| 1 | #include "slalib.h"
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| 2 | #include "slamac.h"
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| 3 | void slaAtmdsp ( double tdk, double pmb, double rh, double wl1,
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| 4 | double a1, double b1, double wl2, double *a2, double *b2 )
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| 5 | /*
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| 6 | ** - - - - - - - - - -
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| 7 | ** s l a A t m d s p
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| 8 | ** - - - - - - - - - -
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| 9 | **
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| 10 | ** Apply atmospheric-dispersion adjustments to refraction coefficients.
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| 11 | **
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| 12 | ** Given:
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| 13 | ** tdk double ambient temperature, degrees K
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| 14 | ** pmb double ambient pressure, millibars
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| 15 | ** rh double ambient relative humidity, 0-1
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| 16 | ** wl1 double reference wavelength, micrometre (0.4 recommended)
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| 17 | ** a1 double refraction coefficient A for wavelength wl1 (radians)
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| 18 | ** b1 double refraction coefficient B for wavelength wl1 (radians)
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| 19 | ** wl2 double wavelength for which adjusted A,B required
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| 20 | **
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| 21 | ** Returned:
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| 22 | ** *a2 double refraction coefficient A for wavelength wl2 (radians)
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| 23 | ** *b2 double refraction coefficient B for wavelength wl2 (radians)
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| 24 | **
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| 25 | ** Notes:
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| 26 | **
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| 27 | ** 1 To use this routine, first call slaRefco specifying wl1 as the
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| 28 | ** wavelength. This yields refraction coefficients a1,b1, correct
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| 29 | ** for that wavelength. Subsequently, calls to slaAtmdsp specifying
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| 30 | ** different wavelengths will produce new, slightly adjusted
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| 31 | ** refraction coefficients which apply to the specified wavelength.
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| 32 | **
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| 33 | ** 2 Most of the atmospheric dispersion happens between 0.7 micrometre
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| 34 | ** and the UV atmospheric cutoff, and the effect increases strongly
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| 35 | ** towards the UV end. For this reason a blue reference wavelength
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| 36 | ** is recommended, for example 0.4 micrometres.
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| 37 | **
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| 38 | ** 3 The accuracy, for this set of conditions:
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| 39 | **
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| 40 | ** height above sea level 2000 m
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| 41 | ** latitude 29 deg
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| 42 | ** pressure 793 mB
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| 43 | ** temperature 17 degC
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| 44 | ** humidity 50%
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| 45 | ** lapse rate 0.0065 degC/m
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| 46 | ** reference wavelength 0.4 micrometre
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| 47 | ** star elevation 15 deg
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| 48 | **
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| 49 | ** is about 2.5 mas RMS between 0.3 and 1.0 micrometres, and stays
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| 50 | ** within 4 mas for the whole range longward of 0.3 micrometres
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| 51 | ** (compared with a total dispersion from 0.3 to 20.0 micrometres
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| 52 | ** of about 11 arcsec). These errors are typical for ordinary
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| 53 | ** conditions and the given elevation; in extreme conditions values
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| 54 | ** a few times this size may occur, while at higher elevations the
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| 55 | ** errors become much smaller.
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| 56 | **
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| 57 | ** 4 If either wavelength exceeds 100 micrometres, the radio case
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| 58 | ** is assumed and the returned refraction coefficients are the
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| 59 | ** same as the given ones.
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| 60 | **
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| 61 | ** 5 The algorithm consists of calculation of the refractivity of the
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| 62 | ** air at the observer for the two wavelengths, using the methods
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| 63 | ** of the slaRefro routine, and then scaling of the two refraction
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| 64 | ** coefficients according to classical refraction theory. This
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| 65 | ** amounts to scaling the A coefficient in proportion to (n-1) and
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| 66 | ** the B coefficient almost in the same ratio (see R.M.Green,
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| 67 | ** "Spherical Astronomy", Cambridge University Press, 1985).
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| 68 | **
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| 69 | ** Last revision: 25 June 1996
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| 70 | **
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| 71 | ** Copyright P.T.Wallace. All rights reserved.
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| 72 | */
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| 73 | {
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| 74 | double tdkok, pmbok, rhok, psat, pwo, w1, wlok, wlsq, w2, dn1, dn2, f;
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| 75 |
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| 76 |
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| 77 | /* Check for radio wavelengths. */
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| 78 | if ( wl1 > 100.0 || wl2 > 100.0 ) {
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| 79 |
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| 80 | /* Radio: no dispersion. */
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| 81 | *a2 = a1;
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| 82 | *b2 = b1;
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| 83 | } else {
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| 84 |
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| 85 | /* Optical: keep arguments within safe bounds. */
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| 86 | tdkok = gmax ( tdk, 100.0 );
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| 87 | tdkok = gmin ( tdkok, 500.0 );
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| 88 | pmbok = gmax ( pmb, 0.0 );
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| 89 | pmbok = gmin ( pmbok, 10000.0 );
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| 90 | rhok = gmax ( rh, 0.0 );
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| 91 | rhok = gmin ( rhok, 1.0 );
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| 92 |
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| 93 | /* Atmosphere parameters at the observer. */
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| 94 | psat = pow ( 10.0, -8.7115 + 0.03477 * tdkok );
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| 95 | pwo = rhok * psat;
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| 96 | w1 = 11.2684e-6 * pwo;
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| 97 |
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| 98 | /* Refractivity at the observer for first wavelength. */
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| 99 | wlok = gmax ( wl1, 0.1 );
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| 100 | wlsq = wlok * wlok;
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| 101 | w2 = 77.5317e-6 + ( 0.43909e-6 + 0.00367e-6 / wlsq ) / wlsq;
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| 102 | dn1 = ( w2 * pmbok - w1 ) / tdkok;
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| 103 |
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| 104 | /* Refractivity at the observer for second wavelength. */
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| 105 | wlok = gmax ( wl2, 0.1 );
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| 106 | wlsq = wlok * wlok;
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| 107 | w2 = 77.5317e-6 + ( 0.43909e-6 + 0.00367e-6 / wlsq ) / wlsq;
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| 108 | dn2 = ( w2 * pmbok - w1 ) / tdkok;
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| 109 |
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| 110 | /* Scale the refraction coefficients (see Green 4.31, p93). */
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| 111 | if ( dn1 != 0.0 ) {
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| 112 | f = dn2 / dn1;
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| 113 | *a2 = a1 * f;
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| 114 | *b2 = b1 * f;
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| 115 | if ( dn1 != a1 )
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| 116 | *b2 = *b2 * ( 1.0 + dn1 * ( dn1 - dn2 ) /
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| 117 | ( 2.0 * ( dn1 - a1 ) ) );
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| 118 | } else {
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| 119 | *a2 = a1;
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| 120 | *b2 = b1;
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| 121 | }
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| 122 | }
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| 123 | }
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