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<H2><A NAME="SECTION003122000000000000000">
The Optopus session</A>
</H2>
After the initial setting up of the Optopus session, the user is now
ready for the ``real thing'', that is to use of the main commands of the
Optopus package:
<UL>
<LI><TT>HOLES/OPTOPUS</TT>,
<LI><TT>MODIFY/OPTOPUS</TT> and <TT>ZOOM/OPTOPUS</TT>,
<LI><TT>REFRACTION/OPTOPUS</TT>.
</UL>
<P>
<TT>HOLES/OPTOPUS</TT> converts the <TT>RA</TT> and <TT>DEC</TT> coordinates in the
MIDAS table created by <TT>CREATE/OPTOPUS</TT> (and precessed by
<TT>PRECESS/OPTOPUS</TT>) into <TT>:X</TT> and <TT>:Y</TT> positions of the holes to
be drilled on the Optopus starplate. It outputs the following information:
<DL COMPACT>
<DT>1.
<DD>objects or guidestars falling outside the plate area;
<DT>2.
<DD>objects or guidestars falling in the so called ``forbidden
area'', that is the thicker part of the plate used to fix it to
the spectrograph;
<DT>3.
<DD>objects which are too close to a guidestar (big or
small);
<DT>4.
<DD>objects of only which are in competition because of their proximity.
</DL>The plate is needed. The user is offered two alternatives: either to
enter pre-determined center coordinates (using <TT>SET/OPTOPUS
CRA=value1 CDEC=value2</TT>, where value1 has the format HH,MM,SS.sss, and
value2 the format +/-DD,AM,AS.ss, and <TT>SET/OPTOPUS ACFLAG=N</TT>, or to use
the command to compute them automatically (<TT>SET/OPTOPUS ACFLAG=Y</TT>.
The automatic determination of the center simply uses the arithmetic mean
of the <TT>:RA</TT> and <TT>:DEC</TT> columns. The result is not always optimal.
To choose the ``best'' center (that is the one which permits to keep the
maximum number of objects inside the plate limits) from this guess, the user
uses the <TT>MODIFY/OPTOPUS</TT> command. This command displays graphically the
position of the holes on the starplate and, if required, permits modifications
of the RA and DEC of the center using <TT>SET/OPTOPUS CRA=value1 CDEC=value2</TT>.
The user should re-run <TT>HOLES/OPTOPUS</TT> followed by <TT>MODIFY/OPTOPUS</TT>
to verify the improvements.
<P>
An important point is that both the center of the plate and the <TT>:RA</TT> and <TT>:DEC</TT> coordinates in the input table must be corrected
to the same equinox. If you decide to input your own pre-calculated
center coordinates, either precessed or not, you also have to remember
to set the value of the parameter <TT>PFLAG</TT> accordingly. In case of
automatic determination of the center, the center is calculated by
averaging the <TT>:RA</TT> and <TT>:DEC</TT> columns in an already precessed
table, so <TT>PFLAG</TT> is by default set to <TT>N</TT>.
<P>
The output table created by the command <TT>HOLES/OPTOPUS</TT> contains
also a column called <TT>:CHECK</TT>. A letter <TT>N</TT> in this column
identifies objects or guidestars with location problems of any kind
(they will be indicated by a square, in the graphic output produced by
<TT>MODIFY/OPTOPUS</TT> and <TT>ZOOM/OPTOPUS</TT>).
<P>
The task of <TT>MODIFY/OPTOPUS</TT> is very simple and twofold:
<UL>
<LI>to visualise the RA and DEC positions of the holes to be drilled
in an Optopus starplate.
<BR>
Care is also taken to distinguish between different kinds of
objects by using different graphic symbols and to permit the
correct identification of every single object by overlaying the
content of the <TT>:IDENT</TT> column of the input table.
<LI>to enable the rejection of objects or guidestars falling in a
(for any reason) ``in-convenient'' position.
For this purpose, a cursor is activated in the graphic display. The
user can click on the objects or guidestars he/she wants to be
ignored the subsequent commands of the Optopus session.
<P>
In case of very crowded fields, the limited physical dimensions of
the outputs of some graphic devices can make it difficult to read
the identification labels of the objects, and hence making the task
of deleting the ``right'' objects a really tricky one. To avoid
undesirable results, some auxiliary information is displayed whenever
the user clicks on an object: <TT>:RA</TT>, <TT>:DEC</TT>, <TT>:IDENT</TT>
and content of the <TT>:CHECK</TT> column. The user is prompted for
substitution the ``N'' already present in this later column with a
``D'' or ``d'' (for delete).
