<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"> <!--Converted with LaTeX2HTML 98.1p1 release (March 2nd, 1998) originally by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds * revised and updated by: Marcus Hennecke, Ross Moore, Herb Swan * with significant contributions from: Jens Lippmann, Marek Rouchal, Martin Wilck and others --> <HTML> <HEAD> <TITLE>Horizon obstructions</TITLE> <META NAME="description" CONTENT="Horizon obstructions"> <META NAME="keywords" CONTENT="vol2"> <META NAME="resource-type" CONTENT="document"> <META NAME="distribution" CONTENT="global"> <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1"> <LINK REL="STYLESHEET" HREF="vol2.css"> <LINK REL="next" HREF="node605.html"> <LINK REL="previous" HREF="node569.html"> <LINK REL="up" HREF="node547.html"> <LINK REL="next" HREF="node582.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html8766" HREF="node582.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html8763" HREF="node547.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html8757" HREF="node580.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html8765" HREF="node1.html"> <IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents" SRC="icons.gif/contents_motif.gif"></A> <BR> <B> Next:</B> <A NAME="tex2html8767" HREF="node582.html">Getting the data</A> <B> Up:</B> <A NAME="tex2html8764" HREF="node547.html">File Formats Required for</A> <B> Previous:</B> <A NAME="tex2html8758" HREF="node580.html">Column label: SLITWID</A> <BR> <BR> <!--End of Navigation Panel--> <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: $\pm 30^{\circ}$ --> <IMG WIDTH="54" HEIGHT="41" ALIGN="MIDDLE" BORDER="0" SRC="img1142.gif" 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--> <A NAME="CHILD_LINKS"> </A> <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> </UL></UL> <!--End of Table of Child-Links--> <HR> <!--Navigation Panel--> <A NAME="tex2html8766" HREF="node582.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html8763" HREF="node547.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html8757" HREF="node580.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html8765" HREF="node1.html"> <IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents" SRC="icons.gif/contents_motif.gif"></A> <BR> <B> Next:</B> <A NAME="tex2html8767" HREF="node582.html">Getting the data</A> <B> Up:</B> <A NAME="tex2html8764" HREF="node547.html">File Formats Required for</A> <B> Previous:</B> <A NAME="tex2html8758" HREF="node580.html">Column label: SLITWID</A> <!--End of Navigation Panel--> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>