\parindent 15pt
\parskip 0pt
\section{THE 2.5-M ISAAC NEWTON TELESCOPE (INT)}
%\section{The 2.5-m Isaac Newton Telescope (INT)}
\label{INT-intro}

\subsection{Introduction}

The Isaac Newton Telescope (INT) is a conventional Cassegrain-type telescope,
mounted on a polar-disk mounting. The telescope building is located
on the Canary Island of La Palma at an altitude of 2336 meters above sea level
(ground floor)
near the highest peak of the Caldera de Taburiente, an extinct volcano. 
Its geographical
position is at longitude 17\DEG\ 52\min 39.5\sec (W) and 
latitude         28\DEG\ 45\min 43.2\sec   (N).
The telescope stands on its own pillar, separate from the rest of the 
building, with its base about level with the second floor
(see Figure~\ref{ch2:fig1}); the axes intersect at 14 meters above ground level.
The observing floor and control room are found on third floor. 
A general desciption
of the INT and the observing site can be found in {\it Vistas in Astronomy},
{\bf 28}, 1985.

The INT has a  2.54-m diameter primary mirror with a focal ratio of f/2.94. 
There are three focal stations, of which the f/3.29 Prime focus (with focal
corrector) and the f/15 Cassegrain focus have been fully commissioned. The
third, f/50 Coude focus was never implemented.
 Both Prime and Cassegrain foci are equipped with 
instrument rotators and autoguiders.    In addition, the Cassegrain 
acquisition and guidance (A\&G) box contains an integrating TV camera for 
field viewing.   A second camera is attached to the wide-field finder 
telescope.   This is used for locating guide stars for the autoguider and, 
to a limited extent, for field verification.

The telescope, the dome and shutters are normally operated by means of the 
dedicated telescope computer, which resides in a 
temperature-controlled clean room on the INT
building's second floor, to the west of the void, the so-called CLIP centre. 
There one also finds power amplifiers for the telescope drive and much of
the telescope, TV and Autoguider electronics. The Control Desk and System
Console are located in the control room on the third floor, 
together with the Engineering Rack,
which can be used, if necessary, for manual operation of the telescope system.
(First-time readers may read section \ref{ch2:croom} at the end of this
chapter before continuing.)

A separate Instrument Computer is located in a small room
on the third floor, adjacent to the control room.


\subsection{Mechanical design and Performance}

The telescope has a polar disc/fork-type equatorial 
mounting, as illustrated in Figure~\ref{ch2:fig2},
which is supported by five axial and three radial hydrostatic oil-bearing 
pads.   The tube, a conventional open Serrurier truss   
structure, supports the Prime-focus assembly or the Cassegrain   
secondary mirror.
\begin{figure}[htp]
\vskip 20cm
\caption[The INT building viewed from the east.]
{The INT building viewed from the east; north-south cut through
the Dome. The ground floor is 2336 metre above sea level.}
\label{ch2:fig1}
\end{figure}

\begin{figure}[htp]
\vskip 20cm
\caption{Schematic views of the INT from the east and north.}
\label{ch2:fig2}
\end{figure}
The telescope is operated under computer control and can
be pointed to between $-6$ and $+6$ hours in hour angle, and from 
$-30$\DEG $\,$ 9\min 30\sec 
to $+90$\degs $\,$ in declination, subject to a maximum
zenith distance of 70\degs. Operation below the pole is possible in
principle but has never been implemented in practice.
Note that the lower windshield causes vignetting for zenith
distances $>$ 55\DEG~ and has to be raised for such observations. Figure
\ref{ch2:fig3} shows the area of sky accessible to the INT.
\begin{figure}[htp]
\vskip 20cm
\caption[The area of the sky accessible to the INT]
{The area of sky accessible to the INT.   Lines of equal 
zenith distance are drawn on a plot of declination against hour angle.}
\label{ch2:fig3}
\end{figure}

