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SUBROUTINE sla_OAP (TYPE, OB1, OB2, DATE, DUT, ELONGM, PHIM,
: HM, XP, YP, TDK, PMB, RH, WL, TLR,
: RAP, DAP)
*+
* - - - -
* O A P
* - - - -
*
* Observed to apparent place
*
* Given:
* TYPE c*(*) type of coordinates - 'R', 'H' or 'A' (see below)
* OB1 d observed Az, HA or RA (radians; Az is N=0,E=90)
* OB2 d observed ZD or Dec (radians)
* DATE d UTC date/time (modified Julian Date, JD-2400000.5)
* DUT d delta UT: UT1-UTC (UTC seconds)
* ELONGM d mean longitude of the observer (radians, east +ve)
* PHIM d mean geodetic latitude of the observer (radians)
* HM d observer's height above sea level (metres)
* XP d polar motion x-coordinate (radians)
* YP d polar motion y-coordinate (radians)
* TDK d local ambient temperature (DegK; std=273.155D0)
* PMB d local atmospheric pressure (mB; std=1013.25D0)
* RH d local relative humidity (in the range 0D0-1D0)
* WL d effective wavelength (micron, e.g. 0.55D0)
* TLR d tropospheric lapse rate (DegK/metre, e.g. 0.0065D0)
*
* Returned:
* RAP d geocentric apparent right ascension
* DAP d geocentric apparent declination
*
* Notes:
*
* 1) Only the first character of the TYPE argument is significant.
* 'R' or 'r' indicates that OBS1 and OBS2 are the observed Right
* Ascension and Declination; 'H' or 'h' indicates that they are
* Hour Angle (West +ve) and Declination; anything else ('A' or
* 'a' is recommended) indicates that OBS1 and OBS2 are Azimuth
* (North zero, East is 90 deg) and zenith distance. (Zenith
* distance is used rather than elevation in order to reflect the
* fact that no allowance is made for depression of the horizon.)
*
* 2) The accuracy of the result is limited by the corrections for
* refraction. Providing the meteorological parameters are
* known accurately and there are no gross local effects, the
* predicted apparent RA,Dec should be within about 0.1 arcsec
* for a zenith distance of less than 70 degrees. Even at a
* topocentric zenith distance of 90 degrees, the accuracy in
* elevation should be better than 1 arcmin; useful results
* are available for a further 3 degrees, beyond which the
* sla_REFRO routine returns a fixed value of the refraction.
* The complementary routines sla_AOP (or sla_AOPQK) and sla_OAP
* (or sla_OAPQK) are self-consistent to better than 1 micro-
* arcsecond all over the celestial sphere.
*
* 3) It is advisable to take great care with units, as even
* unlikely values of the input parameters are accepted and
* processed in accordance with the models used.
*
* 4) "Observed" Az,El means the position that would be seen by a
* perfect theodolite located at the observer. This is
* related to the observed HA,Dec via the standard rotation, using
* the geodetic latitude (corrected for polar motion), while the
* observed HA and RA are related simply through the local
* apparent ST. "Observed" RA,Dec or HA,Dec thus means the
* position that would be seen by a perfect equatorial located
* at the observer and with its polar axis aligned to the
* Earth's axis of rotation (n.b. not to the refracted pole).
* By removing from the observed place the effects of
* atmospheric refraction and diurnal aberration, the
* geocentric apparent RA,Dec is obtained.
*
* 5) Frequently, mean rather than apparent RA,Dec will be required,
* in which case further transformations will be necessary. The
* sla_AMP etc routines will convert the apparent RA,Dec produced
* by the present routine into an "FK5" (J2000) mean place, by
* allowing for the Sun's gravitational lens effect, annual
* aberration, nutation and precession. Should "FK4" (1950)
* coordinates be needed, the routines sla_FK524 etc will also
* need to be applied.
