\parindent 0pt
\parskip 0.3cm
%\section{Acquisition and (Auto)guiding}
\section{ACQUISITION AND (AUTO)GUIDING}
\subsection{Finder telescope} \label{ch12:finder}

A finder telescope is available on the INT. A TV camera can be used to
view a field 33\mins$\times$25\mins\, in size. The finder is in particular
useful for the location of guide stars: the TCS software will generate
overlays on the TV image of the finder field, at either focus, indicating 
the areas of sky where guide stars may be selected. It is therefore important
that the zeropoint of the finder coordinate system is properly determined (see
section \ref{ch11:fzero}), in order to allow accurate mapping of the autoguider
acquisition areas and detector/slit positions onto the finder field.

\subsection{The Acquisition and Guider Units}

A\&G units are provided at both the Cassegrain and prime
foci of the INT.

\subsubsection{Cassegrain A\&G Unit} \label{ch12:AGCass}

The A\&G Unit at Cassegrain is mounted on the turntable at the rear of the
INT primary mirror cell. The turntable can be rotated through 365 degrees.
The Cassegrain A\&G Unit has been 
designed primarily for use with the Cassegrain Intermediate Dispersion 
Spectrograph (IDS) and Faint Object Spectrograph (FOS).  Nevertheless, 
the overall philosophy of the design has been to incorporate a range of 
facilities which would be required by most instruments operating at the 
Cassegrain focus. Figures \ref{ch12:fig1} through \ref{ch12:fig7} show 
the layout of the A\&G unit. The following functions are provided:

\begin{itemize}
\item {\bf Acquisition:} A TV camera views a 1.8\mins$\times$1.8\mins\, element
of the 20\mins\, diameter unvignetted Cassegrain field (on or off axis) via a
flip mirror in the field-viewing position. There are four TV colour filters
available: BG28~(blue), BG38~(green), RG630~(red) and clear (UBK7).
\item {\bf Slit Viewing:} A simple movement of the flip mirror facilitates
viewing of the light reflected from the spectrograph slit jaws by the 
acquisition TV.
\item {\bf Autoguiding:} An autoguider based on an FW-130 image dissector 
scanner with S-20 photocathode is available, which allows offset guiding
using stars anywhere in a 90\mins\, square field, apart from an area near the
centre where the probe obscures the spectrograph slit. Guide stars should have
magnitudes in the range 7$<m_v<$14. There are four colour filters
available for the autoguider beam:  BG28, BG38, RG630 and clear (UBK7).
\end{itemize}

Further, a selection of spectroscopic and photometric calibration lamps and 
a selection of neutral density and colour filters are available 
for use with the attached instrument.
The comparison lamps and filters are only useful over a more restricted field 
of 4\mins\, diameter, matching that of the IDS.

\begin{figure}
\vspace{20.0cm}
\caption[The Cassegrain A\&G Unit (1: elevation)]
{The Cassegrain Acquisition and Guider Unit (side view), showing 
the autoguider, TV camera, comparison system and filters.}
\label{ch12:fig1}
\end{figure}
\begin{figure}
\vspace{20.0cm}
\caption[The Cassegrain A\&G Unit (2: elevation)]
{The Cassegrain Acquisition and Guider Unit (side view).}
\label{ch12:fig2}
\end{figure}
\begin{figure}
\vspace{20.0cm}
\caption[The Cassegrain A\&G Unit (3: plan)]
{The Cassegrain Acquisition and Guider Unit (viewed from above).}
\label{ch12:fig3}
\end{figure}
\begin{figure}
\vspace{20.0cm}
\caption[The Cassegrain A\&G Unit (4: probe positions and encoder readings)]
{The relationship between probe positions and encoder readings for the
Cassegrain A\&G Unit. The IDS slit lies parallel to the $x$ co-ordinate: for
Position Angle 0\degs\, the slit orientation is North-South. Units in this
diagram are microns.}
\label{ch12:fig7}
\end{figure}

\subsubsection{Prime Focus A\&G Unit} \label{ch12:AGPrime}

At Prime focus the A\&G unit is mainly used with the prime focus CCD camera. 
The unit is mounted on a rotator, which allows 
the detector to be rotated to any position angle, although it is 
normally operated at one of the cardinal points. The default orientation
is 180\degs, for which position angle spillage of Liquid Nitrogen is lowest.

Note that {\bf no direct viewing} facility exists at prime. The telescope points
sufficiently accurately that any object with known position can be acquired 
close to the centre of the CCD detector. Further, there are no calibration
lamps provided, but there is a filter wheel which can carry up to 6 filters,
for use with the CCD detector.

