Transformation of camera coordinates far outside the field of view

[maxnoe]

Currently it is possible to transform crazy values of camera coordinates to skycoordinates without a warning.

Then there is a certain point, where it completely breaks down (at pi/2 times the focal length) and the transformation to telescope frame:

from astropy.coordinates import AltAz, SkyCoord, EarthLocation
from ctapipe.coordinates import CameraFrame
import astropy.units as u
from astropy.time import Time

altaz = AltAz(
    location=EarthLocation.of_site('Roque de los Muchachos'),
    obstime=Time.now()
)
pointing = SkyCoord(alt=85 * u.deg, az=0 * u.deg, frame=altaz)
camera_frame = CameraFrame(focal_length=5 * u.m, telescope_pointing=pointing)

# crazy, but less than pi / 2
c1 = SkyCoord(x=5 * u.m, y=0 * u.cm, frame=camera_frame)

# more than pi/2
c2 = SkyCoord(x=1.6 * 5 * u.m, y=0 * u.cm, frame=camera_frame)

print(c1.transform_to(altaz))
print(c2.transform_to(altaz))

We should probably warn or error if something is further of than something like 15 degrees or so.

[kosack]

agreed. Generally this should never happen unless the user is doing something unexpected. So raising an Exception (perhaps RangeError) is the right way to go, since it is an exceptional case.

I think adding a line explaining allowed range to the docstring or in the high-level user docs would also be useful. Something like "The CameraFrame/TelescopeFrame are intended to represent coordinates within the FOV of the telescope; coordinates far outside that range may not transform correctly to general Earth or sky coordinates. "

[maxnoe]

agreed. Generally this should never happen unless the user is doing something unexpected.

It could happen quite regularly for badly reconstructed events. A very small percentage of events will be that bad but there will be lots of events....

[kosack]

Ah, yes, i was thinking the reconstructed events are always in the TelescopeFrame or HorizonFrame, but one might want to plot them in the CameraFrame. In any case, it should still complain.