;----------------------------------------------------------------------
; NAME:
;	update_aimass
; PURPOSE:
;       Computes the airmass for the supplied values.
;
; EXPLANATION:
;       
; CALLING SEQUENCE:
;       update_airmass, ra, dec, lat, long, ut, date
; INPUTS:
;	ra - the ra of the object in todays coordinates in hours
;	dec - the dec of the obj. in the date's coords in degrees
; 	lat - latitude of the observer in degrees
;	long - longitude of the observer in degrees
; 	ut   - time of the observation 
;       date - the date mo/day/year as seen in the header of a MIRSI
;              camera file
;
; OPTIONAL INPUTS:
;       ra2 - the RA of the object precessed to the appropriate epoch
;       dec2 - the Dec of the object precessed to the appropriate epoch
; OPTIONAL INPUT KEYWORDS:
;
; OUTPUTS:
;      returns the calculated airmass.
; OPTIONAL OUTPUTS:
;
; NOTES:
;
; PROCEDURE CALLS:

; REVISION HISTORY:
;-------------------------------------------------------------------------

FUNCTION airmass2, ra, dec, lat, long, ut, date

;constants
pi=3.1415926
deg2rad = pi/180.0
hour2rad = pi/12.0

;put the variables into appropriate units
ra = ra * 12.0 / pi
dec = dec * 180.0 / pi
lat = lat * 180.0 / pi
long = long * 180.0 / pi
ut = ut * 12.0 / pi

;convert the date to yr, mo, day
mo = strmid(date,0,strpos(date, '/'))
day = strmid(date, strpos(date,'/')+1, strpos(date,'/', /REVERSE_SEARCH) - 1 - strpos(date,'/'))
year = strmid(date, strpos(date,'/', /REVERSE_SEARCH)+1)

;determine the number of days between the autumnal equinox and the
;specified date
days = ymd2dn(year,mo,day) - ymd2dn(year,9,21)

;calculate the local siderial time and the hour angle.
st = UT + 360.0/15.0/365.2564*days 
lst = st - long/15.0
hour_angle = lst - ra


;put all of the information in radians.
latrad = lat * deg2rad
hourrad = hour_angle * hour2rad
decrad  = dec * deg2rad


;calculate the altitude angle.
alt = asin(sin(decrad)*sin(latrad) + cos(decrad)*cos(latrad)*cos(hourrad))

;calculate the zenith angle
z_angle = pi/2.0-alt

;calculate the secant of the zenith angle
secant = 1.0 / cos(z_angle)
;calculate the secant angle -1 for the airmass calculation
sec_1 = secant -1.0
;calculate the airmass
;functional form comes from:
; Astronomical observations by G. Walker, Cambridge U. Press, New york 1982
airmass = secant - 0.0018167*sec_1 - 0.002875* sec_1^2 -0.0008083 * sec_1^3
;return the airmass
return, airmass

end