PointingOffset
sky_to_az(Elevation el, PointingOffset angle)
Return the azimuth offset that corresponds to a given great-circle distance.
Elevation el
PointingOffset angle
azscan $sky_to_az(60, 5)
The sky_to_az()
function returns the azimuthal
distance that one needs to turn the telescope, at a given
elevation, to cross a specified great-circle distance on the sky.
Whereas az_angle = sky_angle/cos(elevation)
is a good
enough approximation for small angles and low elevations, the
sky_to_az()
function uses the following formal
spherical trignonometry equation.
cos(az_angle) = 1 + (cos(sky_angle) - 1.0) / cos(el)**2)
Note that the maximum sky distance that can be achieved at
a particular elevation, is the great-circle distance between
azimuths that are 180 degrees apart. Thus the maximum sky
angle that can be accomodated at elevation, el
,
is given by:
max_sky_angle = 2*(90-el)If larger angles than this are requested, then the
sky_to_az()
function returns an azimuth
distance of 180 degrees.