/WcsFormatter.cc b/src/formatters/WcsFormatter.cc
@@ -144,7 +144,11 @@ afwForm::WcsFormatter::generatePropertySet(afwImg::Wcs const& wcs) {
     assert(wcs._wcsInfo); // default ctor is private, so an uninitialized Wcs should not exist in the wild
 
     wcsProps->add("NAXIS", wcs._wcsInfo[0].naxis, "number of data axes");
-    wcsProps->add("EQUINOX", wcs._wcsInfo[0].equinox, "Equinox of coordinates");
+    // EQUINOX is "not relevant" (FITS definition, version 3.0, page 30) when
+    // dealing with ICRS, and may confuse readers. Don't write it.
+    if (strncmp(wcs._wcsInfo[0].radesys, "ICRS", 4) != 0) {
+        wcsProps->add("EQUINOX", wcs._wcsInfo[0].equinox, "Equinox of coordinates");
+    }
     wcsProps->add("RADESYS", std::string(wcs._wcsInfo[0].radesys), "Coordinate system for equinox");
     wcsProps->add("CRPIX1", wcs._wcsInfo[0].crpix[0], "WCS Coordinate reference pixel");
     wcsProps->add("CRPIX2", wcs._wcsInfo[0].crpix[1], "WCS Coordinate reference pixel");

diff --git a/tests/wcs1.py b/tests/wcs1.py
@@ -236,6 +236,21 @@ def refIsSameSkySystem(wcs1, wcs2):
                 wcs2 = makeWcs(coordSys=coordSys2, equinox=equinox2)
                 self.assertEqual(refIsSameSkySystem(wcs, wcs2), wcs.isSameSkySystem(wcs2))
 
+    def testIcrsEquinox(self):
+        """Check that EQUINOX is not written to FITS for ICRS coordinates"""
+        def checkEquinoxHeader(coordSysName, writeEquinox):
+            coordSys = getattr(afwCoord, coordSysName)
+            for dummyWcs in (makeWcs(coordSys=coordSys), afwImage.TanWcs.cast(makeWcs(coordSys=coordSys))):
+                dummyWcs = afwImage.TanWcs.cast(makeWcs(coordSys=coordSys))
+                dummyExposure = afwImage.ExposureF()
+                dummyExposure.setWcs(dummyWcs)
+                with utilsTests.getTempFilePath(".fits") as tmpFile:
+                    dummyExposure.writeFits(tmpFile)
+                    metadata = afwImage.readMetadata(tmpFile)
+                    self.assertEqual(metadata.get("RADESYS"), coordSysName)
+                    self.assertTrue(("EQUINOX" in metadata.names()) == writeEquinox)
+        checkEquinoxHeader("ICRS", False)
+        checkEquinoxHeader("FK5", True)
 
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