SLA_DTPS2C - Plate centre from and

**ACTION:**- From the tangent plane coordinates of a star of known ,determine the 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 .
- 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 non-zero value, and hence it is meaningless to ask where the tangent point would have to be to bring about this combination of and .
- 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 ``over-the-pole'' 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 , but in the ordinary, non-pole-crossing, case, the range is .
- 5.
- RA, DEC, RAZ1, DECZ1, RAZ2, DECZ2 are all in radians.
- 6.
- The projection is called the
*gnomonic*projection; the Cartesian coordinates 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 rather than spherical coordinates, the equivalent Cartesian routine sla_DTPV2C is available.

Starlink User Note 67

P. T. Wallace

12 October 1999

E-mail:ptw@star.rl.ac.uk