
ACTION:
 From the tangent plane coordinates of a star of known $\left[\phantom{\rule{0.3em}{0ex}}\alpha ,\delta \phantom{\rule{0.3em}{0ex}}\right]$,
determine the $\left[\phantom{\rule{0.3em}{0ex}}\alpha ,\delta \phantom{\rule{0.3em}{0ex}}\right]$ of
the tangent point (double precision)

CALL:
CALL sla_DTPS2C (XI, ETA, RA, DEC, RAZ1, DECZ1, RAZ2, DECZ2, N)
GIVEN:
XI,ETA  D  tangent plane rectangular coordinates (radians) 

RA,DEC  D  spherical coordinates (radians) 

RETURNED:
RAZ1,DECZ1  D  spherical coordinates of tangent point, solution 1 

RAZ2,DECZ2  D  spherical coordinates of tangent point, solution 2 

N  I  number of solutions: 

  0 = no solutions returned (note 2) 

  1 = only the first solution is useful (note 3) 

  2 = there are two useful solutions (note 3) 


NOTES:


(1)
 The RAZ1 and RAZ2 values returned are in the range $0\phantom{\rule{0.3em}{0ex}}\phantom{\rule{0.3em}{0ex}}2\pi $.

(2)
 Cases where there is no solution can only arise near the poles. For example, it is
clearly impossible for a star at the pole itself to have a nonzero $\xi $
value, and hence it is meaningless to ask where the tangent point would have to be to
bring about this combination of $\xi $
and $\delta $.

(3)
 Also near the poles, cases can arise where there are two useful solutions. The
argument N indicates whether the second of the two solutions returned is useful.
N = 1 indicates only one useful solution, the usual case; under these circumstances,
the second solution corresponds to the “overthepole” case, and this is reflected in
the values of RAZ2 and DECZ2 which are returned.

(4)
 The DECZ1 and DECZ2 values returned are in the range $\pm \pi $,
but in the ordinary, nonpolecrossing, case, the range is $\pm \pi /2$.

(5)
 RA, DEC, RAZ1, DECZ1, RAZ2, DECZ2 are all in radians.

(6)
 The projection is called the gnomonic projection; the Cartesian coordinates $\left[\phantom{\rule{0.3em}{0ex}}\xi ,\eta \phantom{\rule{0.3em}{0ex}}\right]$ are
called standard coordinates. The latter are in units of the distance from the tangent
plane to the projection point, i.e. radians near the origin.

(7)
 When working in $\left[\phantom{\rule{0.3em}{0ex}}x,y,z\phantom{\rule{0.3em}{0ex}}\right]$ rather
than spherical coordinates, the equivalent Cartesian routine sla_DTPV2C is available.