\subsubsection {Pointing and Tracking}
The telescope's position is determined by (Ferranti)
 Moire-fringe grating encoders 
mounted on each axis.  There are three reading  
heads per axis, moving over a ruled stainless-steel tape.   Each head   
produces a sinusoidal signal which is converted to a pulse train by a   
receiver in the CLIP Centre.   These pulses are counted up (or down) by 
the so-called Grubb Parsons modules, whose values are transferred to a  
CAMAC register on receipt of a clock pulse.   The registers are in turn 
read (at 0.1 s intervals) by the drive program.
One encoder bit corresponds to  
0.3 arcsec in RA and 1.0 arcsec in Dec, although systematic errors are  
currently somewhat larger.   These errors have been partially calibrated,       
but still limit the absolute pointing of the telescope. The pointing accuracy
of the telescope under computer control, using a regularly updated pointing
model, is about 5 arcsec rms. Note that the offset accuracy is unreliable
in declination at a level of 1-2 arcsec.

Because the Ferranti encoders are of the incremental
type, their zero points have to be determined. This is done by means
of two zeroset modules, one for each telescope axis.
The modules contain proximity sensors which detect
targets positioned at HA = 0 and Dec = Latitude ({\it i.e.} corresponding
to the telescope pointing towards zenith).
Once a target is detected, when moving the telescope for this purpose, the
``ZEROSET'' procedure (section~\ref{ch11:pointing}),
a pulse is produced which persuades the hardware to reset the counters to zero.

A pair of powerful motors driving though spur gearing are used for the quick
motion, {\it slew}, of the telescope; they also provide the anti-backlash 
preload. The maximum slewing speeds for hour angle and declination 
are 0.5\DEG~s$^{-1}$ and 1.3\DEG~s$^{-1}$, respectively.

The slow-motion drives on both telescope axes are provided by a pair of
DC servo motors with integral tachogenerators.
The RA axis is driven by a worm/wormwheel assembly and the
Dec axis by a recirculating ball screw and nut. Because of its appearance
the latter mechanism is known as the ``Banjo''.
When the telescope is being driven by the slow-motion motors, {\it e.g.} when
tracking, optical   
incremental (Baldwin) encoders, with resolutions of 0.01875 and 
0.01 arcsec in HA and Dec, respectively, are used.   These encoders are mounted
on the slow-motion drive shafts, which are disconnected from    
the telescope during a slew in quick motion.   Consequently, the
Baldwin encoders are not used unless the slow-motion clamps have been
engaged. The guided tracking accuracy (using the autoguider) is better than
 0.3 arcsec. Unguided tracking is accurate to 1 arcsec over a time period of
 3 minutes, and accurate to 2 arcsec over 10 minutes.

\begin{table}[h]
\centering
\caption{Summary of Mechanical Performance}
\label{ch2:pmech}
\vspace{0.2cm}
\begin{tabular} {|l|c|l|} \hline
 & & \\
Telescope Limits & Hour Angle      & from $-6^{h}$ to $+6^{h}$ \\
                 & Declination     & $> -30$\DEG$\,$09\min 30\sec \\
                 & Zenith Distance & $< 70$\DEG$\,$; beware vignetting \\
                 &                 & by windscreen for ZD $> 55$\DEG \\
 & & \\ \hline
 & & \\
Pointing Accuracy & Global Fit & {\it rms} residual = 5 arcsec. \\
                  & Offset in Dec & unreliable at 1-2 arcsec level \\
 & & \\ \hline
 & & \\
Tracking Accuracy & {\it un}guided & $< 1$ arcsec in 3 minutes \\
 &                                 & $< 2$ arcsec in 10 minutes \\
                  & guided         & $< 0.3$ arcsec \\
 & & \\ \hline
 & & \\
Quick Motion & Right Ascension & up to 0.5\DEG $s^{-1}$ \\
             & Declination     & up to 1.3\DEG $s^{-1}$ \\
 & & \\ \hline
\end{tabular}
\end{table}


