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# GET_AIRM -- Derive airmass value from telescope data if possible
# Otherwise return INDEF
#
# Note that HA must be negative to the East.
# If HA is not reasonable, then ST-RA is used
procedure get_airm (ra, dec, ha, st, latitude, airm)
real ra, dec, ha, st, latitude, airm
begin
# Verify realistic value for HA
if (IS_INDEF (ha)) {
if (IS_INDEF (st) || IS_INDEF (ra))
call error (0, "Can't determine airmass")
ha = st - ra
}
# Now verify DEC
if (IS_INDEF (dec))
call error (0, "Can't determine airmass")
# Everything should be just fine now
# Compute airmass using method of John Ball
call airmass (dec, ha, latitude, airm)
end
# AIRMASS -- Compute airmass from RA, DEC, and HA
#
# Airmass formulation from Allen "Astrophysical Quantities" 1973 p.125,133.
# and John Ball's book on Algorithms for the HP-45
procedure airmass (dec, ha, latitude, airm)
real dec, ha, latitude, airm
real scale, rads, cos_zd, sin_elev
real x
data rads /57.29577951/ # Degrees per radian
data scale/750.0 / # Atmospheric scale height approx
begin
cos_zd = sin (latitude/rads) * sin (dec/rads) +
cos (latitude/rads) * cos (dec/rads) * cos (ha*15/rads)
sin_elev = cos_zd # SIN of elev = cos of Zenith dist
x = scale * sin_elev
airm = sqrt (x**2 + 2*scale + 1) - x
end
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