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<H2><A NAME="SECTION003241000000000000000">
Getting the data</A>
</H2>
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.
<P>
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.
<P>
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
<I>completely outside of</I>, 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.
<P>
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.
<P>
Near
<!-- MATH: $\delta = 90^{\circ} - \phi$ -->
<IMG
WIDTH="116" HEIGHT="41" ALIGN="MIDDLE" BORDER="0"
SRC="img1143.gif"
ALT="$\delta = 90^{\circ} - \phi$">
and
<!-- MATH: $\delta = \phi - 90^{\circ}$ -->
<IMG
WIDTH="117" HEIGHT="41" ALIGN="MIDDLE" BORDER="0"
SRC="img1144.gif"
ALT="$\delta = \phi - 90^{\circ}$">,
the limits change rapidly with declination, and should be gathered
at <IMG
WIDTH="26" HEIGHT="22" ALIGN="BOTTOM" BORDER="0"
SRC="img1145.gif"
ALT="$1^{\circ}$">
intervals.
If there are no irregular obstructions, an interval of
<!-- MATH: $10^{\circ}$ -->
<IMG
WIDTH="38" HEIGHT="22" ALIGN="BOTTOM" BORDER="0"
SRC="img1146.gif"
ALT="$10^{\circ}$">
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.)
<P>
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-15</I>
</ADDRESS>
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