Getting the data

If the photometer has an eyepiece, one can easily check whether the pupil is clear or obstructed by examining the exit pupil formed by the eyepiece. If you have a choice between eyepieces, choose the lowest power available to get the largest pupil. You may need to examine the pupil image with a magnifier; another eyepiece will do.

Telescopes without eyepieces can be checked by removing the instrument, and examining the focal plane by eye. The image of any distant obstruction will appear in the focal plane. Nearby obstructions more than a focal length away will be imaged behind the focal plane. The edge of the dome will be visible as an out-of-focus blur seen on the far side of the primary.

In either case, simply fix the telescope at a given declination (or azimuth, if it has an alt-az mounting), and move it in the other coordinate toward the horizon until an obstruction appears. The ``observing'' limit is a position at which the obstruction is near, but completely outside of, the usable field. The ``moonlight'' limit is the position at which the last speck of sky disappears behind terrestrial obstructions. If the ``observing'' limit is set by mechanical obstructions, you may have to estimate the ``moonlight'' limit, or just adopt the true horizon to be safe.

The measurements can easily be made in daytime, or during the brighter part of twilight. It will be most convenient to determine the ``observing'' and ``moonlight'' limits on the same side of the sky together, and then to move to the other side of the sky. The necessary data can be gathered in a few hours, and will prevent many unpleasant surprises while observing or in reducing observations.

Near $\delta = 90^{\circ} - \phi$ and $\delta = \phi - 90^{\circ}$, the limits change rapidly with declination, and should be gathered at $1^{\circ}$ intervals. If there are no irregular obstructions, an interval of $10^{\circ}$ is probably sufficient near the equator. You should assume that programs using the information in this table will interpolate linearly between the adjacent points, and adjust your spacing accordingly. (A program should be available to produce a blank form to fill in.)