\subsection{Optics and optical performance}

The optical system of the INT is a conventional Cassegrain configuration with a
paraboloid primary mirror and a hyperboloid secondary.   The
primary has a diameter of 2.54 m and a focal length of 7.475 m, giving  
a focal ratio of f/2.94 at the {\it uncorrected} primary focus. 
The mirror is made of Zerodur and has a negligible coefficient of expansion:
$<~10^{-7}/^\circ K.$
On axis, 80 percent of the light from a stellar image lies within a circle of  
0.3 arcsec diameter, as shown in Figure~\ref{ch2:fig6} at the end of this
chapter. Table~\ref{ch2:tab1} and table~\ref{ch2:tab2} summarize
the optical characteristics of the INT.
\newpage

\begin{table} [h]
\centering
\caption{Summary of mirror characteristics for the INT}
\label{ch2:tab1}
\vspace{0.2cm}
\begin{tabular}{|l|l|c|c|c|c|c|} \hline
  &  &  & Working & Focal & Separation &  \\
Element  & Shape & Asphericity & Diameter & length  & from primary & Material \\
  & & & (mm)       & (mm)    & (mm) & \\ \hline
  \\
{\bf Telescope} \\ \hline
%  & \\ \hline
Primary   & Paraboloid  & -1  &  2540      & 7475    &      & Zerodur  \\
Secondary  & Hyperboloid  & -2.214  & 531 & -1813 & 6017& Fused Silica \\ \hline
  \\
{\bf Finder}    \\ \hline
%  & \\   \hline
Primary  & Paraboloid   & -1& 405       & 2104 &   & Pyrex   \\
Flat     & Flat &   &   135      & &  1600& Low expansion \\
   &      &   &    & &      &   glass \\  \hline
\end{tabular}
\end{table}

\begin{table}[h]
\centering
\caption{Optical characteristics of INT foci}
\label{ch2:tab2}
\vspace{0.2cm}
\begin{tabular}{|l|c|c|c|c|c|c|} \hline
              & Focal  & Focal & Field    &       & Central & \\ 
Focal Station & Length & Ratio & Diameter & Scale & Obstruction & Notes \\
   & (mm) & & (arcmin) & (arcsec mm$^{-1}$) & (mm) & \\ \hline
   & & & & & & \\ 
Prime (uncorrected) & 7475 & f/2.94 & & 27.6 & 914 & \\ 
Prime (with corrector) & 8357 & f/3.29 & 40 (52) & 24.7 & 914 & 1 \\ 
Cassegrain & 38130 & f/15 & 20 (22) & 5.41 & 914 & 1,2 \\ 
TV finder & 2104 & f/16 & 15 & 98 & 139 & \\ 
   & & & & & & \\  \hline
\end{tabular}
\vspace{0.2cm}

\raggedright
{\footnotesize \hspace{1.0cm} {\it Notes to Table~\ref{ch2:tab2}\/}: \\
\hspace{1.0cm} 1) Numbers between parentheses indicate the field diameter
 for 50\% vignetting. \\
\hspace{1.0cm} 2) A unit movement of the secondary mirror corresponds to 
a focal shift of 27 mm.}
\end{table}

A focal corrector of the type described  
by Wynne (1974), but with a flat rear surface and increased back focal  
distance, is used at the Prime focus in order to enlarge the useful field,
where the images should be $< \,$0.5 arcsec over the unvignetted field. The 
corrected focal ratio is f/3.29.
The three elements of the Prime-focus corrector are made
of UBK7 glass, coated to minimize reflections and thus improve efficiency and
reduce ghosts.  The first two elements are not interchangeable and have a
broad-band, single layer MgF$_2$ coating.  The rear element of the corrector,
however, is available in several coating options, summarized in 
Table~\ref{ch2:tab3} below.