<P>
In case the wrong object has been selected, that is the :CHECK
column is empty, it is sufficient to hit return to keep everything
as it was. It may also happen that a ``wrong'' object is selected
twice, that is the <TT>:CHECK</TT> column already contains a ``D'' or
``d''. In this case one has to type the same letter (``D'' or
``d'') again, otherwise the object wlll not be rejected. Note,
that in case of close pairs of objects, both of them are surrounded
by the square symbol which means ``candidate for deletion''; however,
both squares will disappear after having deleted only one of the
two object.
<P>
Finally, if for any reason you decide you would rather keep
one of the objects marked by a square, click on it and
type anything but a ``D'' or a ``d'' when prompted. In order to
see what the starplate looks like after this editing, the user first
has to deactivate the graphics cursor (by hitting the spacebar or
pressing the right button of the mouse of your workstation) and
then rerun <TT>HOLES/OPTOPUS</TT> and <TT>MODIFY/HOLES</TT>.
Reverting <TT>HOLES/OPTOPUS</TT> is not compulsory, since it would be
enough just to rerun <TT>MODIFY/HOLES</TT> to have a new plot of the
Optopus starplate. However, it is also useful to check to review
outliers and close pairs repetetively.
<P>
The command <TT>HOLES/OPTOPUS</TT> is reasonably fast, so we advise the
users to frequently switch between <TT>HOLES/OPTOPUS</TT> and
<TT>MODIFY/OPTOPUS</TT> and vice versa.
</UL>
<P>
Some users like to start their Optopus session with an populated field
of candidate source. They then proceed to eliminate objects until a
suitable number is reached. However, care should be taken to avoid
eliminating more objects than necessary in cases where several targets
are closely grouped together. In fact, even if the minimum separation
between adjacent pairs is large enough to pass all the overlap checks
performed by <TT>HOLES/OPTOPUS</TT>, once at the telescope it may become
problematic to physically introduce the fibres into extremely close
holes. It then may happen that one is forced to a late rejection of
more scientific targets than one would have liked. However, this might
turn out less harmful than expected if one had careful enough to have
some ``backup'' holes drilled in the starplate.
<P>
In case of very close groups of objects, the command <TT>ZOOM/OPTOPUS</TT> may also be helpful. If the resolution provided by <TT>MODIFY/OPTOPUS</TT> is not enough, this command permits to actually blow
up a section of the Optopus starplate plotted on the graphic screen by
<TT>MODIFY/OPTOPUS</TT>. The user only has to choose the center of the
section she/he wants to be enlarged with the cursor. In most cases the
default zoom factor of 5 is sufficient to resolve close groups or
pairs. However, should this resolution not to be enough, the
possibility exists to enter the command <TT>ZOOM/OPTOPUS</TT> again, with
a new zoom factor, the center remaining unchanged.
<P>
When all inacceptable objects have been removed, it is time to use the
command
<BR>
<TT>REFRACTION/OPTOPUS</TT> to correct the <TT>X</TT> and <TT>Y</TT>
position of the holes on the starplate for the effect of atmospheric
refraction. For a detailed description of the correction algorithm
and an estimation of such effects in the particular case of La Silla,
we refer to G. Lund, 1986, <I>OPTOPUS, ESO Operating Manual No. 6</I>,
pag. 17-18. Here, we summarise that from coordinated of the plate
center coordinates, the specified temporal observating window and the
wavelength range of interest, <TT>REFRACTION/OPTOPUS</TT> determines:
<UL>
<LI>an optimal differential correction vector, scaled according to the
coordinates of each object;
<LI>an optimal chromatic correction vector for the guidestars.
</UL>
<P>
Note, that the coordinates of the plate center must be the same as the
ones already used with <TT>HOLES/OPTOPUS</TT>. It is not necessary to
reset these since <TT>REFRACTION/OPTOPUS</TT> will get the (precessed)
values from the keyword <TT>PLATECEN</TT> that have been saved by <TT>HOLES/OPTOPUS</TT>.
<P>
In general, the observer will try to observe his/her fields at the
smallest possible overall hour angle (airmass). This optimalisation
has to be made in advance. The window in sidereal time for each of the
plates which will be observed during a single night can be easily
computed knowing that for the date entered by <TT>SET/OPTOPUS
DATE=value</TT>, the command <TT>REFRACTION/OPTOPUS</TT> outputs the sidereal
times at the beginning and end of the night on La Silla. Not more than
4 (in summer) or 5 (in winter) Optopus starplates can be used in one
night. So, just to run <TT>REFRACTION/OPTOPUS</TT> using the default
value for the sidereal time slot (ignore any error messages you may
get, as in this first run you are only interested in the first line of
the output, which will be correct anyway) and divide the night into 4
or 5 exposures (allowing for some start-up time at the beginning,
approx. 20 minutes). An example of the output of <TT>REFRACTION/OPTOPUS</TT> can be found in
Table <A HREF="node542.html#output-refraction">H.3</A>.
