source: trunk/MagicSoft/slalib/oap.c

Last change on this file was 731, checked in by tbretz, 24 years ago
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1#include "slalib.h"
2#include "slamac.h"
3void slaOap ( char *type, double ob1, double ob2, double date,
4 double dut, double elongm, double phim, double hm,
5 double xp, double yp, double tdk, double pmb,
6 double rh, double wl, double tlr,
7 double *rap, double *dap )
8/*
9** - - - - - - -
10** s l a O a p
11** - - - - - - -
12**
13** Observed to apparent place
14**
15** Given:
16** type c*(*) type of coordinates - 'R', 'H' or 'A' (see below)
17** ob1 d observed Az, HA or RA (radians; Az is N=0,E=90)
18** ob2 d observed ZD or Dec (radians)
19** date d UTC date/time (modified Julian Date, JD-2400000.5)
20** dut d delta UT: UT1-UTC (UTC seconds)
21** elongm d mean longitude of the observer (radians, east +ve)
22** phim d mean geodetic latitude of the observer (radians)
23** hm d observer's height above sea level (metres)
24** xp d polar motion x-coordinate (radians)
25** yp d polar motion y-coordinate (radians)
26** tdk d local ambient temperature (DegK; std=273.155)
27** pmb d local atmospheric pressure (mB; std=1013.25)
28** rh d local relative humidity (in the range 0.0-1.0)
29** wl d effective wavelength (micron, e.g. 0.55)
30** tlr d tropospheric lapse rate (DegK/metre, e.g. 0.0065)
31**
32** Returned:
33** rap d geocentric apparent right ascension
34** dap d geocentric apparent declination
35**
36** Notes:
37**
38** 1) Only the first character of the type argument is significant.
39** 'R' or 'r' indicates that obs1 and obs2 are the observed Right
40** Ascension and Declination; 'H' or 'h' indicates that they are
41** Hour Angle (West +ve) and Declination; anything else ('A' or
42** 'a' is recommended) indicates that obs1 and obs2 are Azimuth
43** (North zero, East is 90 deg) and zenith distance. (Zenith
44** distance is used rather than elevation in order to reflect the
45** fact that no allowance is made for depression of the horizon.)
46**
47** 2) The accuracy of the result is limited by the corrections for
48** refraction. Providing the meteorological parameters are
49** known accurately and there are no gross local effects, the
50** predicted apparent RA,Dec should be within about 0.1 arcsec
51** for a zenith distance of less than 70 degrees. Even at a
52** topocentric zenith distance of 90 degrees, the accuracy in
53** elevation should be better than 1 arcmin; useful results
54** are available for a further 3 degrees, beyond which the
55** slaRefro routine returns a fixed value of the refraction.
56** The complementary routines slaAop (or slaAopqk) and slaOap
57** (or slaOapqk) are self-consistent to better than 1 micro-
58** arcsecond all over the celestial sphere.
59**
60** 3) It is advisable to take great care with units, as even
61** unlikely values of the input parameters are accepted and
62** processed in accordance with the models used.
63**
64** 4) "Observed" Az,El means the position that would be seen by a
65** perfect theodolite located at the observer. This is
66** related to the observed HA,Dec via the standard rotation, using
67** the geodetic latitude (corrected for polar motion), while the
68** observed HA and RA are related simply through the local
69** apparent ST. "Observed" RA,Dec or HA,Dec thus means the
70** position that would be seen by a perfect equatorial located
71** at the observer and with its polar axis aligned to the
72** Earth's axis of rotation (n.b. not to the refracted pole).
73** By removing from the observed place the effects of
74** atmospheric refraction and diurnal aberration, the
75** geocentric apparent RA,Dec is obtained.
76**
77** 5) Frequently, mean rather than apparent RA,Dec will be required,
78** in which case further transformations will be necessary. The
79** slaAMP etc routines will convert the apparent RA,Dec produced
80** by the present routine into an "FK5" (J2000) mean place, by
81** allowing for the Sun's gravitational lens effect, annual
82** aberration, nutation and precession. Should "FK4" (1950)
83** coordinates be needed, the routines slaFk425 etc will also
84** need to be applied.
