refrac.c File Reference

Correction for atmospheric refraction in visible bands. More...

#include <math.h>
#include <stdlib.h>
#include "aephem.h"

Functions
double	ae_refrac_visible (double alt, void *param)
	Atmospheric refraction correction (in visible bands). More...
double	ae_refrac_ulich (double alt, void *param)
	Atmospheric refraction correction (in millimetre bands). More...

Detailed Description

Correction for atmospheric refraction in visible bands.

Function Documentation

double ae_refrac_ulich	(	double	alt,
		void *	param
	)

Atmospheric refraction correction (in millimetre bands).

This function is suitable for passing to ae_topocentric().

This function uses the equations from Ulich 1981. It does not cite which millimetre wavelength ranges it is expected to be good for. The ALMA memo 366 does analysis on another model and finds it good to 2% up to frequencies of 1000 GHz. (The worst is at 500 GHz.)

Ulich claims accuracy to 2" above altitudes of 3 degrees.

References: -B. L. Ulich. "Millimeter Wave Radio Telescopes: Gain and Pointing Charactersitics". International Journal of Infrared and Millimeter Waves, 2, 2 (1981).

• J. G. Mangum. "ALMA Memo 366: A Telescope Pointing Algorithm for ALMA". Available at http://www.alma.nrao.edu/memos/html-memos/abstracts/abs366.html.

Parameters

alt	The altitude of the observation, in degrees.
param	For passing the atmospheric pressure, in hPa, the temperature, in degrees centigrade and the surface humidity, as a percentage. These should be stored as doubles, such that: -pressure is in ((double *) param)[0], -temperature is in ((double *) param)[1]. -and humidity is in ((double *) param)[2].

Returns

The correction in degrees to be added to true altitude to obtain apparent altitude.

References AE_DTR, and AE_STD.

double ae_refrac_visible	(	double	alt,
		void *	param
	)

Atmospheric refraction correction (in visible bands).

This function is suitable for passing to ae_topocentric().

For high altitude angle, AA page B61 is used. The accuracy is `usually about 0.1 arcsecond'.

The formula for low altitude is from the Almanac for Computers. It gives the correction for observed altitude, so has to be inverted numerically to get the observed from the true. The accuracy is about 0.2' for -20C < T < +40C and 970mb < P < 1050mb.

Parameters

alt	The altitude of the observation, in degrees.
param	For passing the atmospheric pressure, in millibar, and the temperature, in degrees centigrade. These should be stored as doubles, such that the pressure is in ((double *) param)[0] and the temperature in ((double *) param)[1].

Returns

The correction in degrees to be added to true altitude to obtain apparent altitude.

References AE_DTR.