Specifications

AUTOSTAR CCD PHOTOMETRY 47
Two equations are in common use that take into account not only
the curvature, but also the refraction, of the Earth’s atmosphere:
X = secZ (1 – 0.0012 (sec
2
Z – 1))
X = secZ – 0.0018167 (secZ – 1 ) - 0.002875 (secZ – 1)
2
– 0.0008083 (secZ - 1)
3
Where: The value of secZ depends only on the location of the
observer and the position of the star in the sky and can be
determined by:
secZ = (sinLAT sinδ + cos LAT cosδ cosHA)
-1
In this equation , LAT is the observer's latitude while δ and HA
are the star's Declination and Hour Angle, respectively. All values
are given in decimal degrees.
Note: Z in the first equation above refers to the apparent zenith
distance of a star (i.e., taking into account atmospheric refraction).
In physical terms, this distance is equivalent to the angle between
the optical axis of a telescope and a plumb bob hanging from the
mounting. However, direct determination of this angle is both
cumbersome and inherently inaccurate. In contrast, Z in more
complex equations refers to the true Zenith Distance, which
assumes that no atmospheric refraction is present. Use of the latter
value is preferred, as it can be readily calculated from available
parameters.
Star's Declination (δ)
As illustrated in Figure B-2, the Declination, δ, of a star is the
angular distance above or below the celestial equator (which is the
projection of the Earth's equator on the celestial sphere). The
declination of a star on the celestial equator is 0°, while that of a
star at the North Celestial Pole (NCP) would be +90°. Stars
between the celestial equator and the NCP have declinations of
0° ≤ δ ≤ +90°.