The prime focus autoguider is identical to the one at Cassegrain focus.
However, because there is insufficient space to accommodate a filter wheel or 
slide, a single fixed broad-band green filter is used. The choice of this 
filter is a compromise between a match to the FW-130 response and to the 
spectral response of the CCD detectors used at prime focus. Note that
the autoguider at prime  does not look through the same colour filter as the
CCD, which has implications for focal adjustments, in particular on changing
(CCD) filter (see section \ref{ch11:afocus}).

An important feature of the geometry at Prime is that both the guide probe
and the CCD detector are mounted off-axis. This means that guide stars can
only be selected in an area located about 30\mins\, away from the field viewed
by the CCD, on the opposite side of the rotator axis. This feature makes some
special procedures necessary for the set up of geometry parameters at prime
(see section \ref{ch11:gprime}) to enable easy acquisition of guide stars.
Figure \ref{ch12:fig4} shows the geometry and the area in which
guide stars can be selected. As for Cassegrain,
stars with magnitudes in the range 7$<m_v<$14 are suitable.

%---------------------------PF_GEOM_FIG.TEX----------------------------------

\begin{figure}[h]
\center{\underline{\bf GEOMETRY OF INT PRIME FOCUS UNIT}}
\setlength{\unitlength}{1mm}
\begin{picture}(155,105)

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% N.B. This figure can be updated whenever a new geometry for the prime     %
% focus unit has been determined. Full instructions and model commands are  %
% included here. The scale has been chosen to fit the figure upright on an  %
% A4 sheet as printed on the Cannon laser printer. All values in the        %
% changeable section of this figure are entered in arcseconds to make life  %
% easy. R E Wallis.                                                         %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
   
%\put(125,2){\today}

%Draw autoguider field
%-----------------------
\thicklines
\put(6,68){\makebox[57mm]{Rotator Position 180\degs}}
\put(6,6){\framebox(57,78){autoguider field}}
\put(0,2){(1850,1850)}
\put(52.25,2){(1850,900)}
\put(2.5,85){(550,1850)}
\put(53.5,85){(550,900)}
\put(43,91.5){\vector(-1,0){16}}
\put(23,90){Y}
\put(70,60){\vector(0,-1){16}}
\put(68.75,39.5){X}
\thinlines

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%      ----------------BEGINNING OF CHANGEABLE SECTION----------------      % 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%Let AOX be the x-dist from the top of the A/G field to the chip centre,
%    AOY be the y-dist from the rotator axis to the chip centre,
%    BOX be the x-dist from the top of the A/G field to the rotator axis,
%and BOY be the y-dist from the RH side of the A/G field to the rotator axis.

%At present, for the RCA chip
%AOX = 26arcsec, AOY = 587.5arcsec,  
%        and for the GEC chip,
%AOX = 20arcsec, AOY = 623arcsec, 
%          and
%BOX = -24arcsec, BOY = 564.5arcsec.

\setlength{\unitlength}{0.0618mm}

%------------------------RCA CHIP---------------------------------

%For outline of chip,
%--------------------
%\put(AOY +BOY +1018.7 -189.5, 1358.3 -AOX -118.5){\framebox(379,237){ }}
\put(1981.2,1213.8){\framebox(379,237){ }}

%\put(1754.4 , 1358.3 -AOX +70 -24.25){RCA}
\put(1754.4,1378.05){RCA}

%\put(1891.9 , 1358.3 -AOX +70){\vector(1,0){AOY +BOY +1018.7 -189.5 -1891.9}
\put(1891.9,1402.3){\vector(1,0){89.3}}

%Mark chip centre
%-----------------
%\put(BOY +AOY +1018.7,1358.3 -AOX){\circle*{24.25}} 
\put(2170.7,1332.3){\circle*{24.25}}

%Label distances below top line of chip centre
%---------------------------------------------
%\put(AOY +BOY +1018.7 , 1422.9){\vector(0,-1){64.7}} 
\put(2170.7,1422.9){\vector(0,-1){64.7}}
%\put(AOY +BOY +1018.7 , 1358.3 -AOX -64.7){\vector(0,1){64.7}} 
\put(2170.7,1267.6){\vector(0,1){64.7}}

%\put(AOY +BOY +1018.7 -20 , 1460){AOX\H{ }}  
\put(2150.7,1460){26\H{ }}

%\put(1018.7 +BOY +AOY/2 -72.8 , 1544){\vector(-1,0){AOY/2 - 72.8}}
\put(1804.15,1544){\vector(-1,0){220.95}}
%\put(1018.7 +BOY +AOY/2 +72.8 , 1544){\vector(1,0){AOY/2 - 72.8}}
\put(1949.75,1544){\vector(1,0){220.95}}
%\put(1018.7 +BOY +AOY/2 -56.6 , 1528){AOY\H}
\put(1820.35,1528){587.5\H}

