\parindent 0pt
\parskip 0.3cm
%\section {Observing with the INT: recipes} \label{ch4:observing}
\section {OBSERVING WITH THE INT: RECIPES} \label{ch4:observing}
This section describes the basic operations of observing with the Isaac
Newton Telescope in the the form of itemised ``recipes''. The recipes are
necessarily presented as a checklist of things to do, and are primarily intended
as reference for experienced users of the Isaac Newton Telescope. Many of the 
individual actions are described in full detail in the reference sections of 
this manual; some items themselves stand for another checklist in this section.
\subsection {Starting up the INT}
See for full details Chapter \ref{ch5:INT-start}.
\begin{enumerate}
\item
Confirm handover with Duty Technician.
\item
Fill the cryostats.
\item
If the dome temperature is higher than that    
of the outside air, then: 
\begin{enumerate}
\item
Move the dome to azimuth 90 degrees, and open up 1 metre.
\item
Check that the EXTRACTOR fan by the telescope base is on, and that the 
INPUT fan on the ground floor is on.    
\end{enumerate}
\item
Read the Telescope Logbook and check INT\?BEWARE documentation for 
reported faults or hazards.
\item
Check T/S DOME STOP is active.
\item
Take ICONA Meteorological readings (optional).
\item
Input files of standard stars and target objects into the
 PE\,3220 Telescope Computer (see section \ref{ch8:input}.
\item
EXTRACTOR fan ON, if not already done.
\item
INPUT fan ON, if not already done.
\item
Shut all doors and window blinds.
\item
White lights out.
\item 
Remove dome fan covers, if not already done. Dome fans on.
\item
Dome access doors closed.
\item
Dome lights out.   
\item 
Open Dome fully. 
\item
Finder cover open.
\item
Do a ZEROSET (see section \ref{ch11:zeroset})
\item
Primary mirror plug out (Cassegrain focus only).
\item
Secondary and primary mirrors open. 
\item
Telescope in Computer Mode.
\item
TV Camera(s) and Autoguider on.  
\item
Input SET, GUIDE and INCREMENT rates, if other than default values used.
\item
Focus the Telescope (see section \ref{ch4:focus} or \ref{ch11:focus}).
\item
Do a SNAFU (see section \ref{ch4:snafu} or \ref{ch11:1-snafu})
\item
Determine Finder Zero Point
\end{enumerate}

\subsection{Closing down the INT}
See for full details Chapter \ref{ch6:INT-close}.
\begin{enumerate}
\item TV Camera(s) and Autoguider OFF.
\item Press \button{ZP} to move telescope to zenith.
\item Close PRIMARY and SECONDARY mirror covers IN THAT ORDER.
\item Switch over to Engineering Mode.
\item Close dome shutters.
\item Switch off dome fans; replace covers.
\item EXTRACTOR fan off.
\item INPUT fan off.
\item Close the Finder Cover.
\item Move Telescope to Park Access position.
\item (Cassegrain only) Replace Primary Mirror Plug.
\item (Cassegrain only) Move Telescope to Zenith under computer control.
\item Press the \doublebutton{EMERGENCY}{STOP} button.
\item Switch Oil Pump OFF.
\item Top up the Cryostat(s) with Liquid Nitrogen.
\item Check/close dome access doors.
\item Close down Telescope Control Software.
\item Shutdown ADAM (unless a flatfield is being taken).
\item Complete Log Books.
\end{enumerate}
\newpage
\subsection{Startup and Shutdown of the PE\,3220 Telescope Computer}
\begin{itemize}
\item {\bf Normal Startup:} Type at the Telescope Control System Console:

\PE{D D}

If MSMD: is OFF, then type:

\PE{MA MSMD:,ON,,CD=ALL}

When MSMD: is ON, then type:

\PE{GO INT}

\item {\bf Shutdown:} Type at the Telescope Control System Console:

\PE{FINISH}

\PE{MA MSMD:,OFF}

\item {\bf Reboot after a power cut}
Press INIT on CPU consolette

Boot the computer by answering:

\hspace{2.0cm} ENTRY={\tt 3}

\hspace{2.0cm} FILE={\tt MSMD3220.LPO}

and wait for the message: ``END OF ENVIRONMENT CONFIGURATION''

To start the control system, type at the System Console:

