Fix get_transform for non-celestial WCS input that matches axes WCS

wcsaxes/core.py

@@ -10,7 +10,7 @@
                          CoordinateTransform)
 from .coordinates_map import CoordinatesMap
 from .utils import get_coord_meta
-from .wcs_utils import wcs_to_celestial_frame
+from .wcs_utils import wcs_to_celestial_frame, wcs_is_identical_world_system
 from .frame import RectangularFrame
 import numpy as np
 
@@ -298,19 +298,34 @@ def _get_transform_no_transdata(self, frame):
 
         if isinstance(frame, WCS):
 
-            coord_in = wcs_to_celestial_frame(self.wcs)
-            coord_out = wcs_to_celestial_frame(frame)
+            if frame.is_celestial:
 
-            if coord_in == coord_out:
+                coord_in = wcs_to_celestial_frame(self.wcs)
+                coord_out = wcs_to_celestial_frame(frame)
 
-                return (WCSPixel2WorldTransform(self.wcs, slice=self.slices)
+                if coord_in == coord_out:
+
+                    return (WCSPixel2WorldTransform(self.wcs, slice=self.slices)
+                            + WCSWorld2PixelTransform(frame))
+
+                else:
+
+                    return (WCSPixel2WorldTransform(self.wcs, slice=self.slices)
+                            + CoordinateTransform(self.wcs, frame)
+                            + WCSWorld2PixelTransform(frame))
+
+            elif wcs_is_identical_world_system(self.wcs, frame):
+
+                return (WCSPixel2WorldTransform(self.wcs)
                         + WCSWorld2PixelTransform(frame))
 
             else:
 
-                return (WCSPixel2WorldTransform(self.wcs, slice=self.slices)
-                        + CoordinateTransform(self.wcs, frame)
-                        + WCSWorld2PixelTransform(frame))
+                raise ValueError("Could not determine conversion between the"
+                                 "WCS object specified and the WCS associated"
+                                 "with the axes")
+
+
 
         elif frame == 'pixel':
 

wcsaxes/wcs_utils.py

@@ -139,3 +139,43 @@ def wcs_to_celestial_frame(wcs):
                 return frame
     raise ValueError("Could not determine celestial frame corresponding to "
                      "the specified WCS object")
+
+
+def wcs_is_identical_world_system(wcs1, wcs2):
+    """
+    Determine whether two WCS objects have the same world coordinate system.
+
+    This helps us understand if we can go pixel1 -> world -> pixel2 by chaining
+    the two WCS objects.
+    """
+
+    if wcs1.naxis != wcs2.naxis:
+        return False
+
+    for i in range(wcs1.naxis):
+
+        if wcs1.wcs.ctype[i] != wcs2.wcs.ctype[i]:
+            return False
+
+        if wcs1.wcs.axis_types[i] != wcs2.wcs.axis_types[i]:
+            return False
+
+    if wcs1.wcs.specsys != wcs2.wcs.specsys:
+        return False
+
+    if wcs1.wcs.radesys != wcs2.wcs.radesys:
+        return False
+
+    if (wcs1.wcs.equinox != wcs2.wcs.equinox and
+        np.isnan(wcs1.wcs.equinox) is not np.isnan(wcs2.wcs.equinox)):
+        return False
+
+    if (wcs1.wcs.restfrq != wcs2.wcs.restfrq and
+        np.isnan(wcs1.wcs.restfrq) is not np.isnan(wcs2.wcs.restfrq)):
+        return False
+
+    if (wcs1.wcs.restwav != wcs2.wcs.restwav and
+        np.isnan(wcs1.wcs.restwav) is not np.isnan(wcs2.wcs.restwav)):
+        return False
+
+    return True