\parindent 0pt
\parskip 0.3cm
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%\section{Calibration Tests and Quality Control}
\section{CALIBRATION TESTS AND QUALITY CONTROL}

This section is intended primarily for La Palma staff. It describes several
calibration procedures and quality control tests in detail. Some of these tests
must be done every night (by the Telescope Operator), others only on
occasion. Several procedures require the inspection and/or redefinition of
global Task Common parameters, others are just detailed recipes which can
also be of use to experienced observers.
It is recommended to read the La Palma Technical Note No.~39:
``Telescope Control Software: Global Variables and Configuration Files'',
before having a go at those procedures dealing with Task Common variables
(see also section \ref{ch7:query}).

\subsection{Telescope Pointing} \label{ch11:pointing}
In order to achieve the best performance of the Telescope Pointing Model, the
following zeropoint determinations have to be carried out daily, as part of
the start-up procedure for the INT:
\begin{itemize}
\item A hardware zeropoint, determined by the initialization of the Ferranti
Encoders with the ZEROSET procedure (section \ref{ch11:zeroset}), which 
involves carefully moving the telescope across hardware markers on both axes.
\item A software zeropoint, which has to be determined by pointing the telescope
to a star with known position, and use a SNAFU procedure to find a correction to
the pointing-model's zeropoint. SNAFU comes in two versions: a long one, the
2-Star SNAFU (section \ref{ch11:2-snafu}), which has to be performed
after an instrument change-over, and a short one, the
1-Star SNAFU (section \ref{ch11:1-snafu}), which can be used on subsequent 
nights. The recommended procedure is to do a 1-Star SNAFU at the beginning of
the night. With the improvement in performance of the telescope the longer
version is rarely used.
However, failure to do a SNAFU at the beginning of the night, will seriously
impair the pointing of the telescope. A SNAFU at Prime Focus (section 
\ref{ch11:pfsnafu}) is different from
one at Cassegrain because at prime there is no TV camera for
field viewing.


\end{itemize}
Further, the parameters of the Pointing Model itself have to be determined at
regular intervals of several months (see section \ref{ch11:model}).

In order to achieve the maximum accuracy and consistency in the determination of
zeropoints it is essential that these ZEROSET and SNAFU procedures are adhered
to and that variations are not introduced for purposes of expediency or 
personal preference.

\subsubsection{Encoder Zeropoints: ZEROSET} \label{ch11:zeroset}
To be done daily, as part of the start-up procedure for the INT.
\begin{enumerate}
\item
In engineering mode, position the telescope roughly at zenith, {\it i.e.\/} 
position Hour Angle $0^{h}$ and Declination 28\degs 46\mins\, on the synchros. 
\item
Now drive the telescope in slow motion a few degrees away from the zenith 
in both Hour Angle and Declination
by pressing the {\bf positive} direction buttons \button{+}\,.
\item Type ZEROSET on the User Interface and press \key{RETURN}\,, but only if
you have successfully performed action (2). 
\item
Check that the two red encoder/zeroset-in-progress lights on the Control Desk
are {\bf both} lit. If they are not, it means that the telescope is not far 
enough away from the zeroset markers, so cancel zeroset by pressing
the \button{ACK} button on the control console and start again.
\item
Set the engineering-mode speed controls (on the Engineering Rack) to the 
position markers, and make sure the {\bf slow}-motion drive is selected. It
is worth to move the telescope in slow motion to see that the zeroset is 
triggered. The accuracy of the result is now independent of speed to good
approximation.
%It is very important that the telescope is moved slowly, as this will%old text
%give the highest accuracy (see section \ref{ch11:refpt}).            %old text
\item
Drive the telescope through the zeroset position ({\it i.e.\/} zenith)
by pressing the {\bf negative} direction buttons \button{$-$}\,. The indicator 
lights on the Control Desk should go out and a message:

``Zeroset completed'' 

should appear at the User Interface. 
\end{enumerate}
If for any reason, the procedure does not work, or if you feel it is
giving inconsistent results, report it on a Fault Report Card; in order to
escape from the zeroset procedure at any stage, press the \button{ACK} button.

If the above described zeroset fails, it is still possible to observe by doing
a rough zeroset, as follows:
\begin{enumerate}
\item Position the telescope at zenith, as before, and set the telescope
as accurately as possible on the centre of the position range where the
yellow-green lights are on.
\item Type at the User Interface terminal:

\UI{ZEROSET/MANUAL}
\end{enumerate}
The zeropoint determined in this way is far less accurate than when using the
regular procedure, and it is necessary to do a 1-star SNAFU in order to adjust
the software zeropoint. 

\subsubsection{Software Zeropoint: 1-star SNAFU} \label{ch11:1-snafu}
A single star SNAFU is done as follows:
\begin{enumerate}
\item
At the Telescope Computer system terminal, run INPSCF and check 
(see section \ref{ch7:query}) that the value of parameter HATERM equals
2 (inquire its value by typing: $@$HATERM). It is {\bf essential} to do 
this because if it has 
accidentally been left with a value of 1, the following actions will
completely invalidate the pointing.
\item
Check that PCCH has the correct value from the last 2-star SNAFU
(check the log sheet). If it does not, then enter the appropriate value.
\item
Slew to a star with 0\degs $<$ Dec $<$ 40\degs ; the choice between North
($\delta > $28\degs 46\mins) and South ($delta < $28\degs 46\mins) should 
be made to minimize dome rotation.
\item
Type SNAFU at the User Interface.
\item
Centre the star on the reference point (see end of Section \ref{ch11:refpt}).
\item
Press the \button{ACK} button until it goes out.
\item
Determine and record the values of PCIH and PCID.
\item
Type END at Telescope Control System Console to exit from INPSCF.
\end{enumerate}
If the pointing after a 1-Star SNAFU is unsatisfactory, then it is good practice
to first check if the encoders are performing correctly, before doing anything
else, by running the ENCODERS test programme (see section \ref{ch11:encoders}).
If necessary disable a bad encoder and do again a ZEROSET and 1-Star SNAFU.
If all else fails try a 2-Star SNAFU, but it is more likely that such failure
indicates a serious problem: call the Duty Technician.

\subsubsection{Software Zeropoint and Collimation Error: 2-star SNAFU}
\label{ch11:2-snafu}
To be done at the beginning of a run, but definitely after an instrument change.
\begin{enumerate}
\item
At the Telescope Computer System Terminal, enter INPSCF and set HATERM 1
(section \ref{ch7:query}).
\item
Select slit view mode and enable the TV crosswires.
\item
Select a star with Hour Angle $<$ 3 hours and Declination $>$ 85\degs.
\item
Slew to that object.
\item
Type SNAFU at the User Interface.
\item
Move the object to the SNAFU reference position (see end of Section 
\ref{ch11:refpt}) using the handset.
\item
Press the \button{ACK} button on the Control Desk {\bf until it goes out}.
\item
Select a new star at 30\degs $<$ Dec $<$ 40\degs and $|$HA$|$ $<$ 3$^h$.
\item
Slew to that object.
\item
On Telescope Computer System Terminal change HATERM to 2.
\item
Type SNAFU again on the User Interface.
\item
Position the object as before on the TV monitor.
\item
Press \button{ACK} button until extinguished.
\item
If, in action (6) and (12), the  corrections to the telescope positions
were large ($>$ 10 arcsecs) then repeat the process from (1) (INPSCF will 
already be running).
\item
Record the values of the variables PCIH, PCID and PCCH on the form
provided. (Get their values by typing $@$PCIH {\it etc.\/} within INPSCF).
\item
Type END at the Telescope Control System Console to exit from INPSCF.
\end{enumerate}