\begin{table}[h]
\centering
\caption{Coating options for rear element of INT prime focus corrector}
\label{ch2:tab3}
\vspace{0.2cm}
\begin{tabular}{|l|l|c|} \hline 
          &                    & \\
Waveband  &       Coating      &  Useful Range (\AA)   \\ 
          &                    & \\ \hline
 & & \\
Broad Band/Blue & Single layer MgF$_2$ matching first two elements 
                                                          & 3500-7400  \\ 
UV      & Single layer MgF$_2$                            & $<$5000    \\ 
V and R & Three-layer MgF$_2$ $+$ ZrO$_2$ $+$ Al$_2$O$_3$ & 4300-8000  \\ 
R and I & Three-layer MgF$_2$ $+$ ZrO$_2$ $+$ Al$_2$O$_3$ & 6000-11000 \\ 
 & & \\ \hline
\end{tabular}
\end{table}

\noindent
Single-layer coatings produce reflectivities smaller than 2
percent per refracting surface over the appropriate wavelength range.  
Three-layer coatings give reflectivities of less than 0.5 percent per surface,
but the performance deteriorates very rapidly (to worse than for uncoated
glass !) outside the useful wavelength range. 
Most observations are done using the broad band/blue corrector. Note that 
changing from one rear element to another is a major operation and a day-time
job. Frequent changes are strongly discouraged.

The Cassegrain secondary mirror gives a focal ratio of f/15. Spot diagrams
for this focus are shown in Figure~\ref{ch2:fig7} at the end of this chapter.

\subsection{Telescope control}

The INT is controlled by a Perkin-Elmer 3220 computer interfaced through CAMAC.
The software drives the main HA and Dec axes, the Cassegrain Instrument 
Rotator, the Focus and the Dome and Shutters, and the computer provides an 
interface to the Autoguider.

The Telescope Operator interacts with the telescope via commands
typed at the User Interface terminal, to be found in
the control room, or by pushing buttons on the Control Desk or the handset,
{\it e.g.} to make the telescope slew or adjust the tracking rate.
These buttons are of a special design which ensures that, even when held down,
the actions are only performed once, except for
the guider buttons, which act continuously. Most of the buttons 
acknowledge that they have been pressed by lighting up.

Data can be entered into the control system via the User Interface. In
particular, it is possible to set up a catalogue with position of up to   
500 objects, which is held in memory. The coordinates may be entered with
simple commands or loaded from disc files: catalogues of photometric,
spectrophotometric, and astrometric standards are available.
Further, one may enter telescope parameters such as offsets or tracking rates.

\subsubsection {Control Tasks}
The control software is written in FORTRAN (with a minimum of   
assembler) and is, in fact, applicable to both INT and JKT. The differences
between these telescopes are handled by Control Files with system parameters,
which are read in when the software is being initialized at the start of
an observing session.

The system control is organized as Tasks, which may run simultaneously or
separately. The principal task, ``TNHZ'', is, of course, the one that drives 
the telescope,
and, therefore, is active throughout. Other tasks that control the Dome,
the Cassegrain Instrument Rotator, or Focus are run only intermittently,
obviously because they are not needed all the time, and even then these 
functions
do not need to be controlled to the same accuracy as the telescope itself.
Yet another task is run every two seconds or so and drives the Information
Display, on which such parameters are shown as the source name and its sky
position, aperture offsets, differential tracking rates, the current    
pointing and focus of the telescope, and the location of the dome
aperture.

The main control task for the telescope drives (TNHZ) runs at $10\,$Hz. 
A cycle consists
basically of three steps. First, the actual encoder positions are read and 
the time is obtained from the Time Service rack in the CLIP Centre. 
Subsequently, the desired encoder values (for that moment) are calculated:
The program calculates the position of a virtual telescope, without pointing
defects, taking into account non-sidereal tracking rates, offsets, handset
demands, autoguider error signals, as well as precession, nutation,
aberration and atmospheric refraction. All     
calculations are performed in the proper coordinate system, as specified by
the observer (mean or geocentric apparent). Finally, a parameterized 
model of the telescope's pointing (Section~\ref{ch2:point}) is applied in order
to produce a desired position reading for the encoders, which is compared with
the true reading. If necessary, velocity corrections are
applied to the drive motors in order to zero the position errors.

