fix: add support for AzEl observations

Adds check for empty radesys and functionality to handle AltAz (AzEl) coordinate frame

src/dysh/coordinates/core.py

@@ -11,6 +11,7 @@
 from astropy.coordinates.spectral_coordinate import (
     DEFAULT_DISTANCE as _DEFAULT_DISTANCE,
 )
+from astropy.time import Time
 
 _PMZERO = 0.0 * u.mas / u.yr
 _PMZERORAD = 0.0 * u.rad / u.s
@@ -302,6 +303,21 @@ def sanitize_skycoord(target):
             pm_b=pm_lat,
             radial_velocity=_rv,
         )
+    elif hasattr(target, "az"):  # AzEl or AltAz
+        lon = target.az
+        lat = target.alt
+        pm_lon = target.pm_az_cosalt
+        pm_lat = target.pm_alt
+        _target = coord.SkyCoord(
+            lon,
+            lat,
+            frame=target.frame,
+            distance=newdistance,
+            pm_az_cosalt=pm_lon,
+            pm_alt=pm_lat,
+            radial_velocity=_rv,
+        )
+
     else:
         warnings.warn(f"Can't sanitize {target}")
         return target
@@ -479,6 +495,8 @@ def make_target(header):
         frame=header["RADESYS"].lower(),
         radial_velocity=header["VELOCITY"] * _MPS,
         distance=_DEFAULT_DISTANCE,  # need this or PMs get units m rad /s !
+        obstime=Time(header["DATE-OBS"]),
+        location=gbt_location(),
     )
     # print(f"{t1},{t1.pm_ra_cosdec},{t1.pm_dec},{t1.distance},{t1.radial_velocity}")
     target = sanitize_skycoord(t1)

src/dysh/fits/gbtfitsload.py

@@ -91,6 +91,8 @@ def __init__(self, fileobj, source=None, hdu=None, **kwargs):
         if lsdf > 1:
             warnings.warn(f"Found {lsdf} FITS files")  # or maybe just print()
 
+        self._update_radesys()
+
     @property
     def selection(self):
         """
@@ -1633,3 +1635,13 @@ def write_scans(self, fileobj, scans, output_verify="exception", overwrite=False
         # write it out!
         outhdu.update_extend()  # possibly unneeded
         outhdu.writeto(fileobj, output_verify=output_verify, overwrite=overwrite, checksum=checksum)
+
+    def _update_radesys(self):
+        """ """
+
+        mask = self._index["RADESYS"] == ""
+        if mask.sum() > 0:
+            warnings.warn("No RADESYS specified.")
+            azel_mask = (self._index["CTYPE2"] == "AZ") & (self._index["CTYPE3"] == "EL")
+            azel_radesys = "AltAz"
+            self._index.loc[azel_mask, "RADESYS"] = azel_radesys