\PE{GO INT}

If there has been a power failure initialize the CAMAC crates associated
with the PE\,3210 (see section \ref{ch7:CAMAC}).
\end{itemize}

\subsection{Startup and Shutdown of the PE\,3210 Instrument Computer}
\begin{itemize}
\item {\bf Normal Startup:} Type at the ADAM terminal:

\PE{SI ABC, n, DEF}
\item {\bf Warm Restart} (after a controlled shutdown):

Power up the EGLE: disc (On with 3 lights)

\PE{D D} at System Console

\PE{MA EGLE:,ON,,CD=ALL}

\PE{STARTUP}  to give END OF ENVIRONMENT CONFIGURATATION

\PE{SI ABC, n, DEF} at the ADAM terminal
\item {\bf Cold Restart (after power failure)}

Mains supply ON.

Power up the EGLE: disc (On with 3 lights).

Press the INIT switch on the PE3210 consolette.

Answer {\tt DSC2} and {\tt EGLE3210.LPO} to prompts at system terminal.

Await  END OF ENVIRONMENT CONFIGURATION

\PE{SI ABC, n, DEF} at ADAM terminal
\item {\bf Stopping} the PE 3210:

\PE{SIGNOFF} at all terminals

\PE{STOPALL} at System Terminal

\PE{OFF}

\PE{MA EGLE:,OFF}
\end{itemize}

\subsection{ZEROSET}
For full details see chapter \ref{ch11:zeroset}.
\begin{enumerate}
\item Position the telescope at zenith.
\item Drive the telescope a few degrees away from zenith by pressing the
{\bf positive} direction buttons \button{+} on the handset, in both Right
Ascention and Declination.
\item At the User Interface do:

\UI{ZEROSET}

The two red encoder/zeroset lights on the Control should light up. If not,
go back to step 2 and move the telescope a bit further from zenith (consult 
section \ref{ch11:zeroset}).
\item Set the speed controls on the Engineering Rack to the marked positions.
\item Drive the telescope back through the zenith by pressing the {\bf negative}
direction handset buttons \button{$-$} until the lights on the Control Desk go
out and a message appears on the User Interface to the effect that the ZEROSET
has completed.
\end{enumerate}

\subsection{Acquiring an object}
\subsubsection {Slewing to an object}
If not done before, enter the coordinates of the object, equinox and proper
motions via the User Interface (see section \ref{ch8:enter}):

\UI{SO(URCE) source-name}

\UI{R hh mm ss.ss}

\UI{D dd mm ss.s}

\UI{EQ Byyyy or Jyyyy}

\UI{P/R s.sss} (optional)

\UI{P/D s.sss} (optional)

When the coordinates of your object are stored in the catalogue do:

\UI{NEXT source-name}

\UI{GO} \hspace{2.5cm} , or press the \button{SRCE} button on the 
Control Desk.
 
If the object is within the 
software limits, then the green {\bf go} lamp will light up and the 
telescope will commence slewing. If the object is beyond the limits, then the 
{\bf go} lamp will only flash briefly and the red {\bf stop} lamp will be lit. 
Appropriate messages will also be output to the Information Display (see
section \ref{ch8:display}).


\subsubsection{Using the Handset and Guide Buttons}


The guide buttons on the handset and the `arrow' buttons at the top left-hand
corner of the SCP have identical functions. Those on the handset are 
ostensibly dual-pressure, but the first pressure has been disabled on 
grounds of unreliability. 

Selection between the SET, GUIDE and INCREMENT modes is made with the
buttons at the bottom left-hand side of the Control Desk.
The \button{SET} and \button{GUIDE} modes give continuous motion while
\button{INC} moves the telescope in steps.

The set and guide rates and incremental step size can be defined at the User 
Interface withe the HANDSET command (see chapter \ref{ch8:INT-UI}). Enter 
new values with:

\UI{HA/S set-rate} (\sec/sec)

\UI{HA/G guide-rate} (\sec/sec)

\UI{HA/I increment} (\sec)

However the new value will only become active {\bf after} the \button{SET}\,,
\button{GUIDE} or \button{INC} button is pressed.
 
