Information required

Ideally you need to know the average air mass, $X$, of each observation. Alternatively, the zenith distance, $z$, is just as good. The CURSA applications for calibrating instrumental magnitudes (see the recipe in Section ) can automatically calculate the air mass from the zenith distance. Conversely, if you need to calculate the air mass from the zenith distance yourself then Section  gives the requisite formulæ.

If the auxiliary information for your observations contain neither the air mass nor the zenith distance then you will have to calculate the zenith distance from whatever information is available about the celestial coordinates and times of your observations. The zenith distance, $z$, can be calculated from:

$$\sec z = \frac{1}{(\sin \psi \sin \delta + \cos \psi \cos \delta \cos h)}$$ (23)

where:

$\psi$

is the latitude of observation,

$\delta$

is the Declination of the object observed,

$h$

is the Hour Angle of the object observed.

The Hour Angle is simply:

$$h = s - \alpha$$ (24)

where $\alpha$ is the Right Ascension of the object observed and $s$ is the local sidereal time. Again, the local sidereal time may not be recorded in your observations and it might be necessary to calculate it from whatever information is available about the time of your observations. Most standard textbooks on spherical astronomy give further details of calculating the zenith distance and converting between time systems (see, for example, Spherical Astronomy by Green[31]). Another useful source of information is the explanation and notes for the SLALIB positional-astronomy subroutine library (see SUN/67[75]).

The keywords used to represent these various items of information differ between different observatories. Table  gives some examples. It is based on CCD frames observed with the Jacobus Kapteyn Telescope (JKT) on La Palma. In this case both the air mass and the zenith distance are included and hence there is no need to calculate them. The keywords used at the Anglo-Australian Observatory are available via the World Wide Web (at URL http://www.aao.gov.au/local/www/tjf/fits.html ). The appropriate instrument and observatory manuals should document the keywords used in a given dataset. In case of difficulty staff at the observatory where the dataset was acquired should be able to advise.

Table: Example of some keywords present in a CCD frame acquired with the Jacobus Kapteyn Telescope (JKT) on La Palma

Keyword	Description
AIRMASS	air mass
ZENDIST	zenith distance (degrees)
TIMSTART	start time of exposure
TIMEND	end time of exposure
RA	Right Ascension of the object
DEC	Declination of the object
EQUINOX	equinox of coordinate system
DATE-OBS	date of the observation