
\chapter{The Intermediate Dispersion Spectrograph}

\section{Overview}

 The Intermediate Dispersion Spectrograph (IDS) is the principal common-user
 instrument at the Cassegrain focus of the INT.
 The IDS design is based on the principle of a folded-input, flat-bed 
 instrument. The ``straight through'' position is occupied by the Faint 
 Object Spectrograph (FOS-1; see the Observer's Guide). Figure
 3.1 shows the overall optical layout; selection of either IDS or FOS-1
 operation is determined by manual movement of a single optical element, the 
folding
 prism, so that both spectrographs, and indeed both cameras of the IDS, albeit
 with different detectors, can be used consecutively during the same night.
 Facilities for acquisition and guiding, such as calibration and comparison
 lamps, above-slit neutral-density and colour filters, the autoguider and a
 TV camera, are all located in a separate
 A\&G unit to which the spectrograph is attached (Section 3.2).
 Dekker masks, slit assembly and below-slit filters are within the main body
 of the spectrograph itself. 



\subsection{Design}

 The layout of the IDS spectrograph is shown in Figure 3.1. The following
 components can be discerned:

\begin{figure}[p]
\vskip 20cm
\caption[The IDS]{The Intermediate Dispersion Spectrograph}
\end{figure} 

\begin{itemize} 
\item {\bf Slit and dekker unit.} The same slit and dekker unit is shared by the
    IDS and FOS-1. The maximum slit length is 44.3 mm (4 arcmin); 
    the slit width is variable from 40 $\mu$m (0.216 arcsec) to 1.745 mm 
    (9.43 arcsec) in steps of 5 $\mu$m (0.027 arcsec).  The entire slit 
    assembly can be removed, if required.  Dekker plates are interchangeable 
    in sets; each individual set consisting of 8 apertures and a clear 
position. Several standard dekkers, incorporating
    single slots, pairs of slots, a ``comb'', and a coronographic set-up, are
    always available. 
   
    A multislit unit is also available. This consists of ten parallel 
    slitlets, each 16\sec\ long with a 7\sec\ gap between each pair and with 
    a fixed width of 270 $\mu$m (1.46\sec \ at the Cassegrain focus). Each 
    slit can be moved perpendicular to the slit orientation by up to 
    1.6\min \ in either direction. With FOS-1 a useful field of 
    4\min\ by 3\min\ can be covered, whereas the IDS has a smaller field 
    of 4\min\  by 1\min .  A program is available for finding the optimum 
arrangement and typically one needs 20 or 30 objects in the field to make use 
    of all 10 slits. Astrometry to within 0.3\sec is needed to use the unit 
    effectively. There is only one module that has to be preset using
    a Coradograph xy table in the INT building. Clusters of objects spread 
    over less than 1\min\ are less likely to benefit as they are too small. 
    To install the multislit assembly, the
conventional Cass cluster slit assembly is replaced by a similar one 
which holds the multislit module or a long-slit equivalent. This instrument
change requires lowering the IDS and is a daytime operation.  The long slit 
module allows observations to continue while the multislit module is being 
preset with the Coradograph for a new field. The device should not be regarded
as a general purpose multiobject coupler but rather as a special purpose 
addition to the INT Cassegrain cluster instrumentation. 
 See {\it Using the multislit unit at the 
INT Cassegrain focus (R.~Ellis et~al.~1986)}  for more information.
 
\item {\bf Filter slides.} In addition to the above-slit filter positions in 
    the A\&G unit, there are 3 filter positions available 
    below the slit for both colour and ND filters. One of
    these positions is always clear. Five colour filters are currently 
    available: UG11, BG18, BG24 and BG38, measuring 
    19 mm $\times$ 60 mm $\times$ 2 mm and made from Schott glass. The 
    wavelength dependence of the
    transmission of these filters is shown in figures 2.5 and 2.6. The copper 
    sulphate filter cuts out the part of the spectrum from 600 -- 650 nm 
    redwards and is usually employed to block the red leak of the UG11 filter
or to remove the FOS-1 first order. 
    The neutral density filters 
    below the slit provide a choice of ND = 0.5, 1.0 and 1.5.
    Note that the use of filters below the slit makes it necessary to
    refocus the spectrograph.

\item {\bf Collimator.} The collimator is of 1275 mm focal length, giving a collimated
    beam with a diameter of 85 mm. There is a choice of three different 
    collimators, with different coatings for maximum reflectivity in the 
    wavelength range of interest. The efficiency of the collimators as a 
    function of wavelength is shown in Figure~2.3. The collimators can be 
    exchanged during the night by the support staff.
    The collimator is mounted on a remotely-driven sliding carriage, making
    it possible to focus the spectrograph under computer control. 