\subsubsection{RA/Dec Offsets}
 
The telescope may be offset from its current position with an RA/Dec offset,
for example to move an object with known offset position onto the reference
point or spectrograph slit. The offset is made in the same coordinate system 
as specified for the input source and is currently 
implemented as an offset in arcseconds East and North in a tangent plane 
projection centred on the current tracking centre. The value of the 
offset is entered via the User Interface by typing:
 
\UI{OFFSET/RA value}

\UI{OFFSET/DEC value}

The values are specified in arcseconds.
They are introduced into the telescope tracking when the \button{OFFSET}
button on the Control Desk is pressed. The OFFSET lamp will then be lit 
and the value of the offset will be shown on the Information Display. 
Offsets are {\bf not} cumulative, {\it i.e.\/} pushing the offset button again 
will not cause a further offset. 

Offsets in Declination are notoriously inaccurate. Therefore, some adjustment
afterwards to the position of the translated object may be required; this
implies that that object should be visible on the TV probe. If you want to do 
an offset on an object too weak to be seen on the TV probe, then it is better 
to use the autoguider probe for a blind offset, as described in 
section \ref{ch4:boffset}.
\vfill
\newpage

\subsection{Single-Star SNAFU at Cassegrain} \label{ch4:snafu}

Perform a Single star SNAFU in 
order to determine a software zeropoint for the Pointing Model 
(see section \ref{ch11:pointing} for a full description). It is recommended to
use a star within 2 degrees from the zenith if afterwards the finder zeropoint
has to be determined (using the same star).

The SNAFU at the {\bf Cassegrain} Focus is done as follows:
\begin{enumerate}
\item
Move the telescope to the SNAFU star.
\item
Wait until the telescope is tracking accurately.
\item
Type at User Interface:-

\UI{SNAFU}

\item
Turn up the gain of the Cassegrain TV until the star is clearly  
visible and use the handset to place the star at the reference position on
the spectrograph slit.   
\item
Press the \button{ACK} button.
\end{enumerate}

\subsection{Single-Star SNAFU at Prime}
See section \ref{ch11:pfsnafu} for full details:
\begin{enumerate}
\item If not yet done startup ADAM.
\item Move the rotator to position angle 180\degs \  with the ADAM command
      {\tt ROTPOS 180}.
\item Slew the telescope to the SNAFU star, as for Cassegrain mode.
\item Take a test GLANCE exposure of the field (1-5 seconds; use filters if
      the star is very bright).
      The CCD image will appear on the LEXIDATA and the star should be
      visible near the centre of the chip.
\item Use the Grinnell MOVESTAR command, and locate the star with the cursor.
      The telescope will move and the star will move on the Finder TV image.
\item Mark the position of the star now on the Finder TV screen with a felt
      pen or similar marker.
\item Move the telescope again to the original position by pushing the
      \button{SCRE} button.
\item Type at the User Interface:

      \UI{SNAFU}

      and move the telescope with the handset such that the star 
      image on the finder coincides with the position marker put in step 6.
\item Press the \button{ACK}\,.
\end{enumerate}

\subsection{Focusing the telescope} \label{ch4:focus}
 
\subsubsection{Using the Handset} 

The simplest way of focusing the telescope is to use the focus
\button{+} and \button{$-$} 
buttons on the handset. These are actually dual-pressure buttons, 
designed to give slow and fast focus for first and second pressure, 
respectively. The fast focus drive is currently disabled in computer 
mode, so the handset gives slow drive for either pressure. 
 
\subsubsection{Via the User Interface}
 
Set the telescope focus to a known value as follows.
\begin{enumerate}
\item Type at the User Interface: 
 
\UI{FOCUS value}
 
where the focus {\tt value} is in {\tt mm} (as on the Information Display; 
note that the units on the engineering display are 0.01 {\tt mm}).

\item  Press the FOCUS \button{NOM} button on the Control Desk. 
The focus will then drive to the value defined before, and the red {\bf focus
stop} light will come on when it is in position. 
\end{enumerate}
 
\subsubsection {From the Engineering Rack}
 
This is recommended when the focus is grossly out of position, because it 
provides the only way of driving quickly. 
\begin{enumerate}
\item Either (a) depress the FOCUS \doublebutton{SELECT}{MANUAL} button 
(the button must be illuminated), \\
 or (b) select Engineering Mode by pressing the \doublebutton{ENG}{COMP} button.
\item
Select FOCUS \button{Fast} or \button{Slow} motion as appropriate and move 
the focus using the \button{+} or \button{$-$} button. All buttons are found
on the Engineering rack.
\end{enumerate} 

\subsubsection{Finding the best Focus at Cassegrain}
At Cassegrain, the telescope is focused by eye, using an {\bf unsaturated}
stellar image on the TV. Run through focus a few times, noting the 
value for the smallest image each time and averaging the results. Further:

\begin{itemize}
\item Check that the TV camera itself is focused on the slit (see section
      \ref{ch11:tvfocus}).