85**
86** 6) To convert to apparent RA,Dec the coordinates read from a
87** real telescope, corrections would have to be applied for
88** encoder zero points, gear and encoder errors, tube flexure,
89** the position of the rotator axis and the pointing axis
90** relative to it, non-perpendicularity between the mounting
91** axes, and finally for the tilt of the azimuth or polar axis
92** of the mounting (with appropriate corrections for mount
93** flexures). Some telescopes would, of course, exhibit other
94** properties which would need to be accounted for at the
95** appropriate point in the sequence.
96**
97** 7) The star-independent apparent-to-observed-place parameters
98** in aoprms may be computed by means of the slaAoppa routine.
99** If nothing has changed significantly except the time, the
100** slaAoppat routine may be used to perform the requisite
101** partial recomputation of aoprms.
102**
103** 8) The date argument is UTC expressed as an MJD. This is,
104** strictly speaking, wrong, because of leap seconds. However,
105** as long as the delta UT and the UTC are consistent there
106** are no difficulties, except during a leap second. In this
107** case, the start of the 61st second of the final minute should
108** begin a new MJD day and the old pre-leap delta UT should
109** continue to be used. As the 61st second completes, the MJD
110** should revert to the start of the day as, simultaneously,
111** the delta UTC changes by one second to its post-leap new value.
112**
113** 9) The delta UT (UT1-UTC) is tabulated in IERS circulars and
114** elsewhere. It increases by exactly one second at the end of
115** each UTC leap second, introduced in order to keep delta UT
116** within +/- 0.9 seconds.
117**
118** 10) IMPORTANT -- TAKE CARE WITH THE LONGITUDE SIGN CONVENTION.
119** The longitude required by the present routine is east-positive,
120** in accordance with geographical convention (and right-handed).
121** In particular, note that the longitudes returned by the
122** slaObs routine are west-positive, following astronomical
123** usage, and must be reversed in sign before use in the present
124** routine.
125**
126** 11) The polar coordinates xp,yp can be obtained from IERS
127** circulars and equivalent publications. The maximum amplitude
128** is about 0.3 arcseconds. If xp,yp values are unavailable,
129** use xp=yp=0.0. See page B60 of the 1988 Astronomical Almanac
130** for a definition of the two angles.
131**
132** 12) The height above sea level of the observing station, hm,
133** can be obtained from the Astronomical Almanac (Section J
134** in the 1988 edition), or via the routine slaObs. If p,
135** the pressure in millibars, is available, an adequate
136** estimate of hm can be obtained from the expression
137**
138** hm = -29.3 * tsl * log ( p / 1013.25 );
139**
140** where tsl is the approximate sea-level air temperature
141** in deg K (See Astrophysical Quantities, C.W.Allen,
142** 3rd edition, section 52). Similarly, if the pressure p
143** is not known, it can be estimated from the height of the
144** observing station, hm as follows:
145**
146** p = 1013.25 * exp ( -hm / ( 29.3 * tsl ) );
147**
148** Note, however, that the refraction is proportional to the
149** pressure and that an accurate p value is important for
150** precise work.
151**
152** 13) The azimuths etc used by the present routine are with respect
153** to the celestial pole. Corrections from the terrestrial pole
154** can be computed using slaPolmo.
155**
156** Called: slaAoppa, slaOapqk
157**
158** Last revision: 6 September 1999
159**
160** Copyright P.T.Wallace. All rights reserved.
161*/
162{
163 double aoprms[14];
164
165 slaAoppa ( date, dut, elongm, phim, hm, xp, yp, tdk,
166 pmb, rh, wl, tlr, aoprms );
167 slaOapqk ( type, ob1, ob2, aoprms, rap, dap );
168}
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