%Draw the small vertical lines to mark the y-position of the chip centre
%-----------------------------------------------------------------------
%\put(1018.7 +BOY +AOY , 1520){\line(0,1){48}} 
\put(2170.7,1520){\line(0,1){48}}



%-------------------------------GEC CHIP-----------------------

%For outline of chip,
%--------------------
%\put(AOY +BOY +1018.7 -156 , 1358.3 -AOX -104){\framebox(312,208){ }}
\put(2050.2,1234.3){\framebox(312,208){ }}

%\put(1754.4 , 1358.3 -AOX -70 -24.25){GEC} 
\put(1754.4,1244.05){GEC}

%\put(1891.9 , 1358.3 -AOX -70){\vector(1,0){AOY +BOY +1018.7 -156.0 -1891.9} 
\put(1891.9,1268.3){\vector(1,0){158.3}}

%Mark chip centre
%----------------
%\put(BOY +AOY +1018.7,1358.3 -AOX){\circle*{24.25}} 
\put(2206.2,1338.3){\circle*{24.25}}

%Label distances below top line of chip centre
%---------------------------------------------
%\put(AOY +BOY +1018.7 , 1422.9){\vector(0,-1){64.7}} 
\put(2206.2,1422.9){\vector(0,-1){64.7}}

%\put(AOY +BOY +1018.7 , 1358.3 -AOX -64.7){\vector(0,1){64.7}} 
\put(2206.2,1273.6){\vector(0,1){64.7}}

%\put(AOY +BOY +1018.7 -20 , 1358.3 -AOX -64.7 -40){AOX\H{ }} 
\put(2186.2,1233.6){20\H{ }}

%\put(1018.7 +BOY +AOY/2 -72.8 , 1604){\vector(-1,0){AOY/2 - 72.8}}
\put(1821.9,1604){\vector(-1,0){238.7}}
%\put(1018.7 +BOY +AOY/2 +72.8 , 1604){\vector(1,0){AOY/2 - 72.8}}
\put(1967.5,1604){\vector(1,0){238.7}}
%\put(1018.7 +BOY +AOY/2 -56.6 , 1588){AOY\H}
\put(1838.1,1588){623\H}

%Draw the small vertical lines to mark the y-position of the chip centre
%-----------------------------------------------------------------------
%\put(1018.7 +BOY +AOY , 1580){\line(0,1){48}} for the GEC chip
\put(2206.2,1580){\line(0,1){48}}



%-------------------------BOTH CHIPS------------------------

\put(2069.7,1091.4){outline of}
\put(2142.5,1018.7){chip}

%Mark rotation centre
%-----------------------
%\put(1018.7 +BOY , 1358.3 -BOX){\circle*{24.25}}
\put(1583.2,1382.3){\circle*{24.25}}
%\put(1018.7 +BOY -80 , 1204.6){rotator}
\put(1503.2,1204.6){rotator}
%\put(1018.7 +BOY -40 , 1140){axis}
\put(1543.2,1140){axis}

%Label distances below top line of rot. centre 
%---------------------------------------------

%IF BOX IS POSITIVE,
%\put(1018.7 +BOY , 1422.9){\vector(0,-1){64.7}}
%\put(1018.7 +BOY , 1358.3 - BOX -64.7){\vector(0,1){64.7}}

%IF BOX IS NEGATIVE,
%\put(1018.7 +BOY , 1422.9 -BOX){\vector(0,-1){64.7}}
%\put(1018.7 +BOY , 1358.3 -64.7{\vector(0,1){64.7}}

\put(1583.2,1446.9){\vector(0,-1){64.7}}
\put(1583.2,1293.6){\vector(0,1){64.7}}

%\put(1018.7 +BOY -24.25 , 1460){BOX\H{ }}
\put(1558.95,1460){-24\H{ }}

%Label distances from rotation centre to "park" position
%-------------------------------------------------------
%\put(1018.7 +BOY/2 -72.8 , 1544){\vector(-1,0){BOY/2 -72.8}}
\put(1228.15,1544){\vector(-1,0){209.45}}
%\put(1018.7 +BOY/2 +72.8 , 1544){\vector(1,0){BOY/2 -72.8}}
\put(1373.75,1544){\vector(1,0){209.45}}
%\put(1018.7 +BOY/2 -56.6 , 1528){BOY\H{ }}
\put(1244.35,1528){564.5\H{ }}

%Draw the small vertical lines to mark the y-positions of the RH side of the
% A/G field and the rotator centre.
%--------------------------------------------------------------------------
\put(1018.7,1520){\line(0,1){48}}
%\put(1018.7 +BOY , 1520){\line(0,1){108}}
\put(1583.2,1520){\line(0,1){108}}