 For large errors the velocity correction will be proportional 
to the square root of the error, within the mechanical limits; if the error 
is small, the correction will be approximately
proportional to its magnitude. When a slew to a new object is requested, the
slow-motion clamps are initially released and the telescope is driven   
in quick motion until it is within a few hundred arcsec of the object.  
The clamps are then re-engaged and final acquisition is made in slow    
motion, using the high-resolution Baldwin encoders for smooth       
tracking.       

The telescope RA, Dec and Zenith Distance, as well as the universal and 
sidereal times, are output to the dedicated Beckmann Display on the 
Control Desk at 0.1 s intervals. 

\subsubsection {Pointing Model} \label{ch2:point}
The pointing model for the INT has 17 terms, comprising the standard    
errors of an equatorially-mounted telescope (offsets in HA and Dec,     
collimation error in HA, misalignment of the polar axis and 
non-perpendicularity of axes), together with an empirical (Fourier series   
and polynomial) model of flexure in the telescope structure and large-scale 
errors in the Ferranti encoder readings. The (software) zero point of the
pointing model is determined with the ``SNAFU'' procedure 
(section~\ref{ch11:pointing}). If pointing
tests are made sufficiently frequently in order to update the pointing model,
the {\it rms} error in the absolute pointing of the INT should always be 
better than 5 arcsec: values between 2.8 and 4.5      
arcsec have been measured.   Short-term tracking errors are $<$0.25       
arcsec (there is a small residual oscillation at about 1$\,$Hz in HA).   
Longer-term drifts should be removed by engaging the autoguiders.

\subsubsection {Engineering Mode}
                                                                                
The Engineering mode serves to move the telescope              
hardware without using the computer. This is done by technical staff            
to move the telescope for such jobs as top end changes or by the                
Telescope Operator for the initial setting up at the start of the night.
Engineering Mode is the default mode on powering up the system; 
telescope control is from the Engineering Rack in the control room, placed
conveniently close to a window from which the telescope can be seen. A blind
is drawn over this window when the telescope is under computer control.

The telescope is protected from being driven outside a safe operating
regime by the use of   
limit switches. When under computer control, the telescope is normally
protected from running into these limit switches by the software. If, 
however, due to exceptional circumstances or faults ({\it e.g.} CAMAC or
encoder failure), the telescope runs into a
limit switch when under computer control, that control is lost completely.
Then, the telescope can only be driven out of the switches by changing to
the Engineering Mode and in all likelihood the zeropoint of the encoders
will be lost. Therefore, the encoders will have to be re-initialized
(section~\ref{ch11:zeroset}).
                                                                                
                                                                                
\subsubsection {Alarms}
                                                                                
There is a set of warning lights on the Control Desk which               
signal major changes of state and/or situations which could endanger
the telescope hardware. Alarms can be of           
various types and upon no account should any be ignored. You will be            
unable to ignore an alarm anyway because, when an alarm occurs and a
warning light comes on, the system also trips a relay which sounds a
high-pitched, loud and very irritating whistle.
A typical contingency (which we hope will never occur) would be
a loss of oil pressure in the hydrostatic telescope bearing.

A failure of the air conditioning system will activate an alarm in          
the Air-Conditioning Plant room located outside the Control Room on             
the roof.

Normally, the Telescope Operator or Duty Technician will cope with these
situations. Nevertheless, just in case, there is an emergency STOP button
on the top-lefthand side of the Control Desk. Actually, certain fault 
conditions are best handled by using the emergency stop, such as
encoder or CAMAC failures, which can cause a run away motion of the telescope.

\subsection {Acquisition and Guiding}
\subsubsection {TV Cameras}
                                                                                
Low-light-level, integrating television cameras are mounted on the              
finder and the Cassegrain focus of the main telescope.   Picture                
integration is possible either on the camera target or in digital               
memory.   A Grinnell image display is used.   The limiting magnitudes
(in good seeing and transparancy)
against a dark sky are fainter than V = 21.5 mag and V = 18.5 mag for           
the Cassegrain and finder cameras, respectively and approximate                 
aperture photometry should be possible (in theory).   Given that the            
INT points well, the most important use of the finder is                
the selection of guide stars for the Prime and Cassegrain autoguiders.          
The Cassegrain TV camera is                                                     
used for acquisition and for guiding on light reflected from the                
spectrograph slit jaws (section~\ref{ch12:AGCass}). It is possible to
archive a camera image on disc.   
                                                                                