\subsubsection{SNAFU at Prime Focus} \label{ch11:pfsnafu}
The SNAFU at {\bf Prime} Focus is done completely differently, because there
is no TV camera for field viewing, and involves the
taking of a test GLANCE exposure on the prime focus CCD, under ADAM control.
One approach
would be to simply follow the procedures for Cassegrain focus (sections 
\ref{ch11:2-snafu} and \ref{ch11:1-snafu}), and move the star iteratively to 
the SNAFU reference point on the CCD by means of repeated GLANCE exposures on
which the star's position is determined by using the LIST option of PHOTOM. The
reference point is obviously the centre of the chip. However, the following
procedure is more convenient, since it requires only one GLANCE to be taken, 
and makes use of standard ADAM functions:
\begin{enumerate}
\item Move the rotator to position angle 180\degs \  with the ADAM command
      ROTPOS 180.
\item Select the finder on the TV monitor at the Control Desk.
\item Slew the telescope to the SNAFU star, as for Cassegrain mode.
\item Startup ADAM (See the Prime Focus User Guide).
\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
      obviously visible near the centre of the chip. If it is not, either the
      top-end change over has gone catastrophically wrong (is the telescope
      control software configured for Prime?), or you have done
      something silly, such as mis-typing the SNAFU star coordinates.
\item Use the MOVESTAR command, and locate the star with the cursor.
      ADAM wil now calculate a position correction and send those via the
      inter-processor
      link to the Telescope Control Computer. The telescope will move to
      the corrected position: 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. This position corresponds to the centre of the
      CCD detector.
\item Now send the telescope again to the original position by pushing the
      \button{SRCE} button.
\item Type at the User Interface console:

      \UI{SNAFU}

      and move the telescope with the handset such that the star 
      image on the finder will coincide with the position marker put on the
      screen with action 7.
\item Press the \button{ACK} button on the Control Desk.
\end{enumerate}
The reason behind the last three steps is that although ADAM sends the
corrections to the Telescope Control Software, this does not update the
pointing model zeropoint. That is done in the same way as for Cassegrain
mode during the last three steps.

\newpage
\subsubsection{Reasons behind the ZEROSET and SNAFU procedures} 
\label{ch11:refpt}
{\bf ZEROSET}

ZEROSET is the procedure that must be run when the telescope control
system has been started up, or if for some reason the encoders have become
corrupted during operation. It is a technique for initialising the Ferranti
encoders. These are incremental devices and must be given an absolute 
reference from which to work. The ZEROSET process involves sweeping the 
telescope across hardware markers on both axes. When these markers are 
detected, the encoders are zeroed by the hardware, if this mode is enabled
by having typed ZEROSET. The hardware detector is positioned so that
the ``zeroset position'' is at the zenith.

There are errors inherent in this process, but these can be minimised by the
correct operational procedures. 
\begin{enumerate}
\item
The angle within which the telescope 
is ``detected'' on each axis has a finite size of a few arcsec. This is why
it is possible to position the telescope and keep it stationary ``in the
zeroset position'' with the green zeroset lights above the RA and Dec dials lit.
However, the encoders will be initialized by the hardware if the telescope is 
anywhere within this region and this leads to zeroset errors comparable
to the extent of the zeroset region. For this reason, the technique of setting
the telescope at this position and {\bf then} running ZEROSET is 
unsatisfactorily crude. The technique
capable of giving the highest accuracy and the most repeatable results is to
1) ZEROSET at the User Interface, and then 2) move the telescope slowly
through the zenith in both axes, but always going in the same direction. In this
way the leading edge of the detector area will be used as the reference, and
this is clearly much better determined than any random point within the region.
\item
The speed with which the telescope is driven past the detectors
is now not very important. Although the detection/encoder-reset is done by the 
hardware, and there is some small error associated with this process, tests
indicate that the error is practically independent of speed, but depends on
direction. Markers have been placed on the speed control
potentiometers to indicate a recommended speed at which the ZEROSET 
triggering can easily be observed.
\end{enumerate}
\vspace{1.0mm}

{\bf SNAFU}

The SNAFU procedure is designed to perform a somewhat similar task to ZEROSET.
It does however have various subtleties which should be appreciated if it is
to be used properly.

Originally SNAFU was used to update only the encoder index errors (ID and IH)
of the telescope pointing model, which are simple zeropoint offsets of the 
encoder readings. These are deduced by pointing the telescope at an object whose
position is precisely known. More recently, it was considered desirable to
introduce the possibility of updating the collimation error of the telescope
as well. This error (CH term in the pointing model) is the offset between the
mechanical and optical axes of the telescope and ideally should not change
if the telescope configuration is stable. The collimation error is 
indistinguishable from the index error in Declination, being a constant 
angular offset. But in Right Ascension a  Cos(Dec) term is introduced
by the projection onto the sky. The error in RA as seen on the sky is 
given by:

E(RA) = Index Error $\times$ Cos(Dec) $+$ Collimation error

As the Declination increases, the effect of the index error diminishes
and it is a good approximation to ascribe the pointing error solely
to the collimation error. Note though, that cos 85\degs$\sim$0.09, but
cos 70\degs $\sim$0.34. The SNAFU process can therefore be split into
two separate operations. 
\begin{enumerate}
\item
The telescope is set on a star at {\bf high} Declination and the correction
observed is attributed to the collimation error in RA and  the index error
in Dec. The hour angle of the star observed is not critical (it can be +3 hrs
before other effects become important) but it is essential that the
Declination be greater than 85\DEG, or else the approximation that  
cos(Dec)$\sim$0 implicit in the method is invalid.
\item
An object at {\bf low} declination is observed, and only the index errors in 
both axes are updated.
\end{enumerate}

The SNAFU software therefore has two modes of operation, and depending
on which mode it is in, the coefficients of the pointing model that are
updated are either (ID and IH) {\bf or} (ID and CH). The mode of SNAFU is
set by the Task Common variable HATERM. At the moment, access to this
variable is somewhat cumbersome and only possible through the use of
INPSCF on the telescope control system terminal. The default ({\it e.g.\/}
startup) value for HATERM is 2, which means it is in the mode where the index
errors ID and IH are corrected. The consequence of this is that it is
safe to perform a single star SNAFU at any Declination ($<$ 45\DEG)
at any time without changing the value of HATERM. In fact, with improvements
in the performance of the telescope, serious problems with collimation do
now rarely occur, and most of the time doing a single-star SNAFU is sufficient.
We have more problems with the encoders than with collimation. However,
after an instrument change, a full 2-star SNAFU should
be performed. This involves changing HATERM to value 1, and then
SNAFUing on a star with Dec $>$85\DEG. Again, the star used for this
need not be on the meridian, but it is important to get high declination.
The value of HATERM should be returned to 2 before any SNAFU is repeated
at a lower declination. Since the high-declination SNAFU is only 
an approximate solution, if the corrections needed to bring the stars onto the 
reference point are large ($>$10 arcsec) when performing a 2-star SNAFU, 
then the whole procedure should be repeated at least once. In this case
the declinations chosen for the two stars should be (HATERM=1 , Dec $>$ 85\DEG),
(HATERM=2 , 30\DEG $<$Dec $<$ 40\DEG). This avoids a 180\degs \, rotation of the
dome between SNAFUs. \vspace{1.0mm}

{\bf Reference Point for SNAFU} \label{se:refpt}

It is equally important to be precise and consistent about what reference 
point is
used for SNAFU. At Cassegrain, some point on the A\&G box TV is used.
It should NOT be an arbitrary point, since the software assumes that the 
SNAFU position is fixed relative to the rotator axis and uses this information
to perform aperture offsets and corrections for rotator angle changes. A 
preference has emerged for SNAFUing on the spectrograph slit. The position 
along the slit must be defined, and this is the intersection of the slit and 
the single aperture dekker. This convention must be strictly adhered to, 
because the rotator axes will be determined relative to this point. It
also has the inherent advantage of ensuring that the slit mirror, as
opposed to the field mirror, is used to view the object. If this position
is not optimum for placement of the object on the detector, then use
an aperture offset to move to the required position afterwards ({\it i.e.\/}
define an offset using SNAFU/A and thereafter use the BEAMSWITCH \button{A}
button to move to that point). A change of reference point requires a
2-star SNAFU to be performed, since it implies a collimation shift.