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%         ----------------END OF CHANGEABLE SECTION----------------         %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%Draw scale                                                                   
%----------
\thicklines
\setlength{\unitlength}{1mm}
\put(124,20){\line(-1,0){37.2}}
\put(98.15,16){10 arcmin}
\put(124,20){\line(0,-1){2}}
\put(86.8,20){\line(0,-1){2}}

%Draw compass
%------------
\thicklines
\put(117,32){\vector(0,1){18}}
\put(117,32){\vector(-1,0){18}}
\put(115.75,51){N}
\put(95.5,30.5){E}
%Draw orientation (EJZ)
\thinlines
\put(117,32){\vector(-3,+4){10}}
\put(117,32){\vector(+4,+3){14}}
\put(117,32){\oval(12,12)[tl]}
\put(105.0,47.0){X}
\put(132.0,43.0){Y}
\put(112.5,39.5){\bf pa}

%Draw top line (continuation of top of a/g field)
%------------------------------------------------
\put(63,84){\dashbox(90,0){ }}
%Write scale in a/g field and label "park" position
\put(6,20){\makebox[57mm]{1 A/G unit $\equiv$ 0.97arcsec}}
\put(64.5,80){GD PARK}

\end{picture}
\caption[Geometry of the INT Prime Focus Unit]
 {Scale drawing of locations of chip, rotator axis and guide-star
 acquisition field, showing probe-coordinate limits. The probe's park position
 is at the top right-hand corner (550,900). The orientation indicated
 is for position angle 180\degs; the `compass' shows the general relation
 between guide probe coordinates for any Position Angle on the sky.}
\label{ch12:fig4}
\end{figure}
\eject

\subsection{Using the Autoguiders} \label{ch12:gauto}
\subsubsection{Introduction} 

Both the Cassegrain and Prime Focus A\&G units are equiped with identical
autoguiders. The autoguiders are controlled by a dedicated micro processor,
which continuously analyses the image of a guide star at the centre of the
guide probe and determines its position in the frame of the autoguider.
Position errors, offsets from the probe centre due to tracking errors, are 
communicated to the Telescope Control software which can then make a pointing 
correction. Before the autoguider can lock onto a guide star, that star
has to be positioned on the guide probe near the probe centre, by moving either
the probe or the telescope. If the guide star is within the field of
the probe, the autoguider will locate it in the `field' mode; once the star is
detected, the probe (or telescope) can be moved to centre it, and the 
guider-telescope combination can lock onto the guide star in a closed 
guide loop. The tracking errors are sent to the Telescope Control 
computer (the PE~3220) in order to effectuate pointing or tracking corrections. 

The control of the
probe position (for moving the probe around to acquire a guide star) is via
the Instrument Computer (the PE~3210, because that one controls the A\&G Unit
functions). Therefore, one can centre a guide star on the probe in 
two different ways: (1) by moving
the probe with the ADAM GDXY command (Instrument Computer) while the telescope
is tracking, or (2) by moving the telescope (handset or User Interface) with a
fixed probe position. Each possibility has its own advantages and disadvantages
which are discussed in section \ref{ch12:gmode} below. The GET GUIDE STAR
command of the Grinnell menu has the effect of moving the probe.

The standard way of acquiring guide stars is by locating them on the Finder
TV probe (at either focus). The telescope control software knows how to map
positions on the Finder TV onto the autoguider acquisition field provided
that (1) the finder zeropoint is determined correctly (see section 
\ref{ch11:fzero}), and (2) the geometry parameters for the focus used are set
up correctly (see section \ref{ch11:geometry}). Although guide stars can
be located on the finder TV, it is desirable to select guide stars in advance
of an observing run on survey plates and measure their coordinates, in
particular if there are restrictions on the rotator Position Angle (see
section \ref{ch12:survey}). If a guide star position is known, the coordinates
can be entered directly via the User Interface and the TCS will position the
guide probe straightaway. At Cassegrain, care must be taken to avoid 
selecting a guide star so positioned that the autoguider probe obstructs 
the spectrograph slit.

\subsubsection{Acquisition modes: Probe or Telescope\,?} \label{ch12:gmode}

When the `acquire' button on the autoguider control panel is pressed 
there are two ways of moving the guide star into the centre of the 
autoguider field so that the guiding loop may be locked: by offsetting 
the telescope, leaving the probe stationary, or by moving the probe 
whilst the telescope tracks undisturbed. Their advantages and 
disadvantages are: 
 
\begin{itemize}
\item
Move the probe (after {\tt AC/PR} at the User Interface):
Location of a guide star is normally much 
less accurate than the positioning of the main telescope, because of the
finder flop error. Therefore, this is the default mode, because by moving the
probe any residual error in the mapping will be compensated for. (Of course,
when the guide loop is closed, the probe will be kept stationary.)
Since the probe is moved from the Instrument Computer, but the position errors
are sampled by the Telescope Computer, this mode requires data to be transfered
via the interprocessor link.  It will not work if the link is broken. 
\item
Move telescope (after {\tt AC/TE} at the User Interface): 
If the guide star position is known precisely, either 
in RA and Dec by accurate measurement or in probe $x$,$y$ coordinates from 
a previous 
observation, then this mode can be used to make an accurate blind 
offset. This is not very important with an imaging detector, {\it i.e.\/}
the Prime focus CCD, but does help with the IDS and FOS; acquisition 
is also significantly faster than when the probes are moved and the 
process is independent of the interprocessor link. 
\end{itemize}

