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/node135.html | 319 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 319 insertions(+) create mode 100644 src/slalib/sun67.htx/node135.html (limited to 'src/slalib/sun67.htx/node135.html') diff --git a/src/slalib/sun67.htx/node135.html b/src/slalib/sun67.htx/node135.html new file mode 100644 index 0000000..753bc4c --- /dev/null +++ b/src/slalib/sun67.htx/node135.html @@ -0,0 +1,319 @@ + + + + +SLA_OAP - Observed to Apparent + + + + + + + + + + + + +
+ +next + +up + +previous +
+ Next: SLA_OAPQK - Quick Observed to Apparent +
+Up: SUBPROGRAM SPECIFICATIONS +
+ Previous: SLA_NUTC - Nutation Components +

+

+

SLA_OAP - Observed to Apparent +   +

+
+
ACTION: +
Observed to apparent place. +
CALL: +
CALL sla_OAP ( + TYPE, OB1, OB2, DATE, DUT, ELONGM, PHIM, + HM, XP, YP, TDK, PMB, RH, WL, TLR, RAP, DAP) +

+

+
GIVEN: +
+
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
TYPEC*(*)type of coordinates - `R', `H' or `A' (see below)
OB1Dobserved Az, HA or RA (radians; Az is N=0, E=$90^{\circ}$)
OB2Dobserved zenith distance or $\delta$ (radians)
DATED UTC date/time (Modified Julian Date, JD-2400000.5)
DUTD$\Delta$UT: UT1-UTC (UTC seconds)
ELONGMDobserver's mean longitude (radians, east +ve)
PHIMDobserver's mean geodetic latitude (radians)
HMDobserver's height above sea level (metres)
XP,YPDpolar motion $[\,x,y\,]$ coordinates (radians)
TDKDlocal ambient temperature (degrees K; std=273.155D0)
PMBDlocal atmospheric pressure (mB; std=1013.25D0)
RHDlocal relative humidity (in the range 0D0-1D0)
WLDeffective wavelength ($\mu{\rm m}$, e.g. 0.55D0)
TLRDtropospheric lapse rate (degrees K per metre, +e.g. 0.0065D0)
+

+
RETURNED: +
+
+ + + + + +
RAP,DAPDgeocentric apparent $[\,\alpha,\delta\,]$
+

+
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 $90^{\circ}$) 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 azimuth and elevation should be within about +

$0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.1$ for $\zeta<70^{\circ}$. Even + at a topocentric zenith distance of + $90^{\circ}$, the accuracy in elevation should be better than + 1 arcminute; useful results are available for a further + $3^{\circ}$, 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 microarcsecond 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 $[\,h,\delta\,]$ 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 $[\,\alpha,\delta\,]$ or $[\,h,\delta\,]$ 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 $[\,\alpha,\delta\,]$ is obtained. +
5. +
Frequently, mean rather than apparent + $[\,\alpha,\delta\,]$ will be required, + in which case further transformations will be necessary. The + sla_AMP etc. routines will convert + the apparent $[\,\alpha,\delta\,]$ 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 B1950 + coordinates be needed, the routines sla_FK524 etc. will also + need to be applied. +
6. +
To convert to apparent $[\,\alpha,\delta\,]$ 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 $\Delta$UT 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 $\pm$$0^{\rm s}\hspace{-0.3em}.9$.
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 (as in the Astronomical + Almanac before 1984) 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 + $0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.3$ . 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 mB, is available, an adequate + estimate of HM can be obtained from the following expression: +
HM=-29.3D0*TSL*LOG(P/1013.25D0) +
+ where TSL is the approximate sea-level air temperature in degrees K + (see Astrophysical Quantities, C.W.Allen, 3rd edition, + §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. +
+
+ +next + +up + +previous +
+ Next: SLA_OAPQK - Quick Observed to Apparent +
+Up: SUBPROGRAM SPECIFICATIONS +
+ Previous: SLA_NUTC - Nutation Components +

+

+

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