PointingOffset
sky_to_ra(Declination dec, PointingOffset angle)
Return the Right Ascension offset that corresponds to a given great-circle distance.
Declination dec
PointingOffset angle
radec_offset ra=$sky_to_ra($declination(jupiter), -5) track jupiter
The sky_to_ra()
function returns the Right Ascension
distance that one needs to offset the telescope, at a given
declination, to cross a specified great-circle distance on the
sky.
Whereas ra_angle = sky_angle/cos(declination)
is a good
enough approximation for small angles and low declinations, the
sky_to_ra()
function uses the following formal
spherical trignonometry equation.
cos(ra_angle) = 1 + (cos(sky_angle) - 1.0) / cos(dec)**2)
Note that the maximum sky distance that can be achieved at
a particular declination, is the great-circle distance between
Right Ascensions that are 180 degrees apart. Thus the maximum sky
angle that can be accomodated at declination, dec
,
is given by:
max_sky_angle = 2*(90-dec)If larger angles than this are requested, then the
sky_to_ra()
function returns an Right Ascension
distance of 180 degrees.