\subsubsection {Testing the Encoders} \label{ch11:encoders}

Problems with the telescope pointing are nowadays more often than not due
to problems with the encoders. If encoder problems are suspected, {\it e.g.\/}
because even after a repeated 1-Star SNAFU the pointing remains poor, test
the encoder readings, as follows:

\begin{enumerate} 
\parsep 0.3cm
\item Be sure that the slow-motion drive is engaged, {\it e.g.\/} the
      telescope is tracking (to ensure that the Baldwin encoders are engaged).
\item Stop the telescope and switch to Engineering mode (for safety).
\item Run the task ENCODERS from the Telescope Control System terminal. Type:

 \PE{ENCODERS}

 This produces a status report of the readings from the 4 independent 
 encoders (3 Ferranti, 1 Baldwin) on each axis. Firstly, look at the Ferranti 
 readings. If one of these is grossly different from other two on the same axis
 and is not already disabled, then that individual encoder is baulking. Confirm
 this by looking at the counters in the CLIP Centre. Correct any 
 obvious problems (such as disconnected electronics).

 If the problem persists run the
 INPSCF task (see section \ref{ch7:query}) and disable the faulty encoder by
 resetting the appropriate encoder 
 weight to zero with variables WHAFE{\it n} and WDCFE{\it n} for Hour Angle 
 and Declination Ferranti head {\it n}, respectively. 
 Thus, to disable Hour Angle Ferranti Encoder number 2, do:

 \PE{INPSCF}

 \INPSCF{WHAFE2 0.0}

 \INPSCF{END}

 Note that this procedure only affects the running system - if, 
 for any reason, the TCS is stopped and then restarted you will 
 have to reset again the appropriate weights to zero. A long-term fix
 is to ask software support staff to update the initialization files.

 \item If all of the Ferranti readings appear to be sensible, compare the 
  Baldwin+Ferranti values with those for the Ferrantis alone. If they 
  are more than a few arcseconds different, then the Baldwin encoder has 
  probably broken. It is easy to switch over to using Ferrantis only 
  in Declination (just set Task Common parameter JDCBW to zero using INPSCF).
  However, if the problem is with the Hour Angle encoder, better call out the
  Duty Technician in order to check out the encoder and adjust the servo
  constants.
\end{enumerate}


\subsubsection{Pointing Tests using POINT and AUTOPNT} \label{ch11:model}

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. 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; a small residual oscillation at about 1$\,$Hz 
persists in Hour Angle.

 The data necessary to produce pointing models for the telescope are 
logged by the PE\,3220 running the task POINT. Data are logged by this 
task when the \button{LOG} button on the control console is pressed. Note that
this button lights up while the logging task is active and the telescope should 
not be moved until the light goes out. The pointing program creates four data 
files on the PE\,3220 system disc. The first one is called POINT.DAT and must
not exist when the program is started. Either delete them or rename them if 
they are to be kept. The extra three files created at the INT are called 
POINT1, POINT2 and POINT3 (all .DAT file extension). They contain details 
of the individual Ferranti encoder readings alone so that checks can be made 
for any systematic drifts in the Baldwin encoders and also to provide data from 
which it may be possible at a later stage
to model the individual Ferranti encoder errors and separate 
them from global pointing effects such as a polar axis misalignment. The 
main data file, POINT.DAT, contains the name of the observed source,
the Baldwin and Ferranti encoder readings of RA and Dec and the Local Siderial 
Time of the observation. A grid of objects suitable for use in the pointing 
test is contained in the file PTGRID.CAT (see Appendix \ref{ptgrid}), which 
resides permanently on the system disc.

{\bf To run a pointing test with POINT:} 
\begin{enumerate}
\item Load the PTGRID Catalogue by typing at the User Interface:

\UI{I(NPUT) ptgrid}

\item Run POINT by typing at the TCS terminal:

\PE{POINT INT}

If the computer responds with a message to the effect that file POINT.DAT 
already exists, and that therefore the task can not be started, rename that 
file with:
 
\PE{RENAME POINT.DAT, POLD.DAT}

and similarly for the other three data files:

\PE{RENAME POINT1.DAT, P1OLD.DAT} \hspace{0.5cm}, {\it etc.}
 
Do {\bf not} delete the files, since they may still be of interest. Further,
report this action to the Software Manager.

\item Select a grid star from the catalogue with:

\UI{NEXT sourcename}

\item Move the telescope to the grid star by pressing the \button{SRCE} button.
 Centre the star on the SNAFU reference point (see section \ref{ch11:refpt}).
\item Press the \button{LOG} button on the Control Desk - wait until the 
 light goes out (it should light up for a brief moment). 
\item Repeat steps 3 through 5 until sufficient stars have been 
 observed. Choose your objects all over the sky.
\item Stop the POINT program by typing at the TCS terminal:

\PE{ENDPOINT}

This will stop the program and print out the data files. Subsequently, press
the {\bf on-line} button in order to re-initialise the terminal.
\item Ask the software group to transfer the file POINT.DAT to the POINTING
account on the VAX~8300.
\end{enumerate}
Having transferred the POINT.DAT file to the VAX~8300 it is necessary to add
two lines (with the editor) at the beginning with the date and observer's name, 
and the temperature and atmospheric pressure at the time the measurements were 
made.
The data can then be analysed with the TPOINT software (SUN 100.7, a Starlink
User Note by
P. Wallace, 1989) in order to determine the pointing model coefficients. These
can then be entered in the relevant .STD file (by qualified La Palma staff!).
\vfill
\newpage
{\bf AUTOPNT}

The pointing procedure has now been completely automated and the
procedure AUTOPNT should be used at Cassegrain. The acquisition of pointing
stars is made with the autoguider probe; the stars are put at the 
SNAFU reference position.
Therefore, the guide probe has to be told where the reference position is by 
placing a star on the reference point
and acquiring it with the autoguider in `probe' mode. The
procedure is now as follows:

\begin{enumerate}
\item Place a star on the SNAFU reference position, {\it i.e.} the slit or
      the Grinnell crosswire (press \doublebutton{cross}{hair}\,).
\item Acquire this star with the autoguider using the ACQUIRE/PROBE
      option.

      (Alternative to steps 1 and 2: drive the probe to the centre 
      position. The idea is just to put the probe at the reference point.)
\item At the user interface select ACQUIRE/TELESCOPE.
\item At the Telescope Control System Console type:

      \PE{AUTOPNT INT}
\end{enumerate}

The telescope will work its way through every available star
from PTGRID.CAT from North to South and then back again.
When it has acquired each star it will
guide for a few seconds and then log that position before moving on to
the next star.