\subsubsection {Autoguiders}

Both the Prime and Cassegrain foci are equipped with autoguiders. The two
systems are identical and have FW-130 image dissectors with S-20 photocathodes
as detector.
The autoguiders are controlled by a dedicated Micro Processor, which
continuously analyses the image of a guide star, and provides small
corrections to the telescope tracking (just two numbers) via the interface 
polled by the PE$\,$3220. That is, of course, their main function, but they
also monitor transparency and seeing. The guide-star image and seeing data
can be displayed on a VDU in the Control Desk for inspection.

In order to acquire a guide star, a prism mounted on the end of a      
probe is moved to its position, which either can be measured in advance 
and entered via the User Interface, or it can be determined
by using a cursor on the finder TV picture (see section~\ref{ch12:gauto}).


\begin{figure} [hb]
\vskip 20cm
\caption[Spot diagrams for the INT prime focus]
{Spot diagrams for the prime focus of the INT with the three-element 
corrector at different wavelengths and field radii.
The scale is shown by a circle of radius 1.0 arcsec.}
\label{ch2:fig6}
\end{figure}

\begin{figure} [hb]
\vskip 20cm
\caption[Spot diagrams for the INT Cassegrain focus]
{Spot diagrams for images at the f/15 Cassegrain focus of the
INT, showing the dependence on field radius and the effects of defocussing.}
\label{ch2:fig7}
\end{figure}

\clearpage
\newpage
\parindent 0pt
\parskip 0.3cm
\subsection {A Guided Tour of Telescope Control Room and Environs}
\label{ch2:croom}

This section is intended as an introduction to the ``geography'' of the
Telescope Control Room, in order to familiarize the reader with the location
of various parts of the telescope control hardware and electronics.

For a start, the Telescope Control Room is located on the third floor of 
the INT buiding, on the North side of the observing floor and telescope.
See Figure~\ref{ch2:fig4} for a ground plan.
Access to the control room is via one of two staircases located on the East 
and West side. The staircase on the West side runs near the library on the
first floor. When entering the control room through the West entrance, we
encounter the following pieces of hardware (all on your right):

\begin{itemize}
\item{\bf Instrument Desk and Data Monitor} \\
These units are part of the instrument control hardware ({\it e.g.} for 
the IPCS) and the data acquisition system, and as such, do not function 
in the control of the INT itself. The ``Data Monitor'' consists of an
$\,$ICS terminal of the PE$\,$3210 Instrument Computer and a 
Lexidata display. The astronomer normally sits at this terminal with
students sitting at the observer's 
feet and the support astronomer standing behind the chair.
An important item at the top of the IPCS control desk is the 
instrument MIMIC display. A
MOTOROLA EXORSET stands on a trolley to the right of the desk. It
is not part of the regular instrument-control system, but is a valuable
aid in diagnosing faults on the A\&G box and Spectrographs.

\item{\bf Control Desk} \\
This is the large panel with push buttons, TV monitors, keyboards, displays,
{\it etc}, and has everything to do with the INT. It is the fiefdom of the
Telescope Operator. A diagram of its layout is shown in Figure~\ref{ch2:fig5}.
There are, starting from the left, four sections, which each control a 
separate subsystem:

\begin{enumerate}

\item{The System Status Panel}, which shows you everything about the system's
health and holds the alarm lights. In the top left-hand corner is the
emergency STOP. Here one also finds switches for telescope power, for the 
selection of computer or engineering mode, and the Dome lights. There is
a monitor to which the Operator can route any of a selection of displays:
the Information, Autoguider, or MIMIC display, or the digitized image from
the Grinnell integrating TV system.

\item{The System Control Panel (SCP)}: A group of push buttons and a handset
which enable function such as slew, offset, or tracking adjustments. The monitor
here always carries the Information Display.

