The default name of the file is the ``telescope'' variable name described above, but with the suffix ``hor.tbl'' in place of the focus designation. Thus, for the ESO 1.5-meter telescope, the file name would be ESO15hor.tbl. If mechanical considerations make separate tables necessary for different foci of the same telescope, the ``hor.tbl'' should be appended to the full name of the focus.
For most telescopes, it is most natural to provide the hour-angle limits (east and west) as functions of declination. However, for telescopes with alt-azimuth mountings, the data may most easily be gathered in the form of altitude limits as functions of azimuth. No provision is made at this time for alt-alt mounts, although some are in use.
Notice that telescopes with German equatorial (cross-axis) and other asymmetrical mountings require two groups of columns: one for telescope east of the pier, and one for telescope west. Also, notice that ``telescope east'' is the position usually used in observing the western part of the sky, with the telescope above the polar axis.
Finally, two kinds of columns are required. The first subset specifies the observing limits (i.e., the accessible region of sky in which the telescope pupil is completely illuminated by a star). In this region, no part of the telescope pupil may be shaded by an obstruction. Photometric observations can be made only in this part of the sky.
The second subset specifies the region from which any part of the pupil
may be illuminated by the Moon.
As the Moon can only appear between limits of about
declination, the ``moonlight'' part of the table need only include this range.
Users should remember that trees have a tendency to grow larger, so that the ``horizon'' file should be re-checked from time to time if nearby trees are significant obstructions. Compilers of these tables should also bear this in mind, and leave a little margin for safety near trees.