Thus, in general it is best to move the probe. If, however, you have
an accurate probe position for the guide star (from a previous run), or
if the interprocesor link is broken, then move the telescope. 
 
In order to switch modes, type at the User Interface terminal: 
 
\UI{AC/PR}   \hspace{2.2cm} to move the probe (with fixed telescope).

\UI{AC/TEL}  \hspace{2.0cm} to move the telescope (with fixed probe).

\subsubsection{Locating guide stars on Sky Survey plates} \label{ch12:survey}

Although there is no strict need for the observer to select guide stars in 
advance, since locating them on the finder field is simple and quick,
it is desirable to measure guide star coordinates before going to the telescope,
and at Prime Focus in particular it is worth checking that there is a 
suitable guide star in the field accessible to the probe, especially if there 
are constraints on the exact position angle of the CCD chip on the sky. 

A scale diagram of the Prime Focus detector and guide-probe fields is shown in
figure~\ref{ch12:fig4}; the diagram for Cassegrain is show in 
figure~\ref{ch12:fig5}. In order to check that there is a suitable guide star,
place a scaled version of the overlay
on the POSS with the chip outline centred on the object to be observed. 
Rotate the
overlay to your chosen position angle. If you do not care, use 0\degs\, at 
Cassegrain, and 180\degs\, at Prime,
because this avoids unnecessary loss of liquid nitrogen at large zenith
distances in the North. Check that there
is a star with 7$<m_v<$14 in the probe field. If there is not, try
again with the overlay rotated by 180\degs\, (if you care about the
orientation) or any angle (if you don't, but remember that cardinal
points give better displays). Make a note of the approximate PA to be used; at 
the telescope you can locate the star on the Finder TV, or, if you happen to 
know its coordinates, then these can be entered at the User Interface console.
 
\subsubsection{Locating guide stars at Prime Focus}
 
The standard method of finding guide stars for the Prime focus CCD 
Camera is to locate them on the wide-field Finder display. Having 
reassured yourself in advance that a suitable star exists (see 
\ref{ch12:survey} above) and having zeroed the finder model at the beginning
of the night (section \ref{ch11:fzero}), proceed as follows: 

\begin{enumerate}
\item Move the rotator to your required position angle with the ADAM command
      ROTPOS. The default angle on startup is at 180\degs.
\item Point the telescope to your object to be observed; take a GLANCE
      to check its position.
\item Select \button{FINDER} and \button{DIRECT} on the Grinnell panel to
      route the raw finder image to the monitor.
\item Offset the telescope so that the guide probe field becomes visible on 
      the finder. This done by pressing the BEAMSWITCH \button{A} button on 
      the Control Desk. 
      The offset is made in a coordinate system fixed to the Prime Focus Unit;
      its value on the sky depends therefore on the rotator Position Angle.
\item As a result of pressing \button{A}\,, stars should appear 
      to move along the major axis of the chip as the beamswitch takes 
      effect and this will be visible on the finder image. If they move
      otherwise, or if nothing happens,
      perhaps the default aperture offset values have been overwritten.
      The correct offset values are set up on startup of the control software;
      to check the values, type at the User Interface console:
 
      \UI{AD(VISE)/T(ELESCOPE)}
 
      Aperture A should be set to 0,$-$590 arcsec. If you have accidentally 
      overwritten these values, type:
 
      \UI{AP(ERTURE)/A 0 -590}

      and press \button{A} again.
      The aperture in use is shown on the Information Display with the 
      offsets converted to RA and Dec. 
 
\item Select the Grinnell function GET GUIDE STAR. A display of 
      the field accessible to the probe will appear on the screen. Move the 
      cursor to your chosen guide star in the permitted area and press 
      \button{ENTER} on the Grinnell joystick control box. The coordinates of 
      the star will be transmitted to the instrumentation computer which 
      will then move the guide probe to the correct position. 

      A star will 
      be bright enough for autoguiding if it is visible on the finder with
      4 frames of integration at full TV gain. Avoid very bright ($m_v<$6) 
      stars. 
 