\item The telescope focus is altered by the insertion of an 
 ABOVE SLIT filter. For the neutral density filters currently in use
 with FOS or IDS, the focus is increased by 0.04 {\tt mm}
 when the filter is put in. Correct the focus to the
 by entering the new value (after changing to a filter) via the User Interface.
\end{itemize}
 
\subsubsection{Finding the best focus at Prime}
 
There is no TV camera at Prime, so interactive focussing is not possible. The 
telescope 
is focussed by making a multiple exposure on the CCD (for a full description
see section \ref{ch11:focus}), stepping both 
focus and telescope between exposures.

\begin{enumerate}
\item If not done already, startup ADAM
\item Move the rotator to a cardinal point, preferably 180\degs, with

\ADAM{ROTPOS 180}
\item Move to a not very bright star and take a GLANCE exposure of between 5
 and 10 seconds. After the image appears on the LEXIDATA display go into PHOTOM
and make sure that the star image is not saturated. If it is,
select another object and repeat.
\item Check whether the star is in the correct position on the chip: with the
rotator at 180\degs\, the orientation on the LEXIDATA
display is: North up, and East to the right. The telescope
will step with 20\sec between exposures with a double step between the
first and second exposure. Is there enough space on the chip for your intended
number of steps and stepping direction? If not, move the star to a position 
where it is and confirm with another GLANCE.
\item Start the multiple focus exposure with:

\ADAM{FOCUS}
 
and follow the prompts. Use 7 exposures for a first setup, 5 for subsequent
fine-tuning; provide a negative number of exposures if you want to
use a negative increment; use an exposure time between 5 and 10 seconds.
The focus is stepped by $\pm~$ 0.1 mm between exposures; the current focus is 
in the middle of the range. 
\item After the FOCUS procedure has finished, the multiple image will appear on
the LEXIDATA display. It will normally be obvious which 
images are closest to focus and these should be measured using the 
PROFILE option of PHOTOM and the seeing determinated.
If you find yourself far from focus, then make an educated guess about
its approximate value and move the focus to that value via the User Interface,
followed by a repeat of the FOCUS procedure.
\item Log the current value of the Seeing.
\end{enumerate}

Beware of starting an observation until the focus has stopped driving 
and the red {\bf focus stop} lamp on the Control Desk is lit. 
 
\subsubsection{Using automatic Focus Adjustment at Prime}
 
The Prime focus CCD software allows automatic compensation for the 
changes in focus due to different filter thicknesses. This option must be
explicitly enabled with the SETFOCUS command.

\begin{enumerate}
\item Use the ADAM command WHEEL to set up a table with filter thicknesses (see
 the Prime Focus manual section 3.1).
\item Make sure that the same filter is used for which the focus was 
 determined. Alternatively, determine the focus now for the current filter.
\item To enable the focus adjustment, type, {\bf before changing filter}:
 
\ADAM{SETFOCUS value}
 
Here, {\tt value} is your best estimate of the focus, in {\tt mm}. The focus 
will be driven automatically to the correct position on change of filter.
\item To disable the focus adjustment, type:

\ADAM{SETFOCUS DISABLE}
\end{enumerate}
 
Beware of starting an observation until the focus has stopped driving 
and the red {\bf focus stop} lamp on the Control Desk is lit. 
 
\subsubsection{Autoguider Focus at Prime}
 
It is most unlikely that the autoguider will be detectably out of 
focus if the system has been set up correctly. However, if the 
images look very extended and in particular if they have holes in 
their middles, then the focus should be adjusted. For a full description,
see section \ref{ch11:afocus}:
\begin{enumerate}
\item Focus the telescope without using a filter.
\item Acquire a star on the autoguider.
\item Step the focus through a range of values around the nominal focus 
position (the current one, most likely) using the command:
 
\ADAM{GDFOCPOS value}
 
After each step, press \button{FIELD}\,, then \button{WAIT} on the Autoguider 
Panel. Do not leave the autoguider integrating, 
as the images will get mixed up. Inspect the star image and measure its width 
with the Grinnell PROFILE function; find the {\tt value} for which the
image is narrowest.