At Prime focus the situation is more complicated because it is impossible
to move the autoguider to the rotator axis of the telescope.
The Prime Focus geometry is set up such that if a star
is on the optical axis of the telescope pressing BEAMSWITCH \button{B} will
move the telescope so that the star is centred on the autoguider
in its parked position.   Thus, when using AUTOPNT at Prime you have
to press BEAMSWITCH \button{B} so that the stars fall within the field of the
autoguider.   Unfortunately the Telescope Drive Software has a 
feature that at the beginning of every slew the BEAMSWITCH
automatically reverts to the NOMINAL aperture. Therefore, when running AUTOPNT
at Prime it would be necessary to press BEAMSWITCH \button{B} every time the 
telescope begins a slew to the next star! This would be a cumbersome exercise,
but a simple way
around it is to change the NOMINAL aperture position before starting AUTOPNT
as follows:

\begin{enumerate}
\item Inquire the current BEAMSWITCH B aperture co-ordinates at the 
      User Interface console:

      \UI{ADV/TEL}

      The aperture position is given in arcseconds.
\item Transform the co-ordinates to radians through division by 206265.
\item Inquire the current NOMINAL aperture co-ordinates at the TCS console:

      \PE{INPSCF} \\ \INPSCF{@AOXNOM} \\ \INPSCF{@AOYNOM}

      {\bf Important:} make a note of the current values.
\item Replace the NOMINAL offset values by adding the beamswitch offsets to 
      their current values:

      \INPSCF{AOXNOM new-aoxnom} \\
      \INPSCF{AOYNOM new-aoynom}

      \begin{tabbing}
      where: \= {\tt new-aoxnom} = current aoxnom + beamswitch B.x \\
             \> {\tt new-aoynom} = current aoynom + beamswitch B.y
      \end{tabbing}
\end{enumerate}
{\bf Important}: do not forget to reset AOXNOM and AOYNOM to their original
values after AUTOPNT has finished! \vspace{0.4cm}

{\bf Some NOTES:} 
\begin{itemize}
\item At any time the contents of the data file(s) being created can be 
inspected by typing at the TCS terminal:
 
\PE{PRINT filename}
 
It is worth checking that the program is working by printing the POINT.DAT file
after the first 5 observations have been made with:

\PE{PRINT POINT.DAT}

A listing should then be output to the lineprinter standing in the Instrument
Computer Room, adjacent to the Control Room. Check that it is switched 
on-line. If there does not appear any output, go to the TCS console and try:

\PE{TASK SPOOLER}

\PE{CO(NTINUE)}

If that doesn't work, type

\PE{STARTSPL}

If this still doesn't work write a `Software Error Report' (SER).

\item It is {\bf not} possible to run the task INPSCF or the ENCODERS
test program simultaneously with POINT. 
 
\item
If you have renamed an existing POINT.DAT file,
report this action to the Software Manager so that he can archive or 
delete the file as he sees fit. Don't forget to do this, or the disk 
will get littered with old pointing files. Do NOT delete the 
file yourself unless you are positive that it is of no further interest. 
\item
The POINT program can be used to
keep a record of the directions to which the telescope 
was pointed throughout an observing night. This
can be particularly useful for analysing the geometrical errors of the mounting 
(see for instance RGO/La Palma Technical Note No. 32). POINT is started at
the beginning of the as
for a normal run with PTGRID, but now your own objects are the test points.
Thus, press the \button{LOG} button on the console every time you centre a 
star. At the end of the night type ENDPOINT at the TCS terminal, press
the {\bf on-line} button and issue a request to transfer the file to the 
VAX~8300.
\end{itemize} 
\vfill

\newpage
\subsection{Telescope Focus} \label{ch11:focus}
\subsubsection{Focus Drive}
The focus drive has two drive speeds. The point at which the change from fast 
to slow takes place when moving the focus position to a new value is determined
by the Task Common (TSKCOM) variable FOCS2F, while the stopping window for the 
focus is defined by the variable FOCSTP. These parameters may
need to be adjusted at times (see section \ref{ch7:query})
, especially if any of the encoders is faulty ({\it e.g.\/} losing
bits), because this is liable to cause oscillation of the focus mechanism, in
particular if the limits are too finely set. Adjustment is exclusively done
by La Palma support staff; the mechanism has proved to be reliable.

\subsubsection{Focus and Temperature}
The telescope focus varies with temperature, primarily as a result of thermal 
expansion in the Serrurier trusses. This dependence has been determined
empirically by fitting a simple linear model to the focus position as function
of temperature, and the focus on the INT is now temperature compensated.
Temperature sensors are positioned on the steel work of the tube; their 
analogue output is
digitised in order to be read by the PE\,3220. The temperature of 
the structure is calculated by assuming a linear relation between the ADC output
value and the temperature:

Temperature = ATEL + BTEL $\times$ ADC reading.

The calibration constants ATEL and BTEL 
should be checked occasionally.

The assumed focus-temperature  relation is of the form:

Focus position = FOCT0 + TEMP2F $\times$ temperature

The constants are TSKCOM variables. For Cassegrain mode, FOCT0=0.5 and
TEMP2F=0.07335; for Prime TEMP2F=$-$0.0854. The coefficients for the two focii
have opposite sign because the encoders are mounted in opposite directions.
Their values merely reflect the coefficient of expansion of the steel work, and
once determined should never again have to be measured.

The system checks that the focus is within a margin FOCERR of its demanded 
value, {\bf provided} that the parameter FTRACK is set to either 1 or 2; if
FTRACK is set to 0, the temperature compensation is disabled. FOCERR
approximately equals the value of the stopping window FOCSTP. Similarly, the 
maximum allowed change of temperature
before the focus is moved is set by a parameter TERR (currently 0.5\degs).

\subsubsection {Focusing at Prime}

At Prime Focus the telescope is focused by making a multiple exposure of a star
on the CCD detector, stepping both the focus and the telescope pointing
between exposures, and only reading out the chip at the end of the operation. 
Subsequently, the multiple image is examined to establish the
best focus (accurate to about 0.05\,{\tt mm}).  

The exposure is controlled from ADAM by an automatic procedure on the 
Instrumentation Computer. Select a faint star with no bright star near it on
the TV and move it to the appropriate side of the chip. Saturated images
should not be used for focusing. The idea is to make exposures 
of at least 5 seconds duration in order to smooth out seeing fluctuations.

If you are going to use several filters during the night, you must
first run the WHEEL command (see INT Prime Focus Manual section 3.2.4)
to tell the software the names and thicknesses
of the filters in the filter wheel. This is vital if the automatic focus
adjustment for different filters is to be used.

To start the focus procedure, type at the ADAM terminal:

\ADAM{FOCUS}

The software will prompt for three parameters to set up the focus procedure:
\begin{itemize}
\item The number of exposures, including sign, $+$ or $-$, to indicate
positive or negative focus increments, respectively.
\item The exposure time in seconds per `shot'.
\item The direction of motion of the image on the chip (N, S, E or W).
\end{itemize}
The focus is stepped by $\pm$0.1 mm between exposures, depending on the sign 
specified with the first parameter, and starts at position:

Current focus postion $\pm$0.05(n - 1) mm,                

so that the current focus is in the middle of the range.  The telescope
is offset by 20 arcsec between shots:  make sure that there is enough space on
the chip by looking at a GLANCE run or on the finder display before you
select the direction in which the telescope has to move between exposures.
It helps if the
rotator is at a cardinal point and the CCD's orientation is indicated on the
LEXIDATA display. Check the exposure time by using PHOTOM on the
results of the GLANCE.  At the beginning of the night, try seven exposures
($\pm$0.3 mm around the nominal value).  Fine-tuning during the night can be
done with 5 exposures:  differences in focus of 0.1 mm are easily
distinguishable under good conditions.  

The telescope will be returned to
its starting position at the end of the run but the focus {\bf remains at its
final value}.  A double step between the first and second image will help you to
establishes which image corresponds to which focus position.  It will
normally be obvious which images are closest to focus and these should be
measured using STARFIT option of PHOTOM in order to determine the seeing. If
the calculated focus is out of the range covered by the exposure, you will
have to extrapolate and repeat the procedure at another focus position.

The focus setting is effectively determined by the temperature (variation), for
which a reliable correction procedure is now implemented (see previous section).
In principle only one focus determination at the beginning of each prime focus 
observing run is necessary. You can carry the focus setting from night to night
if you adjust for temperature. In practice, one focus run per night is usually
done.