\item Finally press the BEAMSWITCH \button{NOM} button on the Control Desk to 
      return the telescope to your object and press \button{DIRECT} on the 
      Grinnell panel to unfreeze the TV display.
\end{enumerate}
The telescope is now pointing to your object, which should be positioned close
to the centre of the CCD detector, while the guide probe is positioned such 
that the guide star is at the centre of the acquisition area. The guide star
must now be acquired with the guide probe, and the guide loop enabled
as described in section \ref{ch12:acquire} below.

Note: if the acquisition of a guide star fails, the most likely cause is that
the mapping of the autoguider acquisition field onto the finder has gone wrong.
This will occur if the TV scales are more than about 5 percent off the correct
values, and these should be checked/redefined with the ADAM command 
TV\?SETSCALE.
Nominal values at Prime are 3.38~arcsec/pixel in $x$ and 2.75 arcsec/pixel in
$y$ (see section 3.1.8 of the INT Prime Focus User Guide).

\subsubsection{Locating guide stars at the Cassegrain focus}

The best way to find guide stars is by using the Finder TV probe. 
The Grinnell menu function GET GUIDE STAR draws an overlay on the 
display of the Finder field which shows the locations of 
the spectrograph slit and the area in which guide stars may be 
selected (Figure \ref{ch12:fig5}). It is very important that the zeropoint
of the finder be determined beforehand, in order to
enable the TCS to map the guide probe acquisition area 
accurately onto the finder field.

\begin{figure}
\vspace{15.0cm}
\caption[Guide star acquisition area at Cassegrain]
{The area in which guide stars may be chosen at the Cassegrain focus of
the INT. The orientation on the sky is given for Position Angle 0\degs; at
other angles, the diagram must be rotated anticlockwise by the angle. Stars
may be selected in the shaded area. If the autoguider probe is moved closed
to the centre of the field, then the slit is obscured; partial obscuration
occurs in the area shaded with dashed lines. The field outside the circle is
heavily vignetted, but bright stars may be usable.}
\label{ch12:fig5}
\end{figure}

\begin{enumerate}
\item Slew the telescope to the object to be observed and move it onto the 
      spectrograph slit using the Cassegrain TV camera.
% and MARK its position, {\it i.e.\/} store its 
% coordinates so that the telescope can be returned to this position 
% afterwards, by typing at the User Interface:
%
% \UI{SO(URCE)/M(ARK) name}
% 
% where {\tt name} is your chosen name for the 
% object. If a catalogue entry already exists, then its coordinates will 
% be overwritten; otherwise, a new entry will be created.
 
\item
 Press the \button{FINDER} button followed by the \button{DIRECT} button
 on the Grinnell Panel. 
 
\item
 Select the Grinnell menu function GET GUIDE STAR: a
 horseshoe-shaped acquisition field will appear on the 
 Grinnell TV monitor (figure \ref{ch12:fig5}).
 Any stars which appear within the permitted area of sky
 can be reached by the guide probe
 without obscuring the spectrograph slit.
 Choose a star which is visible without
 integration (otherwise it will be too faint for the autoguider).

\item % step 4
 Move the grinnell cursor to the desired guide star position and press the
 \button{ACCEPT}\, button on the joystick. The probe will now move to this star.

\item
 Select probe mode by typing at the User Interface:

 \UI{AC/PR}
%
% here follows the old method from step 4 on (replaced by Grinnell command)
%\item
% Move the selected guide star to the slit and MARK its position with:
% 
% \UI{SO(URCE)/M(ARK) guidename}
% 
% where {\tt guidename} is your preferred name for the guide star. 
% 
%\item
% The object and its guide star are now stored as User Interface
% catalogue entries {\tt name} and {\tt guidename}. Associate these two 
% entries by typing:
% 
% \UI{name}
% 
% (This selects {\tt name} as the edit source), followed by:
% 
% \UI{GUIDE guidename}
%\item 
% Check with the {\tt LIST} or {\tt ADV/EDIT} commands that {\tt guidename} 
% is indeed attached as guide-object to object {\tt name}.
%
%\item Now return to the original object in the usual way: 
% 
%\UI{NEXT name}
%
%\UI{GO} \hspace{1.0cm}(or press \button{SRCE}\,)
% 
% The object should return to its position on the slit. 
% 
%\item
% Press the \doublebutton{GUIDE}{STAR} button on the A\&G panel. This causes the 
% probe to move to the position of the guide star. 
\end{enumerate}
 
You are now ready to set up the autoguider and acquire the guide star as
described in section \ref{ch12:acquire} furtheron.

