\parindent 0pt
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\section{PLANNING OBSERVATIONS} \label{ch3:planning}
%\section{Planning observations} \label{ch3:planning}
The observer will need to consider target coordinates, finding
charts, guide stars, catalogue formats, and possibly blind offsets.
The requirements are slightly different depending whether observations
are made at Cassegrain or Prime focus.

\subsection{Acquisition and Guiding at Cassegrain} \label{ch3:AGCass}
\begin{itemize}
\item{\bf Acquisition.}
A Westinghouse extended S20 TV camera views a 1.8\mins$\times$1.8\min square
 of the 20\min diameter unvignetted Cassegrain Field (on or off axis).
The TV display includes labels giving orientation and scale. It is possible
to store TV images, which is useful during good seeing for recording high
spatial resolution images of the target. A filter wheel containing up to
4 circular filters is loaded. The filters currently available are
green(BG28), blue(BG38), red(RG630) and clear (UBK7).
 The effective magnitude limit for
the acquisition camera is 21.5 magnitude in integrating mode.

\item{\bf Slit Viewing.}
A simple movement of the flip mirror allows viewing of the light
reflected from the slit jaws using the acquisition TV. This allows 
visual confirmation of the positioning of the the target on the
entrance aperture of the instrument, as well as an  estimate
of the seeing profile.

\item{\bf Autoguider.} The autoguider allows offset guiding  using stars
anywhere in a 90 square arcmin field (see Figure \ref{ch2:fig3} for available 
area). Guide stars should have magnitudes in the range 7$<m_{v}<$14, and 
should ideally be selected in advance of the observing run.
Care must be taken to avoid selecting
a guide star so positioned in the field that the autoguider probe
obstructs the on-axis star-beam.

 Alternatively, an
overlay showing the IDS slit and the area within which guide stars may be
selected can be superposed on the TV image of the finder field.  A guide 
star is then selected visually  and the A\&G unit  autoguider
probe directed to that position.  There is a filter wheel in the
autoguider beam allowing up to four circular filters green(BG28),
blue(BG38), red(RG630) and 
clear (UBK7).
\end{itemize}

\subsection{Acquisition and Guiding at Prime} \label{ch3:AGPrime}
This is mainly used with the prime focus CCD camera. The A\&G unit is mounted 
on a rotator which allows the detector to be rotated to any position,
although it is normally operated at one of the cardinal points. The default
orientation is 180 degrees.
\begin{itemize}
\item{\bf Acquisition.} No direct viewing of the field is available for
the prime focus. The telescope points sufficiently accurately that any
object with a reasonably accurate position can be acquired close to the
centre of the CCD detector. This can be verified using a short CCD exposure.
\item{\bf Autoguider.} The prime focus autoguider is identical to that at
Cassegrain. The detector is an FW-130 image dissector with an S-20 
photocathode. There is a single filter, at present a broad-band green
filter. Ideally guide stars should be selected in advance of the observing
run, using Figure \ref{ch12:fig4} which shows the area of sky within
which guide stars may be selected.   

 Alternatively they may be selected from 
the finder telescope and then acquired by scanning a 2.5\mins$\times$2.5\min
area around the selected position. Stars with magnitudes in the range
7$<m_{v}<$14 will probably be suitable.   There is an overlay
of the TV image which shows the area from which guide stars can be 
chosen.   To get this overlay the TV camera must be on when we start
ADAM.    By default the system displays the Cassegrain overlay.

\item{\bf Important} At prime focus you must select the guide star
before you start the exposure.

\end{itemize}

\subsection{Finding charts, coordinates and blind offsets}
The pointing accuracy of the INT is sufficiently high that acquisition 
onto the centre of the TV field (within 15\secs\, say) is usually
possible.   Positions should therefore be measured to better than 5 arcsec.
A finding chart is necessary for all but the brightest targets. It should
cover an area somewhat larger than the field of the acquisition TV, with
orientation and scale clearly marked.   A 10\min square is about 
right.

Two coordinate systems are supported : mean pre-IAU76 and geocentric
apparent at the time of observation.   Neglect of the difference 
between pre- and post-IAU76 systems gives rise to a maximum error of about 
1 arcsec in practical cases, which is small compared with the pointing 
error.   Optional parameters are proper motions (often needed for
white dwarfs used as spectrophotometric standards,) where the epoch is
assumed to be the same as equinox, and differential tracking rates in 
RA and Dec, (needed for solar system objects). Conversion of target
coordinates can be performed with the COCO package (Starlink User Note
56) beforehand in the UK, or on the VAX~8300 at La Palma.