\subsubsection{Prime Focus Adjustment when using Filters} \label{ch11:wheel}
The filters in front of the CCD have different optical thicknesses and the
software compensates for these by refocusing the telescope when a
filter is changed.  The thickness values are communicated to the instrumentation
computer by the ADAM command WHEEL (see section 3.2.4 of the INT Prime Focus
Manual). WHEEL is run at the beginning of each Prime Focus run or whenever a
filter wheel is changed.

Therefore, after a focus exposure (previous section) and {\bf before} changing
filter, type:

\ADAM{SETFOCUS value}

where {\tt value} is your best estimate for the focus in {\tt mm} as determined 
by FOCUS. This command enables the automatic focal adjustment, and the specified
focus position value will now be use as a reference position by the software.
The focus will from now on be driven automatically to the correct position, 
and adjusted as you change filters in future, and as the temperature changes.

The automatic focus adjustment can be disabled by simply typing:

\ADAM{SETFOCUS DISABLE}

%\newpage
\parindent 0pt
\parskip 0.3cm
\hyphenation{auto-guider}
\subsection{Finder Flop Zeropoint}

The finder flexes with respect to the main telescope by $\pm$110 arcsec in 
Right Ascension and $\pm$75 arcsec in Declination.  
The flex in RA is very reproducible and can be modelled accurately, whereas the
effect in Declination is less reproducible and can be modelled only to
a (maximum) uncertainty of $\pm$45 arcsec.

The finder flex must be calibrated whenever guide stars are to be located using
the finder display.
A simple linear model of the finder flop as function of zenith distance is 
implemented in order to compensate
for the flop in the finder mounting and allows guide stars to be located
all over the sky. The zeropoint of the relation must be determined at the
beginning of the night with the telescope very close to zenith; thereafter 
adjustment is automatic.

The finder flop zeropoint determination is done by positioning a calibration
star at a reference position on the Cassegrain TV camera (normally the SNAFU
position) or on the autoguider probe at Prime. Subsequently, the same test
star is located on the finder display and its position as measured there is
used to calculate a zeropoint (offset) between the main telescope and the
finder. Because the test star is practically at zenith, the zeropoint can  be
derived directly, without invoking an approximate flop model.
 
The values of the zeropoint derived each night should be logged by the
Telescope Operator. However, note the effect of the reference position used
as implied by the procedures described below.

\subsubsection {Finder Zeropoint in Cassegrain Mode} \label{ch11:fzero}
This is most conveniently done after a SNAFU zeropoint determination, with the
telescope still pointing to the SNAFU star, provided that star is close to the
zenith. If it is not, then track a star within 2\degs\, of the zenith.
\begin{enumerate}
\item
If not already done, select Cassegrain on the TV monitor on the Control Desk.
\item
If not already there, move the  telescope in order to place the test star at 
the reference (SNAFU) position, (the centre of the CROSS HAIR).
\item
The test star must now be visible at, or very close to, the centre of the
finder field. Its current position in the finder field corresponds to the 
reference position the the Cassegrain field.
\item
Press the \button{MENU} button on the Grinnell TV Control Panel and select the
 `SET FINDER ZEROPOINT' option from the
Grinnell Menu (see Section \ref{ch10:menu}).
\item
Put the cursor cross upon the test star with the Grinnell joystick 
(on the Control Desk).
\item
Press the \button{Enter} button; this sometimes has to be repeated. 
\end{enumerate}
The finder zeropoint is now defined, and subsequently positions indicated on
the Finder TV can be translated
into autoguider GDXY co-ordinates.

\subsubsection {Zeroing the finder at Prime}
Zeroing the finder at prime is best done by acquiring the test star with the
autoguider probe.  It is best to use the SNAFU star following the
SNAFU.   Proceed as follows:

\begin{enumerate}
\item
Move the guide probe to its park position: press the guide probe
\button{PARK} button on the A\&G GUIDE PROBE panel, don't confuse
it with the TV \button{PARK} , or type at the ADAM terminal:

\ADAM{GDXY 550 900}                         
\item 
Move the rotator to position angle 180\degs\,with:

\ADAM{ROTPOS 180}
\item
Track a test star within about 2\degs\,of the zenith.   This will usually
be the SNAFU star.
\item
Select the Finder TV on the Grinnell display.
\item
Enable cursor 1, select SET FINDER zeropoint on the Grinnell MENU, and
press the \button{ENTER} button twice.  This puts up a
cursor at the reference point used before.
(If you have an ink mark from the previous night move the cursor to that
position and press \button{ENTER} again; but this isn't essential.)
\item
Select DIRECT mode to unfreeze the display.   Press the BEAMSWITCH \button{B}
button and the star will begin to move to the left of the screen.   Wait until
the star is apparently stationary and the telescope is tracking.
\item
Turn on the autoguider high voltage (with the red-white button below the
mini-monitor at the top right-hand corner of the control desk) and
select the autoguider display (A\&G) on one of the TV channels.  
\item
Do a FIELD integration (section~\ref{ch12:acquire}).
The test star should now be visible
on the autoguider display.  If it is not, do a spiral search around
the position, moving the telescope (NOT THE PROBE)
by 60 arcsec increments until you find it.
\item
When you have located the star, type AC/TEL on the User Interface
terminal and use the autoguider to ACQUIRE the star (again, see
section~\ref{ch12:acquire}).
You are moving the telescope so that the star is at the centre of 
the autoguider probe when at its \button{PARK} position.
\item
Select SET FINDER ZEROPOINT on the Grinnell again, but this time
move the cursor to the star's position on the Finder and press the
\button{ENTER} button.
\item
Put the autoguider into WAIT state and type AC/PR on the User Interface
console for later use.
\end{enumerate}

The purpose of this elaborate procedure is to mark the position
corresponding to a reference point fixed with respect to the main
telescope (the guide probe centre) on the finder field at a known
position on the sky (the zenith).  From now on, the software will
recompute the position of the CCD chip and guide probe field overlays on
source change and will always show them in their correct place on the
finder display.

%\newpage
\subsection{A\&G Box Geometry} \label{ch11:geometry}

Some aspects of the A\&G box geometry should be tested or measured occasionally
in order to ensure the optimum performance of the INT.
\begin{itemize}
\item The position of the axis of rotation of the
A\&G box turntable (the rotator centre position) in the coordinate 
system of the TV camera or guide probe. They serve to inter-relate the
coordinate systems of various A\&G box components.
\item 
The offset from the rotator centre of the reference position used for the SNAFU,
the nominal aperture,
which is usually defined as some point on the instrument or detector. This 
reference position does not coincide with the rotator centre and its offset
must be known to the system, so that the position of an object placed on the 
reference position is invariant with rotator position angle.
\end{itemize}

Different techniques are required for measuring these quantities at
Cassegrain and Prime. At Cassegrain the probes can reach the on-axis 
position but at Prime they cannot, and furthermore at Prime the detector itself
is mounted off-axis.

A simple quick-look test to check if the geometry is set up correctly consists
of placing a star on the reference position and rotating the turntable. The
star should remain stationary on the TV probe (at Cassegrain) or the CCD (at
Prime) during the process.

\subsubsection{Geometry at Cassegrain}
{\bf Rotator Centre}

The relevant Task Common parameters are TVXCTR, TVYCTR, GDXCTR and GDYCTR,
which represent the position of the rotator centre in the TV probe coordinates,
where the TV cross-wires define the probe position, and in guide probe units,
where the tracking centre defines the probe position, respectively.

