AIRMASS

Name

airmass

Purpose (one Line)

Compute airmass for one or more times.

Description

This is should be a pretty good function for computing the air mass factor.
The default is to use the cosine based formula derived by David Tholen
but the older secant based formula from Hardie is still available. The
zenith angle is corrected for refraction before using either formula (see
REFRAC). The defaults on the atmospheric conditions are STP. This function
should be quite good up to 5 airmasses. This formula will work up to
a zenith angle of 80 degrees after which the computation id not done.

Calling Sequence

am = airmass(jd,ra,dec,lat,lon,wave,pressure,temp,relhum)

Inputs

jd - Julian date (must be double precision to get nearest second).
ra - Right ascension (of date) in radians.
dec - Declination (of date) in radians.
lat - Latitude of observatory in radians.
lon - West longitude of observatory in radians.

Optional Input Parameters

wave - wavelength of light, in microns (default=0.56)
pressure - atmospheric pressure in mm of Hg (default=760.0)
temp - atmospheric temperature in degrees C (default=0.0)
relhum - Relative humidity (in percent) (default=0.0)

Keyword Input Parameters

UT - Time, in hours to add to JD to get the correct Universal Time.
HARDIE - Flag, if set causes Hardie formula to be used.

Keyword Output Parameters

ALT - Optional return of the altitude for each airmass.
LHA - Optional return of the local hour angle.
LST - Optional return of the local sidereal time.
AZI - Optional return of the azimuth (west from south).

Outputs

Return value is the airmass in single precision.

Common Blocks

None.

Side Effects

Restrictions

Any input may be a vector. If more than one is a vector then the
lengths must match. The return will have the same dimensions as
the input.

Procedure

Modification History

Written 1992 March, by Marc W. Buie, Lowell Observatory
94/05/05 - MWB, modified to split out the LST calculation, added LST and
LHA optional keyword outputs and UT keyword input.'
97/03/03 - MWB, added the Tholen airmass equation as the default. Hardie
is still included as an option. Also added refraction
to the calculation which added numerous inputs.
97/10/23 - MWB, added the AZI keyword.
2002/01/08, MWB, now allows 2-d inputs on jd,ra,dec