\item{Autoguider Panel}: The autoguider is controlled by its own micro
processor with the buttons in the lower part. Above those is the data-logging
monitor, which is driven by the IBM PC standing on top of the desk, and
interfaces with both the MIMIC display and the Information Display. Further,
at the top of the desk are the Beckmann displays of RA, Dec, Zenith Distance,
LST and UTC.

\item{Acquisition and Guidance Panel (AGP)}: A group of push buttons which
control the movement of the TV camera and the Guide Probes in the A \& G box.
On top are four miniature monitors which carry the analogue TV pictures. Below
them is the Grinnel digitising TV image display.

\end{enumerate}

\item{\bf User Interface} \\
The terminal used by the Telescope Operator to interact with telescope-control 
software. It consists of a VT320 terminal and currently lives on a
trolley standing to the left of the Control Desk. Pointing instructions
and Data input, {\it e.g.} catalogues, to the telescope-control program 
are via this terminal.

\item{\bf Engineering Rack} \\
A tall blue rack standing on the east side near
the observing floor window. It serves for manual control of telescope
functions during start up or a top-end or instrument change. The monitor
on top is slaved to the one in the left-hand bay of the Control Desk. Standing
right beside the Rack is the:

\item {\bf Weather Monitor} \\
Which tells you all about how things are outside. Most annoying here are the
humidity meter and the anemometer, which sometimes persuade the Telescope 
Operator to close the dome, {\it i.e.\/} when the relative humidity is more
than 90~percent, and/or the wind speed rises above 80\,km/h.

\item{\bf System Console} \\
The VT\,320 terminal used by the Telescope Operator to communicate with 
the PE$\,$3220 telescope computer
at the system level, {\it e.g.} to start up the telescope
control software. The terminal currently stands on a desk on the far side
of the Control Room window, as seen from the Control Desk.

\item{\bf VAX terminal and Instrument Computer terminal} \\
Both stand on a desk at the far (east) end of the control room. The terminal to
the PE\,3210 Instrument Computer should {\it never\/} be used during observing,
as using it can slow down, or even halt, the data taking. The VAX terminal has 
nothing to do with the INT. However, you can access the VAX$\,$8300 with it, so
that you can read your email during long exposures.
\end{itemize}

Further, notice against the wall opposite the Control Desk the bookcase 
with various manuals and other documentation, and the notice board (read it !).
Please return the manuals tidily after use.
\vspace{0.4cm}

Apart from the Control Room one might familiarize oneself with:

\begin{itemize}
\item{\bf Observing Floor}\\
During observing, access to the observing floor is normally only needed to
change optical components of the IDS. In order to keep those components at
ambient temperature, they are stored in a steel cupboard standing against
the SE wall. Unless observers are thoroughly familiar with the IDS, they 
should leave such changes to their support astronomer.

\item{\bf CLIP Centre} \\
This is a temperature-controlled clean room on the floor below the console 
room, west of the void, near the west staircase, which houses the
Perkin Elmer 3220
Telescope Computer (See section~\ref{ch7:PE3220}) and other telescope hardware 
and electronics, such as the power amplifiers for the telescope drive and
the autoguider electronics. There is a second Local (System) Terminal for
local control of Telescope Computer. The CLIP Centre also houses the Time
Service.

\item{\bf The Instrument Computer Room}
This is the room directly on your left when you enter the Control Room. It
houses the Perkin Elmer 3210 Instrument Computer, which handles the data 
acquisition (section~\ref{ch9:PE3210}). The computer comes with two tape decks,
an EGLE disk drive, a line printer and a system console.
There is also a rack with a fresh supply of magnetic tapes,
for you to take home with excellent data.
\end{itemize}

\begin{figure}[hb]
\vskip 20.0cm
\caption{Ground plan of Observing Floor and INT Control Room}
\label{ch2:fig4}
\end{figure}

\begin{figure}[hb]
\vskip 20.0cm
\caption{The Telescope Control Desk}
\label{ch2:fig5}
\end{figure}

\clearpage
