update for cos(dec)

docs/jwst/cube_build/arguments.rst

@@ -131,24 +131,25 @@ A parameter only used for investigating which detector pixels contributed to a c
   To print information to the screeen about the x = 10, y = 20, z = 35 spaxel the parameter string value is '10 20 35'.
 
 .. _offsets:
-The offset file is an ASDF formated file : <https://asdf-standard.readthedocs.io/>`_ stands for "Advanced Scientific Data. For each
-input file in the spec3 assocation used to build the IFU cubes, an ra and dec offset, in arc seconds, is provided. 
+The offset file is an ASDF formated file :`<https://asdf-standard.readthedocs.io/>`_ stands for "Advanced Scientific Data. For each
+input file in the spec3 assocation used to build the IFU cubes, there is a corresponding  right ascension  and declination offset,
+given arc seconds.
 Below is an example of how to make an ASDF offset file. It is assumed the user has determined the
-offsets to apply for each file. The offsets are stored in a python dictionary, `offsets`. The items of this dictionary are `filenames`, `raoffset` and `decoffset`. The  cube_building code is expects this dictionary to hold the information
-for storing the file names and the associated ra and dec offsets. 
+offsets to apply the data in each file. The offsets are stored in a python dictionary, `offsets`. The items of this dictionary
+are `filenames`, `raoffset` and `decoffset`. The  IFU cube building code  expects this dictionary to hold the information
+for storing the file names and the associated ra and dec offsets. The file names should not contain the directory path.
 
-It is assumed there exists a list of files, ra and dec offsets that are feed to this routine. The ra and dec offsets
-provided in arcseconds. The cube_building code will apply the ra offsets after multiplying by the  cos(crval2), where crval2 is the
+It is assumed there exists a list of files, ra and dec offsets that are feed to this method. The ra and dec offsets
+provided in arcseconds. The cube building code will apply the ra offsets after dividing by  cos(crval2), where crval2 is the
 declination center of the IFU cube. 
-`num` is the number of files.
-y
+Below `num` is the number of files.
+
 import asdf
 offsets = {}
 offsets['filename'] = []
 offsets['raoffset'] = []
 offsets['decoffset'] = []
 for i in range(num):
-
     offsets['filename'].append(file[i])
     offsets['raoffset'].append(ra_center1[i])
     offsets['decoffset'].append(dec_center1[i])

jwst/cube_build/ifu_cube.py

@@ -114,7 +114,8 @@ def __init__(self,
         self.naxis3 = None
         self.cdelt3_normal = None
         self.rot_angle = None  # rotation angle between Ra-Dec and IFU local instrument plane
-
+        self.median_dec = None
+
         self.a_min = 0
         self.a_max = 0
         self.b_min = 0
@@ -1227,6 +1228,7 @@ def setup_ifucube_wcs(self):
 # _____________________________________________________________________________
         self.cdelt1 = self.spatial_size
         self.cdelt2 = self.spatial_size
+        deg2rad = math.pi / 180.0
         if self.linear_wavelength:
             self.cdelt3 = self.spectral_size
 
@@ -1303,6 +1305,25 @@ def setup_ifucube_wcs(self):
             log.debug(f'Working on data from {this_a}, {this_b}')
             n = len(self.master_table.FileMap[self.instrument][this_a][this_b])
             log.debug('number of files %d', n)
+
+            # find the median center declination if we have an offset file
+            if self.offsets is not None:
+                decs = []
+                for k in range(n):
+                    input_file = self.master_table.FileMap[self.instrument][this_a][this_b][k]
+                    input_model = datamodels.open(input_file)
+                    spatial_box = input_model.meta.wcsinfo.s_region
+                    s = spatial_box.split(' ')
+                    cb1 = float(s[4])
+                    cb2 = float(s[6])
+                    cb3 = float(s[8])
+                    cb4 = float(s[10])
+                    m = (cb1 + cb2 + cb3 + cb4)/4
+                    decs.append(m)
+
+                self.median_dec = np.nanmedian(decs)
+                print('Median declination ', self.median_dec)
+
             for k in range(n):
                 lmin = 0.0
                 lmax = 0.0
@@ -1322,6 +1343,7 @@ def setup_ifucube_wcs(self):
                     log.info("Ra and dec offset (arc seconds) applied to file :%5.2f, %5.2f,  %s",
                              raoffset, decoffset, filename)
                     raoffset = raoffset/3600.0 # convert to degress
+                    raoffset = raoffset /np.cos(self.median_dec *deg2rad)
                     decoffset = decoffset/3600.0
 # ________________________________________________________________________________
                 # Find the footprint of the image
@@ -1731,6 +1753,7 @@ def map_miri_pixel_to_sky(self, input_model, this_par1, subtract_background,
         slice_no = None  # Slice number
         dwave = None
         corner_coord = None
+        deg2rad = math.pi / 180.0
 
         raoffset = 0.0
         decoffset = 0.0 
@@ -1743,6 +1766,7 @@ def map_miri_pixel_to_sky(self, input_model, this_par1, subtract_background,
             log.info("Ra and dec offset (arc seconds) applied to file :%5.2f, %5.2f,  %s",
                      raoffset, decoffset, filename)
             raoffset = raoffset/3600.0 # convert to degress
+            raoffset = raoffset /np.cos(self.median_dec *deg2rad)
             decoffset = decoffset/3600.0
 
         # check if background sky matching as been done in mrs_imatch step