<P>
<BR>
<DIV ALIGN="CENTER"><A NAME="29362"> </A>
<TABLE>
<CAPTION><STRONG>Table H.3:</STRONG>
Output of REFRACTION/OPTOPUS command</CAPTION>
<TR><TD>
<DIV ALIGN="CENTER"><TABLE CELLPADDING=3 BORDER="1" ALIGN="CENTER">
<TR><TD ALIGN="LEFT">Darkness will begin at ST:</TD>
<TD ALIGN="LEFT">20.37</TD>
</TR>
<TR><TD ALIGN="LEFT">and end at ST:</TD>
<TD ALIGN="LEFT">5.16</TD>
</TR>
<TR><TD ALIGN="LEFT"> </TD>
<TD ALIGN="LEFT"> </TD>
</TR>
<TR><TD ALIGN="LEFT">Sidereal time for observation:</TD>
<TD ALIGN="LEFT">21.00</TD>
</TR>
<TR><TD ALIGN="LEFT">Hour angle:</TD>
<TD ALIGN="LEFT">-27.49 degrees</TD>
</TR>
<TR><TD ALIGN="LEFT">Zenith distance:</TD>
<TD ALIGN="LEFT">24.84</TD>
</TR>
<TR><TD ALIGN="LEFT">Maximum refraction correction:</TD>
<TD ALIGN="LEFT">0.23 arcsec</TD>
</TR>
<TR><TD ALIGN="LEFT">Position angle of correction vectors:</TD>
<TD ALIGN="LEFT">-106.56 degrees</TD>
</TR>
<TR><TD ALIGN="LEFT"> </TD>
<TD ALIGN="LEFT"> </TD>
</TR>
<TR><TD ALIGN="LEFT">Chosen length for exposure:</TD>
<TD ALIGN="LEFT">60 minutes</TD>
</TR>
<TR><TD ALIGN="LEFT">Approx. optimal obs. slot (ST):</TD>
<TD ALIGN="LEFT">20h 30m to 21h 30m</TD>
</TR>
<TR><TD ALIGN="LEFT">Approx. optimal obs. slot (UT):</TD>
<TD ALIGN="LEFT">24h 6m to 25h 6m</TD>
</TR>
<TR><TD ALIGN="LEFT">Corresp. range of corr. vectors:</TD>
<TD ALIGN="LEFT">from -99 to -116 deg.</TD>
</TR>
<TR><TD ALIGN="LEFT"> </TD>
<TD ALIGN="LEFT"> </TD>
</TR>
<TR><TD ALIGN="LEFT">Wavelength range for optimisation:</TD>
<TD ALIGN="LEFT">3800 to 5500 Ångstroms</TD>
</TR>
<TR><TD ALIGN="LEFT">Optimal correction at wavelength:</TD>
<TD ALIGN="LEFT">4329 Ångstroms</TD>
</TR>
<TR><TD ALIGN="LEFT">Chromatic correction needed in X:</TD>
<TD ALIGN="LEFT">-46. microns</TD>
</TR>
<TR><TD ALIGN="LEFT">Chromatic correction needed in Y:</TD>
<TD ALIGN="LEFT">-14. microns</TD>
</TR>
</TABLE>
</DIV>
<A NAME="output-refraction"> </A></TD></TR>
</TABLE>
</DIV>
<BR>
<P>
The sidereal time for which the corrections are finally calculated can
either be enforced by the user, by setting the parameters <TT>ASTFLAG=N</TT> and <TT>OST=value</TT>, or automatically determined by the
command. In the latter case <TT>ASTFLAG</TT> must be set to <TT>Y</TT>.
<P>
<TT>REFRACTION/OPTOPUS</TT> produces an output table quite alike the one
created by <BR> <TT>HOLES/OPTOPUS</TT>. The most obvious differences are
that now the <TT>:X</TT> and <TT>:Y</TT> columns contain coordinates
corrected for the atmospheric refraction effects, and the column <TT>:NUMBER</TT>, has been added. This new column will later be needed to
identify the holes on the starplate by a sequential number.
<P>
Another important characteristic of the table produced by <TT>REFRACTION/OPTOPUS</TT> is that, being the final table generated in the
Optopus session and the one which the observer will presumably bring
along to the telescope, it contains all relevant output information
(<I>e.g.</I> see Table <A HREF="node542.html#output-refraction">H.3</A>) in its descriptors.
Besides, as already remarked, the user has the possibility to save all
parameters used in the session as well, by using the command <TT>SAVE/OPTOPUS tablename</TT>.
<P>
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