\subsubsection{Using guide stars of known RA and Dec} 
  
If you happen to know the coordinates of a suitable guide star, you can
enter them directly in the object catalogue.
Enter the object {\tt name} and its guide star {\tt guidename} at the User 
Interface in the usual way and associate the two entries by typing:  

\UI{name}
 
(This selects {\tt name} as the edit source), followed by:
 
\UI{GUIDE guidename}

This information can, of course, be entered from a prepared disc file.  
Note that the coordinate systems used for an object and its guide 
star must be the same (both Apparent or Mean with the same Equinox).  
  
In order to move the guide probe to the correct position, press the 
\doublebutton{GUIDE}{STAR} button on the A\&G panel. 
  
\subsubsection{Using guide stars with known probe coordinates}
\label{ch12:known}
  
If you know the probe coordinates from a previous observation (these 
should have been noted in the observations log), type at the ADAM terminal:

\ADAM{GDXY(POS) x y}

where {\tt x} and {\tt y} are the probe coordinates.
These values are returned by the PFSTATUS command (for the Prime focus) or 
the Mimic Display (at Cassegrain focus). Units are 40 microns at Prime
(despite PFSTATUS's allegation that they are in {\tt mm}) and microns at 
Cassegrain.  
  
In this case, you may well wish to acquire the guide star by moving 
the telescope rather than the guide probe (section \ref{ch12:gmode}), 
as this ensures that your  
field is in exactly the same position on the detector (or slit) as it was on  
an earlier run.

\subsubsection {Acquiring a guide star} \label{ch12:acquire}
 
This procedure is the same whether the telescope or the probe is to be 
moved. Route the autoguider probe image to one of the TV monitors. If the 
autoguider has not been turned on (resulting in a blank screen) press 
the button below the fourth mini-monitor on the extreme top right of 
the Control Desk (red = on; white = off). This switches the High Tension
power supply on. The HT supply is interlocked with the main dome lights: 
if the latter are switched on, the HT is disabled in order to protect the
guide probe; conversely, it is not possible to power the guide probe with
the dome illumination on. The autoguider display should come up with a 
frame and an `idle' message (Figure \ref{ch12:fig6}). If it does not, 
press the \button{SHUTDOWN} button on the Autoguider Panel. If there are still 
no signs of life and the autoguider panel 
appears to be active, call the Duty Technician. 
\begin{figure}
\vspace{17.0cm}
\caption{The Autoguider Display} \label{ch12:fig6}
\end{figure}

If the autoguider is alive, press the \button{FIELD} button 
on the autoguider panel. This causes the autoguider 
to scan a field of about 2\mins$\times$2\mins\, and to lock onto the brightest 
pixel (in software; the probe does not move),
which in all likelihood is the intended guide star (if the proper procedure
was followed to move the probe to the guide star position).
Note that the outer parts of the scanned area are heavily vignetted at Prime, 
but not so at Cassegrain. An image will appear on the 32$\times$32, 
3-bit (gray-scale)
autoguider sub-display (figure \ref{ch12:fig6}), with the position of the 
brightest pixel marked with a cross in the diagram below.

If the image occupies a single pixel, the intended guide star is probably very
weak or strongly vignetted. In such a case, first press \button{WAIT} to stop
the integration, and subsequently repeat the \button{FIELD} integration.
Random events look very similar to faint stars. If, however, an image is 
found consistently in the same place, then it is probably real. 

The ``magnitude'' of the star indicated on the autoguider display is alleged 
to be roughly $m_v$. A magnitude $m_v<$14 is acceptable for guiding.
 
If you are happy with the guide star selection, press \button{ACQUIRE}\,; if 
not, press \button{WAIT} and pause for thought (see section 
\ref{ch12:problems}, furtheron). 

Pressing \button{ACQUIRE}
passes the coordinates of the guide star measured by the probe to the TCS
computer and requests that now either the probe or the telescope be moved,
depending on the acquisition mode selected, such as 
to position the star at the centre of the autoguider field. This is 
done either by sending a message over the interprocessor link to the 
Instrumentation Computer (for probe movements) or directly by moving 
the telescope. When the positioning of the probe/telescope is completed, 
the autoguider will do
another field integration. It then switches from raster to 
cross scan and starts to measure the position of the star with greater 
accuracy. This is the ``pulling-in'' phase, as is indicated on the 
monitor. Messages are sent to the telescope computer, which attempts 
to move the probe/telescope to minimise the error. When the error 
drops below a preset threshold, scanning ceases and the $x$ and $y$ 
profiles of the star are displayed with a message `OK to Guide?'. 
 
At this point, press \button{GUIDE} unless the profile is double-peaked, 
indicating a narrow double star or out of focus image, or if the object is
very weak. Wait until the autoguider display 
is completely redrawn and then press the \doublebutton{LOOP}{ENABLE} button 
on the A\&G panel which should turn green. Note that the loop 
is locked by the action of switching from LOOP DISABLE to LOOP ENABLE.
Wait until the cross (the measured star position) is centred in the 
2 arcsecond diameter circle. You are now ready to observe. 