If the target is fainter than about 20 magnitude, it will not be visible on
the acquisition TV and a Blind Offset will have to be performed,
(see Section \ref{ch4:boffset} for details of indirect methods).
The offset star should preferably be within an arcmin of
the target, and should be in the magnitude range 9 to 16 mag.
The difference in position between the
 target and blind offset must be measured to arcsecond accuracy
before arriving at the telescope.
Blind offsets by moving the telescope are not currently reliable
 in Dec  (errors of 1 $-$ 2 arcsec are possible) on the INT.
However subarcsecond accuracy is easily achieved by moving the probe
of the autoguider.

\subsection{Observation Catalogues}

Target coordinates are normally supplied to the telescope computer
by means of an observation catalogue.  This must be typed in
directly through the user interface
or using an editor. If you have an extensive catalogue, this
is best done in the afternoon before you start observing.
Catalogues can of course be saved to disc, so they are available from 
night to night, but will normally be deleted at the end of an observing
run. The telescope computer now has a tape drive and files
can be transferred from the VAX 8300 on a scratch tape.
This is a clumsy procedure and advance notice is necessary.

Positions of guide stars selected in advance of the observing run
should be entered in the catalogue in the normal way.

\subsubsection{Catalogue format}
A catalogue for the telescope computer is a set of User Interface commands, 
(see chapter \ref{ch8:INT-UI}) exactly as typed in at a keyboard. 
Minimum matching is used, and portions of commands which may be omitted are 
given in brackets ({\it e.g.\/} the command SOURCE may be shortened to SO).   
Spaces or semi-colons may be used as separators, the latter being essential 
if the command is abbreviated (see RA and DECLINATION, below).   Anything 
following an asterisk is treated as a comment and is ignored. The following
user interface commands are relevant:

\begin{itemize}
\item {\bf SO(URCE)} defines a new catalogue entry.   The format is
       SOURCE name. A source name may have up to 20 alphanumeric 
       characters.  Embedded spaces are forbidden, unless the name 
       is enclosed in double quotes.

\item {\bf R(A)} enters the Right Ascension.   The format is R hours, 
      minutes, seconds. The last two are optional, but the command 
      must be terminated by a semi-colon if they are omitted. Spaces  
      are not required between the hours, minutes and seconds but
      it doesn't matter if you put them in.

\item {\bf D(ECLINATION)} enters a declination.   The format is D degrees, 
      minutes, seconds.   Again, the minutes and seconds may be omitted 
      if the command is terminated by a semi-colon. Spaces are not 
      required between the degrees, minutes and seconds but it
       doesn't matter if you put them in.

\item {\bf EQ(UINOX)} for mean positions, a leading letter indicates the 
      coordinate system: \\*
        \hspace*{0.5cm} B implies mean pre-IAU76 or FK4 (default) \\*
        \hspace*{0.5cm} J implies mean post-IAU76 or FK5

       A number indicates the epoch of the mean equator and equinox of that 
       system ({\it e.g.\/} B1950.0, J1987.5).

       The remaining option is  APPARENT (abbreviable to A), for which no 
       equinox is required.

\item {\bf P(ROPER-MOTION)} sets the proper motions.   The corresponding 
      epoch is assumed to be the same as the equinox.

\item {\bf /R(A)} proper motion in RA in sec yr$^{-1}$

\item {\bf /D(ECLINATION)} proper motion in Declination in arcsec yr$^{-1}$.
\item or {\bf /B(OTH)} both proper motions
\end{itemize}

A catalogue may be in free format. For legibility and neatness, however, it
is worth adhering to a more rigid tabular format with one object per line
of the file.  The format should be as follows:-

\begin{itemize}
\item The file should consist of 80-byte records ({\it i.e.\/} maximum of
        80 characters per line).

\item The filename must consist of up to 8 alphanumeric characters,
        starting with a letter, and must have the extension `.CAT'.

\item There should be no non-alphanumeric characters such as tabs 
         anywhere in the file.

\item  A line specifying default parameters and the coordinate type 
        should be given at the beginning of the file, 
        for example:
     
        {\tt .DEFAULT EQUINOX B1950 P/R 0.0 P/D 0.0}

        sets the default equinox to 1950 and the default proper motions
        to zero and specifies mean coordinates.   If these parameters are
        not specified explicitly for objects in the file, then they will
        take these values. These are in fact the default values if 
        DEFAULT is not included.

\item The format should be:

        SO OBJECT R hh mm ss.ss D +dd mm ss.s P/R +s.sss P/D +s.sss EQ Byyyy.y

        with the coordinate type, proper motions and equinox omitted if 
        they do not change from the default values.
\end{itemize}

\subsection{System Catalogues}

The catalogues currently available at the INT are:

\begin{itemize}
\item {\bf PTGRID}: a grid of stars for pointing calibration.

\item {\bf SPECPHOT}: a catalogue of spectrophotometric standards with 
      accurately known positions and proper motions.

\item {\bf UBVRIxxx}: a set of catalogues of photometric standards for 
      imaging, based on the Landolt equatorial standards.
\end{itemize}