These four quantities can be measured as follows:
\begin{enumerate}
\item Set the nominal aperture offset parameters AOXNOM and AOYNOM to zero.
\item Centre a star on the probe (either the TV or the autoguider).
\item Rotate the turntable to the cardinal points in turn (0\degs, 90\degs, 
      180\degs, 270\degs) and at each position angle move the probe so
      that the object is centred on the probe position (as defined above).
\item At each position note the ($x$,$y$) probe position as shown on the MIMIC
      display.
\end{enumerate}
The rotator centre position in probe coordinates is now the centre of the 
circle on which the measured points lie, {\it i.e.\/} the mean of $x$ and 
$y$ measurements if the cardinal points have been used.

{\bf Nominal Aperture Offset}

The parameters of concern here are AOXNOM and AOYNOM, which are used to 
allow for the fact that the reference position is off-axis on the rotator.
Their values must be redetermined if the A\&G box is somehow shifted.
Define the relevant reference point as follows:
\begin{enumerate}
\item For the IDS and FOS, use the intersection of the slit and the dekker.
      All dekker apertures, in the IPCS setup dekker at least, are concentric 
      apart from the comb dekker.
\item For other instruments and non common-user instruments, which do not have 
      any easy reference positions, use the TV crosswires (with the TV probe
      centred) with the flip mirror in the field position, or use the centre
      of the detector.
\end{enumerate}

Measure AOXNOM and AOYNOM as follows:
\begin {enumerate}
\item Set up the system so that the values of AOXNOM and AOYNOM are roughly
      correct, {\it i.e.\/} so that a star image stays in the field of the 
      probe when the turntable is rotated. In fact, at Cassegrain it is good 
      enough to set these parameters to zero.
\item Point the telescope to a star near zenith (and track, of course).
\item Rotate the turntable to the cardinal points and measure or mark the 
      position of the star on the TV probe with Grinnell cursors (see chapter
      \ref{ch10:tv}).
\item Determine the centre of rotation from the measured positions.
\item The {\bf corrections} to the values of AOXNOM and AOYNOM already in the 
      system are the distances between the reference position and the apparent
      centre of rotation. To determine this, put the star on the calculated 
      position of the centre of rotation and use SNAFU/A to determine the 
      offset to the reference position. Using the BEAMSWITCH \button{NOM} and
      \button{A} buttons, it should now be possible to offset and switch back
      and forth between the two positions (circle centre and reference point).
      The required values of AOXNOM and AOYNOM will then be displayed
      on the information display.
\item Enter the correct values for the parameters and test their validity by
      centering an object on the reference position and seeing whether its
      position alters as the turntable is rotated.
\end{enumerate}

\subsubsection{Geometry at Prime} \label{ch11:gprime}

The Prime Focus Camera Unit is now mounted and dismounted
with the probe `park' position defined by a mechanical template, which ensures
a very reproducible geometry from one run to another. It is therefore very
unlikely that the geometry parameters at Prime will have to be modified:
In other words, {\bf be sure to have a very good reason for changing any 
parameters.} If you find something odd with the geometry try first to attribute 
it to some other cause. In particular, check that the telescope was initialized
with the correct configuration 
file for the CCD chip mounted: either with \PE{GO INT, PFRCA, COMPILE}
or \PE{GO INT, PFGEC, COMPILE} (see section \ref{ch7:compile}).

The relevant geometry parameters at Prime, as shown in figure 
\ref{ch11:fig1}, are:

\begin{figure}[ht]
\setlength{\unitlength}{1mm}
\begin{picture}(155,123)

%Draw autoguider field
\thicklines
\put(6,68){\makebox[57mm]{N.B. Rotator Position 180$^\circ$}}
\put(6,6){\framebox(57,78){autoguider field}}
\put(22,116.9){\vector(-1,0){16}}
\put(24,115.6){Y}
\put(114.7,22){\vector(0,-1){16}}
\put(113.5,23){X}
\thinlines

%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,
%    BOY be the y-dist from the RH side of the A/G field to the rotator axis.
%    GDXCTR be the x-coord of the rotator axis in arcsec
%and GDYCTR be the y-coord of the rotator axis in arcsec

%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.
%GDXCTR = 509.5arcsec, GDYCTR = 272.5arcsec
 
\setlength{\unitlength}{0.0618mm}

%For outline of chip,
\put(1981.2,1213.8){\framebox(379,237){ }}

%Mark chip centre
\put(2170.7,1332.3){\circle*{24.25}}

%Label distances below top line of chip centre
\put(2170.7,1422.9){\vector(0,-1){64.7}}
\put(2170.7,1422.9){\line(2,5){32}}
\put(2202.7,1502.9){\line(1,0){32}}
\put(2170.7,1267.6){\vector(0,1){64.7}}
\put(2236,1482.9){TDATA(13)}
\put(2404,1412){[+ve]}

\put(1674.75,1544){\vector(-1,0){91.55}}
\put(2079.15,1544){\vector(1,0){91.55}}
\put(1698.75,1528){AOYNOM[-ve]}

%Draw the small vertical lines to mark the y-position of the chip centre
\put(2170.7,1520){\line(0,1){48}}
\put(2142.5,1163.7){chip}

%Mark rotation centre
\put(1583.2,1382.3){\circle*{24.25}}

%Label distances below top line of rot. centre 
\put(1583.2,1446.9){\vector(0,-1){64.7}}
\put(1583.2,1446.9){\line(-1,0){705}}
\put(458,1434){TDATA(11)[-ve]}
\put(1583.2,1269.6){\vector(0,1){88.7}}
\put(1551.2,1230){rot}
\put(1534,1174){axis}

%Label distances from rotation centre to "park" position
\put(1084.9,1544){\vector(-1,0){66.2}}
\put(1517,1544){\vector(1,0){66.2}}
\put(1099,1528){TDATA(12)[-ve]}

%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(1583.2,1520){\line(0,1){48}}
\put(1620.7,1267.6){\vector(0,1){64.7}}
\put(1604.7,1332.3){\line(1,0){32}}
\put(1620.7,1446.9){\vector(0,-1){64.7}}
\put(1604.7,1382.3){\line(1,0){32}}
\put(1620.7,1267.6){\line(1,0){32}}
\put(1669,1240){AOXNOM}
\put(1755,1180){[+ve]}

%Put in the y-dist between GDPARK and the chip centre, TDATA(14)
\put(1392.2,1095){TDATA(14)[+ve]}
\put(1360.2,1115){\vector(-1,0){341.5}}
\put(1829.2,1115){\vector(1,0){341.5}}
\put(2170.7,1082.65){\line(0,1){64.7}}

%Draw Origin
\put(1855.7,1891.8){\circle{81}}
\put(1900,1875){origin of GD coords}
\put(1855.7,1891.8){\vector(-1,0){272.5}}
\put(1583.2,1859.45){\line(0,1){64.7}}
\put(1855.7,1891.8){\line(0,-1){313}}
\put(1855.7,1512){\vector(0,-1){129.7}}
\put(1823.35,1382.3){\line(1,0){64.7}}
\put(1432.7,1943.5){GDYCTR[+ve]}
\put(1875.7,1698){GDXCTR[+ve]}

\setlength{\unitlength}{1mm}

%Draw compass
\put(97,32){\vector(0,1){18}}
\put(97,32){\vector(-1,0){18}}
\put(95.75,51){N}
\put(75.5,30.5){E}