*
* 6) To convert to apparent RA,Dec the coordinates read from a
* real telescope, corrections would have to be applied for
* encoder zero points, gear and encoder errors, tube flexure,
* the position of the rotator axis and the pointing axis
* relative to it, non-perpendicularity between the mounting
* axes, and finally for the tilt of the azimuth or polar axis
* of the mounting (with appropriate corrections for mount
* flexures). Some telescopes would, of course, exhibit other
* properties which would need to be accounted for at the
* appropriate point in the sequence.
*
* 7) The star-independent apparent-to-observed-place parameters
* in AOPRMS may be computed by means of the sla_AOPPA routine.
* If nothing has changed significantly except the time, the
* sla_AOPPAT routine may be used to perform the requisite
* partial recomputation of AOPRMS.
*
* 8) The DATE argument is UTC expressed as an MJD. This is,
* strictly speaking, wrong, because of leap seconds. However,
* as long as the delta UT and the UTC are consistent there
* are no difficulties, except during a leap second. In this
* case, the start of the 61st second of the final minute should
* begin a new MJD day and the old pre-leap delta UT should
* continue to be used. As the 61st second completes, the MJD
* should revert to the start of the day as, simultaneously,
* the delta UTC changes by one second to its post-leap new value.
*
* 9) The delta UT (UT1-UTC) is tabulated in IERS circulars and
* elsewhere. It increases by exactly one second at the end of
* each UTC leap second, introduced in order to keep delta UT
* within +/- 0.9 seconds.
*
* 10) IMPORTANT -- TAKE CARE WITH THE LONGITUDE SIGN CONVENTION.
* The longitude required by the present routine is east-positive,
* in accordance with geographical convention (and right-handed).
* In particular, note that the longitudes returned by the
* sla_OBS routine are west-positive, following astronomical
* usage, and must be reversed in sign before use in the present
* routine.
*
* 11) The polar coordinates XP,YP can be obtained from IERS
* circulars and equivalent publications. The maximum amplitude
* is about 0.3 arcseconds. If XP,YP values are unavailable,
* use XP=YP=0D0. See page B60 of the 1988 Astronomical Almanac
* for a definition of the two angles.
*
* 12) The height above sea level of the observing station, HM,
* can be obtained from the Astronomical Almanac (Section J
* in the 1988 edition), or via the routine sla_OBS. If P,
* the pressure in millibars, is available, an adequate
* estimate of HM can be obtained from the expression
*
* HM ~ -29.3D0*TSL*LOG(P/1013.25D0).
*
* where TSL is the approximate sea-level air temperature in
* deg K (see Astrophysical Quantities, C.W.Allen, 3rd edition,
* section 52). Similarly, if the pressure P is not known,
* it can be estimated from the height of the observing
* station, HM as follows:
*
* P ~ 1013.25D0*EXP(-HM/(29.3D0*TSL)).
*
* Note, however, that the refraction is proportional to the
* pressure and that an accurate P value is important for
* precise work.
*
* 13) The azimuths etc used by the present routine are with respect
* to the celestial pole. Corrections from the terrestrial pole
* can be computed using sla_POLMO.
*
* Called: sla_AOPPA, sla_OAPQK
*
* P.T.Wallace Starlink 6 September 1999
*
* Copyright (C) 1999 P.T.Wallace and CCLRC
*-
IMPLICIT NONE
CHARACTER*(*) TYPE
DOUBLE PRECISION OB1,OB2,DATE,DUT,ELONGM,PHIM,HM,
: XP,YP,TDK,PMB,RH,WL,TLR,RAP,DAP
DOUBLE PRECISION AOPRMS(14)
CALL sla_AOPPA(DATE,DUT,ELONGM,PHIM,HM,XP,YP,TDK,PMB,RH,WL,TLR,
: AOPRMS)
CALL sla_OAPQK(TYPE,OB1,OB2,AOPRMS,RAP,DAP)
END
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