From d54fe7c1f704a63824c5bfa0ece65245572e9b27 Mon Sep 17 00:00:00 2001 From: Joseph Hunkeler Date: Wed, 4 Mar 2015 21:21:30 -0500 Subject: Initial commit --- src/slalib/sun67.htx/node185.html | 168 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 168 insertions(+) create mode 100644 src/slalib/sun67.htx/node185.html (limited to 'src/slalib/sun67.htx/node185.html') diff --git a/src/slalib/sun67.htx/node185.html b/src/slalib/sun67.htx/node185.html new file mode 100644 index 0000000..0313e77 --- /dev/null +++ b/src/slalib/sun67.htx/node185.html @@ -0,0 +1,168 @@ + + + + +SLA_TPS2C - Plate centre from and + + + + + + + + + + + + +
+ +next + +up + +previous +
+ Next: SLA_TPV2C - Plate centre from and x,y,z +
+Up: SUBPROGRAM SPECIFICATIONS +
+ Previous: SLA_TP2V - Tangent Plane to Direction Cosines +

+

+

    +
+SLA_TPS2C - Plate centre from $\xi,\eta$ and $\alpha,\delta$

+
+
ACTION: +
From the tangent plane coordinates of a star of known $[\,\alpha,\delta\,]$,determine the $[\,\alpha,\delta\,]$ of the tangent point (single precision) +

CALL: +
CALL sla_TPS2C (XI, ETA, RA, DEC, RAZ1, DECZ1, RAZ2, DECZ2, N) +

+

+
GIVEN: +
+
+ + + + + + + + + +
XI,ETARtangent plane rectangular coordinates (radians)
RA,DECRspherical coordinates (radians)
+

+
RETURNED: +
+
+ + + + + + + + + + + + + + + + + + + + + + + + + +
RAZ1,DECZ1Rspherical coordinates of tangent point, +solution 1
RAZ2,DECZ2Rspherical coordinates of tangent point, +solution 2
NInumber 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 $0\!-\!2\pi$.
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 non-zero $\xi$ 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 + ``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 $\pm \pi$, but in the ordinary, non-pole-crossing, 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 $[\,\xi,\eta\,]$ 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 $[\,x,y,z\,]$ rather than spherical coordinates, the + equivalent Cartesian routine sla_TPV2C is available. +
+
+ +next + +up + +previous +
+ Next: SLA_TPV2C - Plate centre from and x,y,z +
+Up: SUBPROGRAM SPECIFICATIONS +
+ Previous: SLA_TP2V - Tangent Plane to Direction Cosines +

+

+

+SLALIB --- Positional Astronomy Library
Starlink User Note 67
P. T. Wallace
12 October 1999
E-mail:ptw@star.rl.ac.uk +
+ + -- cgit