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CO_REFRACT

CO_REFRACT()

## Purpose

Calculate correction to altitude due to atmospheric refraction.

## Description

CO_REFRACT can calculate both apparent altitude from observed altitude and
vice-versa.

## Calling Sequence

new_alt = CO_REFRACT(old_alt, [ ALTITUDE= , PRESSURE= , \$
TEMPERATURE= , /TO_OBSERVED , EPSILON= ])

## Input

old_alt - Observed (apparent) altitude, in DEGREES. (apparent if keyword
/TO_OBSERVED set). May be scalar or vector.

## Output

Function returns apparent (observed) altitude, in DEGREES. (observed if
keyword /TO_OBSERVED set). Will be of same type as input
altitude(s).

## Optional Keyword Inputs

ALTITUDE : The height of the observing location, in meters. This is
only used to determine an approximate temperature and pressure,
if these are not specified separately. [default=0, i.e. sea level]
PRESSURE : The pressure at the observing location, in millibars.
TEMPERATURE: The temperature at the observing location, in Kelvin.
EPSILON: When keyword /TO_OBSERVED has been set, this is the accuracy
to obtain via the iteration, in arcseconds [default = 0.25
arcseconds].
/TO_OBSERVED: Set this keyword to go from Apparent->Observed altitude,
using the iterative technique.
Note, if altitude is set, but temperature or pressure are not, the
program will make an intelligent guess for the temperature and pressure.

## Description

Because the index of refraction of air is not precisely 1.0, the atmosphere
bends all incoming light, making a star or other celestial object appear at
a slightly different altitude (or elevation) than it really is. It is
important to understand the following definitions:
Observed Altitude: The altitude that a star is SEEN to BE, with a telescope.
This is where it appears in the sky. This is always
GREATER than the apparent altitude.
Apparent Altitude: The altitude that a star would be at, if *there were no
atmosphere* (sometimes called "true" altitude). This is
usually calculated from an object's celestial coordinates.
Apparent altitude is always LOWER than the observed
altitude.
Thus, for example, the Sun's apparent altitude when you see it right on the
horizon is actually -34 arcminutes.
This program uses couple simple formulae to estimate the effect for most
(from an observation), and want the apparent altitude. To go the other way,
this program uses an iterative approach.

## Example

The lower limb of the Sun is observed to have altitude of 0d 30'.
Calculate the the true (=apparent) altitude of the Sun's lower limb using
mean conditions of air pressure and temperature
IDL> print, co_refract(0.5) ===> 0.025degrees (1.55')
WAVELENGTH DEPENDENCE:
This correction is 0 at zenith, about 1 arcminute at 45 degrees, and 34
arcminutes at the horizon FOR OPTICAL WAVELENGTHS. The correction is
NON-NEGLIGIBLE at all wavelengths, but is not very easily calculable.
These formulae assume a wavelength of 550 nm, and will be accurate to
about 4 arcseconds for all visible wavelengths, for elevations of 10
degrees and higher. Amazingly, they are also ACCURATE FOR RADIO
FREQUENCIES LESS THAN ~ 100 GHz.
It is important to understand that these formulae really can't do better
than about 30 arcseconds of accuracy very close to the horizon, as
variable atmospheric effects become very important.

## References

1. Meeus, Astronomical Algorithms, Chapter 15.
2. Explanatory Supplement to the Astronomical Almanac, 1992.
3. Methods of Experimental Physics, Vol 12 Part B, Astrophysics,
Radio Telescopes, Chapter 2.5, "Refraction Effects in the Neutral
Atmosphere", by R.K. Crane.

## Dependencies

CO_REFRACT_FORWARD (contained in this file and automatically compiled).

## Author

Chris O'Dell
Assistant Professor of Atmospheric Science
Email: odell@atmos.colostate.edu

## Revision History

version 1 (May 31, 2002)
Update iteration formula, W. Landsman June 2002
Corrected slight bug associated with scalar vs. vector temperature and
pressure inputs. 6/10/2002
Fixed problem with vector input when /TO_OBSERVED set W. Landsman Dec 2005
Allow arrays with more than 32767 elements W.Landsman/C.Dickinson Feb 2010

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