add pixel scale factor to metadata and API translation

jdaviz/configs/cubeviz/plugins/spectral_extraction/spectral_extraction.py

@@ -3,6 +3,7 @@
 
 import numpy as np
 import astropy
+from astropy import units as u
 from astropy.utils.decorators import deprecated
 from astropy.nddata import (
     NDDataArray, StdDevUncertainty
@@ -332,6 +333,10 @@ def collapse_to_spectrum(self, add_data=True, **kwargs):
         fname_label = self.dataset_selected.replace("[", "_").replace("]", "")
         self.filename = f"extracted_{selected_func}_{fname_label}.fits"
 
+        # per https://jwst-docs.stsci.edu/jwst-near-infrared-camera/nircam-performance/nircam-absolute-flux-calibration-and-zeropoints # noqa
+        pix_scale_factor = self.aperture.scale_factor * spectral_cube.meta.get('PIXAR_SR', 1.0)
+        collapsed_spec.meta['_pixel_scale_factor'] = pix_scale_factor
+
         if add_data:
             self.add_results.add_results_from_plugin(
                 collapsed_spec, label=self.results_label, replace=False
@@ -519,3 +524,13 @@ def _live_update(self, event={}):
         for mark in self.marks.values():
             mark.update_xy(sp.spectral_axis.value, sp.flux.value)
             mark.visible = True
+
+    def translate_units(self, collapsed_spec):
+        # remove sr
+        if u.sr in collapsed_spec._unit.bases:
+            collapsed_spec._data *= collapsed_spec.meta['_pixel_scale_factor']
+            collapsed_spec._unit *= u.sr
+        # add sr
+        elif u.sr not in collapsed_spec._unit.bases:
+            collapsed_spec._data /= collapsed_spec.meta['_pixel_scale_factor']
+            collapsed_spec._unit /= u.sr

jdaviz/configs/cubeviz/plugins/spectral_extraction/tests/test_spectral_extraction.py

@@ -10,6 +10,7 @@
 from regions import (CirclePixelRegion, CircleAnnulusPixelRegion, EllipsePixelRegion,
                      RectanglePixelRegion, PixCoord)
 from specutils import Spectrum1D
+from astropy.wcs import WCS
 
 
 def test_version_after_nddata_update(cubeviz_helper, spectrum1d_cube_with_uncerts):
@@ -370,3 +371,50 @@ def test_cube_extraction_with_nan(cubeviz_helper, image_cube_hdu_obj):
     extract_plg.aperture = 'Subset 1'
     sp_subset = extract_plg.collapse_to_spectrum()  # Default settings but on Subset
     assert_allclose(sp_subset.flux.value, 12)  # (4 x 4) - 4
+
+
+def test_unit_translation(cubeviz_helper):
+    # custom cube so we have PIXAR_SR in metadata, and flux units = Jy/pix
+    wcs_dict = {"CTYPE1": "WAVE-LOG", "CTYPE2": "DEC--TAN", "CTYPE3": "RA---TAN",
+                "CRVAL1": 4.622e-7, "CRVAL2": 27, "CRVAL3": 205,
+                "CDELT1": 8e-11, "CDELT2": 0.0001, "CDELT3": -0.0001,
+                "CRPIX1": 0, "CRPIX2": 0, "CRPIX3": 0, "PIXAR_SR": 8e-11}
+    w = WCS(wcs_dict)
+    flux = np.zeros((30, 20, 3001), dtype=np.float32)
+    flux[5:15, 1:11, :] = 1
+    cube = Spectrum1D(flux=flux * u.MJy, wcs=w, meta=wcs_dict)
+    cubeviz_helper.load_data(cube, data_label="test")
+
+    center = PixCoord(5, 10)
+    cubeviz_helper.load_regions(CirclePixelRegion(center, radius=2.5))
+
+    extract_plg = cubeviz_helper.plugins['Spectral Extraction']
+
+    extract_plg.aperture = extract_plg.aperture.choices[-1]
+    extract_plg.aperture_method.selected = 'Exact'
+    extract_plg.wavelength_dependent = True
+    extract_plg.function = 'Sum'
+    # set so pixel scale factor != 1
+    extract_plg.reference_wavelength = 0.000001
+
+    # collapse to spectrum, now we can get pixel scale factor
+    collapsed_spec = extract_plg.collapse_to_spectrum()
+
+    assert collapsed_spec.meta['_pixel_scale_factor'] != 1
+
+    # store to test second time after calling translate_units
+    mjy_sr_data1 = collapsed_spec._data[0]
+
+    extract_plg._obj.translate_units(collapsed_spec)
+
+    assert collapsed_spec._unit == u.MJy / u.sr
+    # some value in MJy/sr that we know the outcome after translation
+    assert np.allclose(collapsed_spec._data[0], 8.7516529e10)
+
+    extract_plg._obj.translate_units(collapsed_spec)
+
+    # translating again returns the original units
+    assert collapsed_spec._unit == u.MJy
+    # returns to the original values
+    # which is a value in Jy/pix that we know the outcome after translation
+    assert np.allclose(collapsed_spec._data[0], mjy_sr_data1)