%Draw top line (continuation of top of a/g field)
\put(63,84){\dashbox(83,0){ }}

%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 }
\put(64.5,76){PARK}
\end{picture}
\caption{Prime Focus geometry parameters} \label{ch11:fig1}
\end{figure}

\begin{tabular} {lll}
AOXNOM & $x$-distance of chip centre below rotator axis & in radians\\
AOYNOM & $y$-distance of chip centre from rotator axis  & in radians\\
TDATA(13) & $x$-distance of chip centre below GD~PARK & in arcseconds\\
TDATA(14) & $y$-distance of chip centre from GD~PARK  & in arcseconds\\
\end{tabular}

for the centre of the CCD chip.
The GD~PARK position is at the top right-hand corner of autoguider field.
Further, for the position of the rotator axis, we have:

\begin{tabular} {lll}
TDATA(11) & $x$-distance of rotator axis below GD~PARK & in arcseconds \\
TDATA(12) & $y$-distance of rotator axis from GD~PARK  & in arcseconds\\
GDXCTR & the rotator axis $x$-coordinate on the guide probe & in $\mu$m \\
GDYCTR & the rotator axis $y$-coordinate on the guide probe & in $\mu$m \\
\end{tabular}

The current values of these parameters can be inquired at the TCS terminal 
with the INPSCF task (see section \ref{ch7:query}).
Note that figure \ref{ch11:fig1} refers to a rotator position of 180\degs.
The sign convention is such that distances measured in $x$ below GD~PARK
(as shown in the diagram) are positive, and distances
measured in $y$ towards the left from GD~PARK are positive. This means that
AOYNOM and TDATA(12) are normally negative. The $x$ measures are close to zero,
and therefore unpredictable.

Because at prime focus both the detector and the autoguider probe are 
positioned off-axis, the default beamswitch apertures A and B are defined
such as to enable viewing of the autoguider field with the Finder TV camera 
and the CCD detector itself:

\begin{itemize}
\item The aperture A (offset) coordinates represent the offsets of the rotator 
axis from the GD~PARK position ({\it i.e.\/} the top right-hand corner of the
autoguider field) with the same sign convention as before, and equals the 
amount the telescope will move
{\bf North} and {\bf West} in order to view the autoguider field on
the Finder TV. The offset values are (must be) the same as TDATA(11), TDATA(12).
\item
The aperture B coordinates are the offsets of the GD~PARK position from the 
centre of the chip, and equal the amount the telescope will move {\bf North} and
{\bf West} in order to transfer a star at the chip centre to the {\bf parked}
autoguider probe. The offset values are (must be) the same as 
\{AOXNOM + TDATA(11)\} , --\{AOYNOM + TDATA(12)\} (the sign of the
$x$ coordinate is due to the sign convention).
\end{itemize}
The current values of the aperture offsets can be queried via the User Interface
by typing:

\UI{AD(VISE)/T(ELESCOPE)}

In order to test and re-determine the geometry parameters, proceed as follows:
\begin{enumerate}
\item
\begin{itemize}
\item Perform a GLANCE to display a brightish star at about the centre of the
chip. 
\item Do a STARFIT to measure on which pixel the star is centred. 
\item Park the autoguider probe by typing GDXY 550 900 at the ADAM terminal.
\item Type AC/TEL at the User Interface terminal. 
\item Press the BEAMSWITCH \button{B} button and acquire the star with the 
      autoguider. 
\item Press the \button{ACQ} button on the Autoguider Panel, and when the small
      circle on the monitor has disappeared, stop the autoguider and press the
      BEAMSWITCH \button{NOM} button.
\item Do a STARFIT to find on which pixel the star is centred,
      and subtract from the measured coordinates those of the chip centre.
\item Change the sign of the $x$ coordinate (because the detector $x$~coordinate
      runs against the autoguider $x$~coordinate. 
\item Convert the offset values thus obtained into arcseconds by multiplication
      with 0.74~(RCA) or 0.54~(GEC). 
\end{itemize}

We now have determined corrections to the APERTURE B offset coordinates.

\item  In order to correct the rotator centre coordinates, continue:
\begin{itemize}
\item Rotate the rotator through 180\degs. The telescope will move in an 
      attempt to keep the star centred on the chip, using the current
      (approximately correct) values of AOXNOM and AOYNOM. 
\item Perform another GLANCE and do a STARFIT to find which pixel
      the star is now on. 
\end{itemize}

The mean of the two sets of measured pixel coordinates gives the
coordinates of the {\em apparent} rotator centre. The differences between these
and the pixel coordinates of the chip centre, converted to arcseconds as in the
preceeding paragraph, and converted to radians by dividing by 206264.8, result
in the corrections to the current values of AOXNOM and AOYNOM. Update these
parameters in the sense that, if the apparent
rotation centre is above the chip centre, ADD the difference to
AOXNOM, and, if the apparent rotation centre is to the right of the chip
centre, ADD the difference to AOYNOM.

\item Calculate the new APERTURE A coordinates by subtracting the new AOXNOM
from the new APERTURE B $x$ coordinate, and by adding the new AOYNOM and the
new APERTURE B $y$ coordinate and changing the sign.

\item Calculate the new rotator axis coordinates from the following expressions:

      GDXCTR = 39.3 $\times$ (550 + TDATA(11)/0.97) \\
      GDYCTR = 39.3 $\times$ (900 + TDATA(12)/0.97)
 
\item Enter the new geometry parameters into the Telescope Controle Software
      with the INPSCF task. Subsequently, inquire the new aperture offset
      values by typing at the User Interface terminal:

      \UI{AD/T}

      and check that:

      AP/A equals TDATA(11) , TDATA(12)\\
      AP/B equals TDATA(13) , TDATA(14) 

\item Finally, communicate the new geometry to the ADAM data acquisition
      software by typing at the ADAM terminal:

      \ADAM{TV\?OFFSETS}

      and follow the prompts (Beware: defaults are not accepted; you must 
      enter values, even if they are unchanged):
      \begin{itemize}
      \item On ``offsets (arcsecs) from rotator centre to parked probes'':
            Take the Aperture A coordinatess and enter them as 
            $-$Ap.A$y$  Ap.A$x$.
      \item On ``offsets (arcsecs) from rotator centre to CCD centre'':
            Enter $-$AOYNOM +AOXNOM.
      \item Chip size in arcseconds: Enter 237 379 for the RCA chip or 
            208 312 for the GEC chip.
      \item Finally, the angle offset in milliradians between TV axes and 
            RA, Dec is currently 10.
      \end{itemize}
\end{enumerate}

You should now be able to find guide stars in any orientation of the rotator.
If you can't, report it to the Astronomy Group. 

All default values are loaded automatically at startup. If you are sure the new
geometry is correct, inform the Software Group, who will enter the new values
as defaults in the proper `.STD' configuration  file. Figure \ref{ch12:fig4}
shows the current geometry of the INT Prime Focus Unit.

\subsubsection{Rotator Mechanics}
{\bf Rotator Drive}

The rotator drive has two drive speeds. The point at which the drive changes
form fast to slow, when driving to a new position, is determined by the
Task Common variable TTS2F. This parameter may need to be adjusted occasionally.
In order to ensure that the turntable does not spontaneously
rotate when the telescope is moved (until now this has never been observed),
the system also checks that its position angle is 
within an error TTERR of the requested one. 

{\bf Encoder Zeropoint}

For the IDS/FOS check that with the rotator nominally at 
90\degs\, or 270\degs\, a star trailed in RA moves accurately along the slit.
A misalignment of 1\degs\, is easily detectable. In general, the rotator offset
is defined so that East$-$West ({\it i.e.\/} PA 90\degs\, or 270\degs) is along
one of the detector axes, in particular the IDS slit.
An allowance can subsequently be made for any offset/rotation
between this and the A\&G box coordinate system (see section
\ref{ch11:coords}). Note that any 
gross change in the rotator zeropoint requires a change in the 
software limits. (If the zeropoint changes, the limits appear to
be at a different Position Angle.)