\item When you have discovered the correct focus, type:
 
\ADAM{SETGDFOCUS value}
 
where value is your best focus in {\tt mm}.
\end{enumerate}

\subsection{Acquiring a Guide Star}
For a full description refer to section \ref{ch12:gauto}. The acquisition of
guide stars proceeds in two steps: (1) location a suitable guide star on the
Finder and moving the probe to the corresponding position, and (2) locking
the probe onto the guide star and closing the guide loop. 

\subsubsection{Locating guide stars at Prime Focus}
 
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.
Proceed as follows:

\begin{enumerate}
\item Move the rotator to your required position angle with {\it e.g.\/}:

      \ADAM{ROTPOS 180}
\item Slew the telescope to your object to be observed; confirm with a GLANCE
      exposure.
\item Press the \button{FINDER} button on the Grinnell panel.
\item Press the \button{DIRECT} button on the Grinnell panel.
\item Press the BEAMSWITCH \button{A} button on the Control Desk.

      The guide probe field will now become visible on the finder; stars 
      should appear to move along the major axis of the chip as the beamswitch
      takes effect.

      If nothing happens, check at the User Interface (with {\tt AD/T}) that 
      the aperture~A position is correctly defined: it should be 0,-590. If 
      it is not, then type:
 
      \UI{AP/A 0 -590}

      in order to define the aperture position, and press \button{A} again.
 
\item Select the Grinnell function GET GUIDE STAR. A display of 
      the field accessible to the probe will appear on the screen.

\item Move the cursor to your chosen guide star in the permitted area and press 
      \button{ENTER} on the Grinnell joystick control box.
 
\item Finally press the BEAMSWITCH \button{NOM} button on the Control Desk
      and press \button{DIRECT} on the Grinnell panel.
\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. Subsequently,
the guide star will have to be acquired with the guide probe, by going through
the {\bf field} - {\bf acquire} - {\bf guide} - {\bf loop/enable} sequence
described below in section \ref{ch4:acquire}.

\subsubsection{Locating guide stars at the Cassegrain focus}
\begin{enumerate}
\item Move the object to be observed onto the slit using the Cassegrain TV 
      camera.
 
\item Press the \button{FINDER} button on the Grinnell Panel. 

\item Press 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.

\item Choose a star within the permitted area which is visible without
      integration (otherwise it will be too faint for the autoguider).
 
\item Move the Grinnell cursor to the desired guide star and press
      \button{ACCEPT} on the joystick.
 
\item If you have been operating the autoguider in Telescope Mode, or just to
      be sure, switch to Probe Mode by typing at the User Interface: 

      \UI{AC/PR}

\item Acquire the guide star by going through the {\bf field} - {\bf acquire}
      - {\bf guide} - {\bf loop/enable} sequence
      described below in section \ref{ch4:acquire}.
\end{enumerate}

\subsubsection{Using guide stars of known RA and Dec (either focus)} 

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:  

\UI{name}

\UI{GUIDE guidename}

In order to move the guide probe to the correct position, point the telescope
to your object and press the \doublebutton{GUIDE}{STAR} button on the A\&G 
panel. Subsequently acquire the guide star.

\subsubsection {Acquiring a guide star} \label{ch4:acquire}
This procedure discribes the acquisition of a guide star with the guide probe
after a guide star is positioned near the centre of the probe field. A full
description is given in section \ref{ch12:acquire}.
\begin{itemize}
\item If not yet done, switch on the autoguider. If no `idle' message appears
      press the \button{SHUTDOWN} button on the Autoguider Panel.

\item Select the autoguider probe image on one of the TV monitors (there is
      also a dedicated mini-monitor).

\item Press the \button{FIELD} button on the Autoguider Panel. An image will
      appear in the autoguider sub-display.

\item If you are happy with the selected guide star, press \button{ACQUIRE}\,.
      Either the probe or the telescope is moved,
      depending on the chosen acquisition mode, in order to
      position the star at the centre of the autoguider field.
      On the monitor a message `pulling-in' will appear after a while.

