Merge pull request #2867 from astrofrog/small-delta-compass

Use small angular offsets when determining compass properties
spacetelescope · May 10, 2024 · 37eba85 · 37eba85
2 parents f985a7f + afef68a
commit 37eba85
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Showing 2 changed files with 23 additions and 15 deletions.
diff --git a/jdaviz/configs/imviz/tests/test_wcs_utils.py b/jdaviz/configs/imviz/tests/test_wcs_utils.py
@@ -24,9 +24,9 @@ def test_simple_fits_wcs():
     w = WCS()
     result = wcs_utils.get_compass_info(w, (100, 100), r_fac=0.25)
     assert_allclose(result[:-1], (50, 50,
-                                  50, 73.57533083557558,
-                                  73.57809873817803, 47.53791379556649,
-                                  0, 95.96136875946559))
+                                  50, 73.573199,
+                                  73.573199, 47.538975,
+                                  0, 95.960048))
     assert result[-1]
 
     # https://learn.astropy.org/tutorials/celestial_coords1.html
@@ -44,9 +44,9 @@ def test_simple_fits_wcs():
              'NAXIS2': 1024})
     result = wcs_utils.get_compass_info(w, (1024, 1024), r_fac=0.25)
     assert_allclose(result[:-1], (512.0, 512.0,
-                                  512.2415718047745, 767.9895741540789,
-                                  255.98982015749573, 512.240013842715,
-                                  0.054068767449065434, -90.00035302773995))
+                                  512.2416, 768.0007,
+                                  256.0009, 512.24,
+                                  0.05406877, -90.00035))
     assert not result[-1]
 
 
@@ -103,9 +103,9 @@ def test_simple_gwcs():
 
     result = wcs_utils.get_compass_info(w, (1024, 2048), r_fac=0.25)
     assert_allclose(result[:-1], (1024.0, 512.0,
-                                  1131.0265005852038, 279.446189124443,
-                                  1262.0057201165127, 606.2863901330095,
-                                  155.2870478938214, -86.89813081941797))
+                                  1131.026544, 279.446094,
+                                  1262.004542, 606.285924,
+                                  155.287048, -86.898131))
     assert not result[-1]
 
 

diff --git a/jdaviz/configs/imviz/wcs_utils.py b/jdaviz/configs/imviz/wcs_utils.py
@@ -98,18 +98,26 @@ def calc_compass(image_wcs, x, y, len_deg_e, len_deg_n):
 
 
 def calc_compass_radius(image_wcs, x, y, radius_px):
-    xe, ye = add_offset_xy(image_wcs, x, y, 1.0, 0.0)
-    xn, yn = add_offset_xy(image_wcs, x, y, 0.0, 1.0)
+
+    # Define an angular length we can use to determine the pixel scale
+    # along east and north - we want to make sure we use a small length so
+    # that the offset points still fall inside the image in case they have
+    # a bounding box set.
+
+    delta = 0.1 / 3600  # 0.1 arcsec
+
+    xe, ye = add_offset_xy(image_wcs, x, y, delta, 0.0)
+    xn, yn = add_offset_xy(image_wcs, x, y, 0.0, delta)
 
     # now calculate the length in pixels of those arcs
     # (planar geometry is good enough here)
-    px_per_deg_e = math.sqrt(math.fabs(ye - y) ** 2 + math.fabs(xe - x) ** 2)
-    px_per_deg_n = math.sqrt(math.fabs(yn - y) ** 2 + math.fabs(xn - x) ** 2)
+    px_per_delta_e = math.sqrt(math.fabs(ye - y) ** 2 + math.fabs(xe - x) ** 2)
+    px_per_delta_n = math.sqrt(math.fabs(yn - y) ** 2 + math.fabs(xn - x) ** 2)
 
     # now calculate the arm length in degrees for each arm
     # (this produces same-length arms)
-    len_deg_e = radius_px / px_per_deg_e
-    len_deg_n = radius_px / px_per_deg_n
+    len_deg_e = radius_px / px_per_delta_e * delta
+    len_deg_n = radius_px / px_per_delta_n * delta
 
     return calc_compass(image_wcs, x, y, len_deg_e, len_deg_n)