The coordinates of the guide star will be recorded in the automatic observing
log and displayed on the MIMIC display. If the same star is observed later,
the probe can be sent to the guide star following section \ref{ch12:known}.
 
The autoguider signal should be monitored during observing in case the 
signal becomes weak (indicated by the message: CLOUDY). On the right-hand 
side of the autoguider display (figure \ref{ch12:fig6}) are histograms of the 
last 128 guiding errors in $x$ and $y$ 
On the left-hand side are the current state (GUIDING), 
warnings ({\it e.g.} CLOUDY), an approximate visual magnitude, a measure of 
the relative transparency (RT, relative to the measurement of the first scan) 
and a measure of seeing estimated from the profiles ({\bf beware:} not equal to
or linearly related to the true seeing! It is a function of both seeing and
brightness of the guide star).

The cross should always remain within the circle. If it consistently strays, 
you have probably lost the guide star. Check whether there is any signal. 
 
On source change press \button{WAIT}\,;
the autoguider is automatically switched to the LOOP DISABLE status.
If you need light in the dome (also with
tungstens, unless these are very dim), turn off the HT. 
 
\subsubsection{Problems with guide-star acquisition} \label{ch12:problems}

\begin{itemize}
\item There is no star in the autoguider field. Is the beam obscured? 
Can you see anything through the telescope? Are mirror covers, dome and shutters
open. Did you provide the correct sky- or probe-coordinates of the guide star,
and are they within the limits (otherwise the probe 
will not have been moved)?   Is the probe in a sensible place? Has it 
moved? (Use PFSTATUS to check this at Prime; Mimic display at Cassegrain). 
Is the star visible on the finder in direct mode? If not, 
then it may be too faint for the autoguider. 
 
 At Prime: Was the star close to the boundary of the box on the 
 finder? If so, it might actually be over the limits. Have you 
 gone back to BEAMSWITCH \button{NOM}\,? It sometimes happens that, 
 because of hysteresis in the finder, the guide star is out of 
 position in Declination. Try moving the probe by $\pm$50\secs\, in an 
 appropriate direction. 
 
 At Cassegrain: Is the probe within the unvignetted field of 
 the telescope? 
 
\item The autoguider selects the wrong star ({\it e.g.} a close double) when 
there is an alternative and acceptable guide star in the field. Provide a 
better position.
 
\item Double-peaked or other non-Gaussian profiles. Is the star 
double? (look on the finder!). If so, you probably cannot guide on 
it. If both profiles are double-peaked or if a field integration 
results in images with holes in their centres, then the autoguider and/or
the telescope itself is probably out of focus:

Ensure that the telescope is focused.

At Prime, refocus the guider using the ADAM commands GDFOCPOS and SETGDFOCUS.
Refer to section \ref{ch11:afocus}.

At Cassegrain, call for help. 

Asymmetries in the profiles often result when the autoguider does a 
scan before an acquisition movement has quite finished. This is nothing to 
worry about; guiding should not be affected.
 
\item The autoguider seizes up whilst guiding. 
Press \button{SHUTDOWN} to re-initialise the autoguider, and repeat the
acquisition sequence, but make sure that the telescope, not the probe,
is moved during acquisition by first typing at the User Interface console:
 
\UI{AC/TEL}

Assuming that the telescope has not drifted by more than 
a few arcseconds, the object should be returned to exactly the same 
position on the detector and observing can continue. 
 
\item The autoguider refuses to acquire by moving the probes. Have 
you asked the probe to go somewhere illegal (is it close to a limit)? 
Is the interprocessor link active on both computers? If not, try 
reloading it (see chapter \ref{ch13:guider}). 
 
\item The autoguider or A\&G panels do not respond. Refer to chapter
\ref{ch13:guider}.

\item At Prime: the Grinnell is re-initialised during normal operation,
 {\it e.g.\/} by pressing the \button{RESET} button by accident. In this
 case, the Prime-Focus overlay may not appear because the Grinnell thinks
 that the Cassegrain system is required. To fix this, type:

 \ADAM{RESETTV}

\item At prime: if acquisition of a guide star fails while everything seems to 
 work properly, check whether the TV finder scale has been altered ({\it e.g.\/}
 by replacement of the TV camera). Nominal values
 are 3.38 arcsec/pixel in $x$ and 2.75 arcsec/pixel in $y$. In order to
 alter the TV scale values type at the ADAM terminal:

 \ADAM{TV\?SETSCALE} 

 and follow the prompts. Alternatively, it may be necessary to measure the
 scale anew: refer to section \ref{ch11:tvscale}.
\end{itemize}