\item When the error drops below a preset threshold, scanning ceases and a
      message `OK to Guide?' will appear on the monitor. 
      If you are satisfied with the image press \button{GUIDE}.

\item Wait until the autoguider display is completely redrawn and then press 
      the \doublebutton{LOOP}{ENABLE} button which should turn green. 

      When the cross (the measured star position) is centred in the circle 
      you are ready to observe.
\end{itemize}
 
On source change, press \button{WAIT}\, (the guide probe is automatically 
switched to LOOP DISABLE).

\subsection{Beamswitching and Aperture Offsets} \label{ch4:sbeam}
 
An APERTURE offset is made in a coordinate system fixed in the 
instrument rotator, so its value in RA and Dec changes as a function 
of rotator angle. This is done to facilitate moving an object between points on 
the detector itself, rather than defining offsets on the sky for which OFFSET
should be used.

There are three apertures: NOMINAL, A and B:
\begin{itemize}
\item 
The NOMINAL aperture is defined to be the position of the reference 
point used for SNAFU: the Grinnell crosswire or spectrograph slit 
for the Cassegrain focus,
or the centre of the CCD chip at Prime focus.

The reference point need not coincide with the axis of the
instrument rotator, and the system knows the displacement between them,
if the software is setup correctly. Thus,
an image placed on the reference point will remain stationary when 
the rotator is turned.
\item
Apertures A and B may be used for other positions on the detector, TV 
or autoguider. At Cassegrain a preference has developed to put the
nominal position on the slit, and use A and B for separate offsets, but if
the crosswire were used as SNAFU reference point, it would be useful,
for example, to define either A or B to be the centre of the slit.
The default value of aperture A at Prime 
focus is defined to bring the guide star acquisition field into the finder 
display.
\end{itemize}

Aperture offsets are applied as {\it pointing corrections}: the displayed 
position of the source does not change when they are introduced (except 
briefly, as the telescope moves). This is because aperture offsets 
are used to move the same object to a different place on the detector, 
rather than a different object to the same place on the detector (for which 
operation, an RA/Dec OFFSET should be used). There is no reason why both kinds
of displacement should not be used 
simultaneously. For example, if the crosswire is used as reference point for
SNAFU, the following sequence can be useful: (1) slew to a bright 
reference star, (2) centre it on the reference point, (3) RA/Dec OFFSET to 
a faint galaxy, and (4) beamswitch A to move that galaxy onto the spectrograph 
slit. 
 
\subsubsection{Using known aperture offsets}
\begin{enumerate}
\item
To define the offset(s)  type at the User Interface:
 
\UI{AP(ERTURE)/A x-value y-value}
 
or 
 
\UI{AP(ERTURE)/B x-value y-value}
 
The {\tt x} and {\tt y} values are given in arcsec. 
\item
In order to introduce the offset, press the BEAMSWITCH \button{A} or 
\button{B} button on the Control Desk; to return to the reference position, 
press the \button{NOM} button. The offsets are displayed on 
the Information Display. 
\end{enumerate}
 
\subsubsection{Setting apertures interactively}
 
The commands SNAFU/A and SNAFU/B are used to determine the 
positions of apertures A and B interactively. For 
example, when an object has been located  
on a particular detector pixel, one might want to keep the offset for 
future use.

\begin{enumerate}
\item Position the object on the SNAFU reference point.
\item Type at the User Interface:

\UI{SNAFU/A}
 
\item Move the object to the intended position of aperture A using the handset 
 or guide buttons.
\item Press the \button{ACK} button (acknowledge) on the telescope Control Desk.
\item Press the BEAMSWITCH \button{NOM} button to return the star to the 
 reference point.
\item If wanted, repeat the procedure for aperture B.
\end{enumerate}
 
Hereafter, pressing the BEAMSWITCH \button{A} or \button{B} button will cause 
the offset to be made. If you want to record the offsets for future reference, 
inquire their values by typing at the User Interface:
 
\UI{AD(VISE)/TEL(ESCOPE)}

\subsection{Blind Offsets at Cassegrain} \label{ch4:boffset}
It frequently happens that an observer working with the FOS wants to take
a spectrum of an object too faint to be seen.   Such objects
should be measured astrometrically before leaving for La Palma
relative to a comparatively bright reference star close to the object.
An accuracy substantially better than an arcsecond is recommended.
Blind offsets are made with the autoguider, because it is more accurate
than simple offsetting the telescope especially in Declination. 

