# Licensed under a 3-clause BSD style license - see LICENSE.rst
from astropy.coordinates import SkyCoord, SkyOffsetFrame
__all__ = ["fov_to_sky", "sky_to_fov"]
[docs]def fov_to_sky(lon, lat, lon_pnt, lat_pnt):
"""Transform field-of-view coordinates to sky coordinates.
Parameters
----------
lon, lat : `~astropy.units.Quantity`
Field-of-view coordinate to be transformed.
lon_pnt, lat_pnt : `~astropy.units.Quantity`
Coordinate specifying the pointing position.
(i.e. the center of the field of view.)
Returns
-------
lon_t, lat_t : `~astropy.units.Quantity`
Transformed sky coordinate.
"""
# Create a frame that is centered on the pointing position
center = SkyCoord(lon_pnt, lat_pnt)
fov_frame = SkyOffsetFrame(origin=center)
# Define coordinate to be transformed.
# Need to switch the sign of the longitude angle here
# because this axis is reversed in our definition of the FoV-system
target_fov = SkyCoord(-lon, lat, frame=fov_frame)
# Transform into celestial system (need not be ICRS)
target_sky = target_fov.icrs
return target_sky.ra, target_sky.dec
[docs]def sky_to_fov(lon, lat, lon_pnt, lat_pnt):
"""Transform sky coordinates to field-of-view coordinates.
Parameters
----------
lon, lat : `~astropy.units.Quantity`
Sky coordinate to be transformed.
lon_pnt, lat_pnt : `~astropy.units.Quantity`
Coordinate specifying the pointing position.
(i.e. the center of the field of view.)
Returns
-------
lon_t, lat_t : `~astropy.units.Quantity`
Transformed field-of-view coordinate.
"""
# Create a frame that is centered on the pointing position
center = SkyCoord(lon_pnt, lat_pnt)
fov_frame = SkyOffsetFrame(origin=center)
# Define coordinate to be transformed.
target_sky = SkyCoord(lon, lat)
# Transform into FoV-system
target_fov = target_sky.transform_to(fov_frame)
# Switch sign of longitude angle since this axis is
# reversed in our definition of the FoV-system
return -target_fov.lon, target_fov.lat