Read image unit

ccdproc/ccddata.py

@@ -11,7 +11,7 @@
 from astropy.io import fits, registry
 from astropy.utils.compat.odict import OrderedDict
 from astropy import units as u
-import astropy
+from astropy import log
 
 from .utils.collections import CaseInsensitiveOrderedDict
 
@@ -262,8 +262,10 @@ def fits_ccddata_reader(filename, hdu=0, unit=None, **kwd):
     hdu : int, optional
         FITS extension from which CCDData should be initialized.
 
-    unit : astropy.units.Unit
-        Units of the image data
+    unit : astropy.units.Unit, optional
+        Units of the image data. If this argument is provided and there is a
+        unit for the image in the FITS header (the keyword ``BUNIT`` is used
+        as the unit, if present), this argument is used for the unit.
 
     kwd :
         Any additional keyword parameters are passed through to the FITS reader
@@ -286,7 +288,19 @@ def fits_ccddata_reader(filename, hdu=0, unit=None, **kwd):
             prefix = 'Unsupported keyword: {0}.'.format(key)
             raise TypeError(' '.join([prefix, msg]))
     hdus = fits.open(filename, **kwd)
-    ccd_data = CCDData(hdus[hdu].data, meta=hdus[hdu].header, unit=unit)
+    hdr = hdus[hdu].header
+
+    try:
+        fits_unit_string = hdr['bunit']
+    except KeyError:
+        fits_unit_string = None
+
+    if unit is not None and fits_unit_string:
+        log.info("Using the unit {0} passed to the FITS reader instead of "
+                 "the unit {1} in the FITS file.", unit, fits_unit_string)
+
+    use_unit = unit or fits_unit_string
+    ccd_data = CCDData(hdus[hdu].data, meta=hdus[hdu].header, unit=use_unit)
     hdus.close()
     return ccd_data
 

ccdproc/tests/test_ccddata.py

@@ -48,6 +48,21 @@ def test_initialize_from_FITS(ccd_data, tmpdir):
         assert cd.meta[k] == v
 
 
+def test_initialize_from_fits_with_unit_in_header(tmpdir):
+    fake_img = np.random.random(size=(100, 100))
+    hdu = fits.PrimaryHDU(fake_img)
+    hdu.header['bunit'] = u.adu.to_string()
+    filename = tmpdir.join('afile.fits').strpath
+    hdu.writeto(filename)
+    ccd = CCDData.read(filename)
+    # ccd should pick up the unit adu from the fits header...did it?
+    assert ccd.unit is u.adu
+
+    # An explicit unit in the read overrides any unit in the FITS file
+    ccd2 = CCDData.read(filename, unit="photon")
+    assert ccd2.unit is u.photon
+
+
 def test_initialize_from_FITS_bad_keyword_raises_error(ccd_data, tmpdir):
     # There are two fits.open keywords that are not permitted in ccdpro:
     #     do_not_scale_image_data and scale_back

docs/ccdproc/ccddata.rst

@@ -45,8 +45,9 @@ A `~ccdproc.ccddata.CCDData` object can also be initialized from a FITS file:
 
     >>> ccd = ccdproc.CCDData.read('my_file.fits', unit="adu")  # doctest: +SKIP
 
-but for the moment you need to set the unit explicitly, even if it is in the
-FITS header.
+If there is a unit in the FITS file (in the ``BUNIT`` keyword), that will be
+used, but a unit explicitly provided in ``read`` will override any unit in the
+FITS file.
 
 There is no restriction at all on what the unit can be -- any unit in
 `astropy.units` or that you create yourself will work.