At the telescope, proceed as follows:

\begin{enumerate}
\item Place the reference star on the slit of the spectrograph.
\item Acquire an autoguide star in the ordinary way with the guide probe.
\item Switch over to ACQUIRE/TELESCOPE mode.
\item Enter the offset values at the User Interface console with:

\UI{AU/B RA-offset,Dec-offset}

where both offsets are in arcseconds. The telescope will
be offset by the appropriate amount to put the invisible object (approximately)
on the slit, while the guide probe is moved to the precise position where
the autoguide star (step 2) should be after the telescope offset.

%     Move the autoguider probe, in steps of about 2 arcseconds (300 units),
%      by an amount equal
%      to the object-reference-star position offset in each co-ordinate.
%      The Cassegrain scale of the INT is 5.41 arcsec/mm and the autoguider
%      probes are calibrated in microns so that 184 units of autoguider
%      travel is one arcsec.
\end{enumerate}

The autoguider will now lock onto the guide star at the offset position to
centre it on the probe. When the measured star position is centred in the
circle, the invisible object will be on the slit. On big offsets the autoguider
may lose the guide star because the telescope moves faster than the probe.
However, you can always re-acquire the guide star (with the probe still fixed).

One important drawback to blind offsetting is that when the autoguider probe 
is offset it may actually be carried into an area where it either vignettes the
spectrograph slit or is itself vignetted by the ironmongery of the A\&G box.
For this reason one should try to find a reference star as close to the 
invisible object as possible. If no suitable reference star is found, one has
to fall back on the simple telescope offset/beamswitch method referred to in
section \ref{ch4:sbeam}, which is inherently less accurate.

\subsection{Rotating the Cassegrain turntable}
 
\subsubsection{Under Computer Control}
\begin{enumerate}
\item Enter the desired position angle on the user interface:

\UI{TU(RNTABLE) position-angle}

where the angle is in degrees. 

\item Press the TURNTABLE \button{NOM} button on the Control Desk. 

The turntable will move in the correct direction to avoid the limit switch at
Position Angle 140\degs. 

\item If the table stops an arcminute or two away from the correct
position, press the \button{NOM} button again. 
\end{enumerate}
 
If you end up in a turntable limit (this should never occur 
when operating in this way), then you will have to escape from it in 
manual mode (see the section immediately below). 

\subsubsection{Under Manual Control} 
\begin{enumerate}
\item Either (a) Depress the TURNTABLE \doublebutton{SELECT}{MANUAL} control 
button on the Control Desk (the button must be illuminated), \\
or (b) Switch over to Engineering Mode by pressing the \doublebutton{ENG}{COMP}
button.
\item 
Select fast or slow motion on the Engineering Rack. 
\item Press the rotation \button{+} or \button{$-$} buttons as appropriate.
Note that the motion as indicated on the Engineering Rack is counter to the
change of position angle! The correct value of the position angle on the sky is
 shown
on the Information Display; the value on the Engineering desk equals 
360\degs$-$PA and in addition includes a small encoder zero-point correction.
\end{enumerate}
{\bf Beware:} do not drive through angle 220\degs\, on the Engineering Rack,
{\it i.e.\/} Position Angle 140\degs\, on the sky (and
Information Display), because the turntable will hit a limit switch
 and you will have to go all the way round in the opposite 
 direction. If you do hit the limit switch somehow, signalled with the red light
 on the Engineering Rack below the ``position angle'' display, back out of it 
 in {\bf slow} motion. 
 
\subsection{Making a Hard Copy of a Grinnell TV image}
\begin{enumerate}
\item Select TV monitor 1 or 2 on the Control Desk.
\item Freeze and Fill the image (essential for a star field).
\item Push the button on the copy machine.
\item {\bf Wait} untill the copying is completed before switching back to the
Grinnell.
\end{enumerate}
\subsection{Archiving a Grinnell TV image}
\begin{enumerate}
\item Freeze and Fill the image.
\item Push the Archive button on the Grinnel Panel.
\item Fingers crossed while a `.BDF' file is created which can be displayed 
on the LEXIDATA and output with FITSOUT.
\end{enumerate}
