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<H1><A NAME="SECTION003240000000000000000">
Horizon obstructions</A>
</H1>
Information on horizon obstructions and limitations to telescope motion
should be in a table file, which we can call the ``horizon'' file.
Usually, the same restrictions apply to all foci of a telescope, and the
same table file can be used for all of them.
<P>
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.
<P>
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.
<P>
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 <I>western</I> part of the sky, with the telescope above the polar axis.
<P>
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 <I>completely</I> 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.
<P>
The second subset specifies the region from which <I>any part</I> of the pupil
may be illuminated by the Moon.
As the Moon can only appear between limits of about
<!-- MATH: $\pm 30^{\circ}$ -->
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ALT="$\pm 30^{\circ}$">declination, the ``moonlight'' part of the table need only include this range.
<P>
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.
<P>
<BR><HR>
<!--Table of Child-Links-->
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<UL>
<LI><A NAME="tex2html8768"
HREF="node582.html">Getting the data</A>
<LI><A NAME="tex2html8769"
HREF="node583.html">Descriptor for the ``horizon'' table</A>
<LI><A NAME="tex2html8770"
HREF="node584.html">MOUNTING=<TT>'FORK'</TT></A>
<UL>
<LI><A NAME="tex2html8771"
HREF="node585.html">Column label: DEC</A>
<LI><A NAME="tex2html8772"
HREF="node586.html">Column label: OBSE</A>
<LI><A NAME="tex2html8773"
HREF="node587.html">Column label: MOONE</A>
<LI><A NAME="tex2html8774"
HREF="node588.html">Column label: OBSW</A>
<LI><A NAME="tex2html8775"
HREF="node589.html">Column label: MOONW</A>
</UL>
<LI><A NAME="tex2html8776"
HREF="node590.html">MOUNTING=<TT>'GERMAN'</TT></A>
<UL>
<LI><A NAME="tex2html8777"
HREF="node591.html">Column label: DEC</A>
<LI><A NAME="tex2html8778"
HREF="node592.html">Column label: TEOBSE</A>
<LI><A NAME="tex2html8779"
HREF="node593.html">Column label: TEMOONE</A>
<LI><A NAME="tex2html8780"
HREF="node594.html">Column label: TEOBSW</A>
<LI><A NAME="tex2html8781"
HREF="node595.html">Column label: TEMOONW</A>
<LI><A NAME="tex2html8782"
HREF="node596.html">Column label: TWOBSE</A>
<LI><A NAME="tex2html8783"
HREF="node597.html">Column label: TWMOONE</A>
<LI><A NAME="tex2html8784"
HREF="node598.html">Column label: TWOBSW</A>
<LI><A NAME="tex2html8785"
HREF="node599.html">Column label: TWMOONW</A>
<LI><A NAME="tex2html8786"
HREF="node600.html">Special remarks on German mountings</A>
</UL>
<LI><A NAME="tex2html8787"
HREF="node601.html">MOUNTING=<TT>'ALTAZ'</TT></A>
<UL>
<LI><A NAME="tex2html8788"
HREF="node602.html">Column label: AZI</A>
<LI><A NAME="tex2html8789"
HREF="node603.html">Column label: OBSALT</A>
<LI><A NAME="tex2html8790"
HREF="node604.html">Column label: MOONALT</A>
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-15</I>
</ADDRESS>
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