\subsubsection{A\&G box coordinate system} \label{ch11:coords}
In theory, the A\&G box coordinate system could be misaligned with the
slit, although such an effect has not been observed until now. To check
the alignment proceed as follows:
\begin {enumerate}
\item
Set the turntable to a known position angle with respect to the slit.
\item
Move the guide probe (and/or TV - both systems should be checked) to centre on
stars of accurately known position ({\it e.g.\/} SA57 or Pleiades field) whilst
guiding on the other probe.
\item
Record all the star positions in guide probe coordinates and go away and
do an astrometric analysis to get the scale, rotation and any field 
distortions.
\end{enumerate}
{\bf NB:} at present none of this is catered for in the control software, but at
least a rotation between the A\&G box and the detector should be allowed
for so that Cassegrain imaging can be set up correctly. \vspace{1.0cm}

\newpage
\subsection{The Autoguiders}
\subsubsection{The Autoguider Focus at Cassegrain}  
The Cassegrain autoguider focus should be checked occasionally. The autoguider
is focussed manually, which involves the removal of the cover of the A\&G box
on the autoguider side, unclamping the autoguider assembly, screwing the camera
head in and out until the profiles (of a point source) produced by the 
autoguider software are as narrow as possible, reclamping the device and 
replacing the cover. All the TV filters, including `clear' are parfocal.

\subsubsection{The Autoguider Focus at Prime} \label{ch11:afocus}
The autoguider focus at Prime appears to be stable, but should
be checked occasionally. There are no autoguider (colour) filters at 
Prime, and also the autoguider does not look through the same filter as the CCD.
However, it is most unlikely that the autoguider will be detectably out of 
focus,
provided that it is initially set up at the nominal value for the (CCD) filter
in use. Nevertheless, if the images look very extended and in particular if
they have holes in their middles, then the focus should be adjusted.

The Prime autoguider focus is determined by performing an interactive
`focus run' under ADAM control. To do this, first focus the telescope with
{\bf no} filter. Subsequently, acquire a star on the autoguider and step the 
autoguider focus through a range of values around the nominal one using the 
GDFOCPOS command:

\ADAM{GDFOCPOS value}

where {\tt value} is the test focus position in {\tt mm}. Recommended values are
from  -3.3 to +3.3 {\tt mm}.  The autoguider focus drives much
more quickly than the telescope focus, so you need not worry about
starting an exposure with it out of position.
After each step, press the \button{FIELD} button
followed by \button{WAIT} on the Autoguider Panel and inspect 
the image.  Select the focus position where the profiles produced (of a point
source) are narrowest.
Do {\bf not} leave the autoguider integrating, otherwise the 
images will get mixed up. When you have discovered the correct focus position
type:

\ADAM{SETGDFOCUS value}     

where {\tt value} is your best focus position in {\tt mm}. This command
enables the automatic focus compensation, such that the autoguider focus is
changed automatically on change of (CCD) filter.
(If you do not type this command at the
beginning of an observing session, the software will assume that the
autoguider focus in the absence of a filter is +2.7 {\tt mm}). 


\subsubsection {Autoguider Gain}
The autoguider runs on its own cycle time of approximately one second duration
and presents its error signals to the PE~3220 TCS computer via the CAMAC
interface. The TNHZ task on the PE~3220 samples these errors ten times a
second; the errors are multiplied by the ``open loop'' gains AGGRA and AGGDEC
in order to scale the autoguider errors to the encoder errors to which they are
then added. The gains AGGRA and AGGDEC are derived from the expression:

Gain = (sample time)/(autoguider update time)

Autoguider gains only need to be changed
if there is a substantial alteration in the telescope response to small
offsets.

\subsection{TV cameras}
\subsubsection{TV scale} \label{ch11:tvscale}
The TV system has nominal scales for various focal setups. The actual scale
is defined by a multiplicative correction factor, which is downloaded by
the Instrument Control Software on startup. An incorrect scale of the Finder TV
will cause difficulties with the location of guide stars.

\begin{enumerate}
\item
To check a TV scale, observe 2 stars of known position (so that both are
in the field of view of the TV in question) and use the CALCULATE DISTANCE
option of the Grinnell TV system menu to compare the measured separation with 
the true value (in both coordinates). Alternatively offset the telescope in
order to displace the same star over a known distance on the TV screen.
\item
To redetermine the scales, set the scale factor to 1.0 and measure the
separation as above. The scale factor is the ratio between the true and measured
values in each coordinate.
\end{enumerate}

\subsubsection{TV Focus} \label{ch11:tvfocus}
The focus should be checked regularly for all the TV cameras. Inspect the 
profiles of unsaturated images for any unusual broadening. If a broadening
is suspected, the camera focus should be adjusted, which is done manually.

\begin{itemize}
\item Focussing the Finder TV camera is
a delicate and sometimes very cumbersome operation in which the
camera head is positioned by hand. Care should be taken not to displace the 
field centre. If in the process the
finder is moved, it must be returned to a position so that
the (prime) CCD chip and guide star overlay can be seen. The most important
requirement is that the autoguider reference point (550,900) in guide 
coordinates is on the screen. Thus, take care that:

\begin{enumerate}
\item {\bf For Cassegrain} - with TV centred, a star centred on the Cassegrain
 TV appears near the centre of the Finder TV screen.
\item {\bf For Prime} - a star recorded on the centre of the CCD chip appears
near the middle of the right-hand side.
\end{enumerate}

\item The Cassegrain TV camera should be focussed on the spectrograph slit, which
should be done under low illumination. It involves the removal of the cover
of the A\&G box on the TV side, unclamping the TV camera, screwing the head
in and out until the optimum focus is obtained, followed by reclamping the 
camera and replacing the cover.
\end{itemize}

\subsection{Dome Encoder Zeropoints}
The dome encoders 
tend to come loose and are checked daily by Duty Technicians. 

\begin{enumerate}
\item
Dome (AZOFF). Should be monitored daily by setting dome to known azimuth
(AP position on INT) and check against information display value. The
easiest way to check for vignetting when on the sky is to observe a
grossly out of focus bright star.
\item
Shutters (TOPOFF and BOTOFF). As above, use out-of-focus star to check
for and to define onset of vignetting and thus compare with telescope
zenith distance (ZD). The dome shutter encoders seem to give sensible answers
now. The shutters are usually positioned at their extreme positions, where 
the precise encoder values are not very relevant anyway.
\end{enumerate}

\subsection{Servo Constants}
The servo velocity demands for the telescope drive depend on the distance
of the telescope to the source it has to go to. There are three regimes of
velocity control ensuring that for small position error (PE) the correction is 
critically damped, whereas for larger distances the velocity increases with
increasing distance (but within the physical specification of the drive):

\begin{tabular}{rl}
$|$PE$|$ $>$ a    & velocity proportional to $\frac{PE}{\sqrt{|PE|}} $\\
a $>$ $|$PE$|$ $>$ b   &velocity proportional to PE (linear regime)\\
b $>$ $|$PE$|$ $>$ 0   &velocity =
                     k$_{1}$.PE + k$_{2}\int PE\,dt$ + k$_{3}$(v$_d$ - v$_a$)\\
\end{tabular}

where k$_{1}$ and k$_{2}$ are linear gains and k$_{3}$ is the integrator gain. 
The relevant Task Common variables are TGLH, TGLD, HGI and DGI. These
are likely to need resetting after any alteration to the telescope
drives. The objective is to obtain parameter values for which a critically 
damped offset with minimal fluctuations is realised when tracking. 
There is a certain art involved in assessing the correct values, the telescope
performance being judged from the TV, position error displays and the
velocity demand display. A basic requirement is, of course,
that the telescope should track with approximately zero position error.
The autoguider errors are logged by the Telescope Control Software and the
servo constants are adjusted until the guiding errors are minimised.
If any of the DACs on the axes get changed, then the parameters HDAC and/or 
DDAC will need to be checked. 
\vfill
\eject