\item {\bf Cameras.} Two cameras are available with the Intermediate Dispersion 
    Spectrograph. Both are of the folded short-Schmidt type, described 
    in detail by Wynne {\it (M.~N.~R.~A.~S.,{\bf 180}, 485, 1977)}. The focal 
    lengths of the two cameras are 235 mm and 500 mm. 

    Along the direction of the slit (i.e. perpendicular to the dispersion 
    direction), the focal lengths of 235 and 500 mm imply slit to detector 
    reduction factors of 5.43 and 2.55 respectively, and hence scales 
    at the detector of 29.4 and 13.8 arcsec mm$^{-1}$.

    Along the dispersion direction, the slit to detector reduction factor
    is more complicated, since it depends on the grating angle. Tables 3.1 
    and 3.2 give the slit width in arcsec which projects to 30~$\mu$m at the 
    detector, when each grating is used at its central wavelength.

    Tables 3.1 and 3.2 also list the dispersion provided by each grating and 
    camera combination. It can be seen that a wide range of dispersions
    is available, from 7 \AA~mm$^{-1}$ to 270 \AA~mm$^{-1}$.

\begin{table}
\caption{Characteristics of the IDS gratings with the 235 mm camera}
\vskip 0.5cm
\begin{tabular}{lccccccc}
Name$^1$ & Origin$^2$ & Ruling           & Blaze & Efficiency$^3$ & Central    & Dispersion$^4$ & Entrance slit \\
         &            &(lines mm$^{-1}$) & (\AA) &  (\%)         & wavelength & (\AA\ mm$^{-1}$) & for 30 $\mu$m \\
         &            &                  &        &               &  (\AA)    &                & at detector \\
         &            &                  &        &               &            &                &  (arcsec) \\
\\
R150V &   B+L   & 150    &  5250    & 66   &  5500     & 271.3      &    1.03   \\
R300V &   NPL   & 300    &  5460    & 72   &  5500     & 138.5      &    1.05 \\
R400B &   B+L   & 400    &  3900    & 50   &  4000     & 104.5      &    1.06 \\
R400V &   B+L   & 400    &  5100    & 72   &  5500     & 104.5      &    1.08 \\
R400R &   B+L   & 400    &  7250    & 59   &  8000     & 104.4      &    1.11 \\
R600R &   B+L   & 600    &  6700    & 65   &  7000     &  69.8      &    1.16 \\ 
R600IR &   B+L  & 600    &  10000   & 70   &  8500     &  70.2      &    1.19 \\
R632V &   NPL   & 632    &  5460    & 72   &  5500     &  66.5      &    1.11 \\
R831R  &  B+L   & 831    &  7500    & 65   &  8000     &  50.7      &    1.27 \\
R900V  &  B+L   & 900    &  5100    & 69   &  5000     &  46.4      &    1.17 \\
R1200U &  J-Y   & 1200   &  3500    & 68   &  3500     &  35.3      &    1.15 \\
R1200B &  J-Y   & 1200   &  4000    & 76   &  3500     &  35.3      &    1.15 \\
R1200Y &  B+L   & 1200   &  6000    & 68   &  6500     &  35.2      &    1.33 \\
R1200R &  B+L   & 1200   &  8000    & 66   &  7000$^4$  &  34.8      &    1.37 \\
H1800V & PTR/NPL & 1800  &  5300    & 64   &  5000$^{5,6}$ & 23.2      &    1.40 \\
H2400B &  J-Y   & 2400   &  4000    & 60   &  3500$^{5,7}$ & 17.5      &    1.37 
\end{tabular}
\vskip 0.5cm
1) Prefix R for ruled grating; prefix H for holographic grating. \\*
2) Key: NPL = National Physical Laboratory; B+L = Bausch and Lomb; \\*
\hspace*{0.5cm} J-Y = Jobin-Yvon; PTR/NPL = PTR replica from NPL master. \\*
3) Absolute efficiency values measured at RGO. \\*
4) At low and intermediate dispersions the dispersion changes little with central wavelength. \\*
5) Near maximum useful angle of incidence \\*
6) Optimized for 5000-7000 \AA. \\*
7) Optimized for 3500-5500 \AA. \\
\end{table}

\begin{table}
\caption{Characteristics of the IDS gratings with the 500 mm camera}
\vskip 0.5cm
\begin{tabular}{lccccccc}
Name$^1$ & Origin$^2$ & Ruling           & Blaze & Efficiency$^3$ & Central    & Dispersion$^4$ & Entrance slit \\
         &            &(lines mm$^{-1}$  ) & (\AA) &  (\%)         & wavelength & (\AA\ mm$^{-1}$) & for 30 $\mu$m \\
         &            &                  &        &               &  (\AA)    &                & at detector \\
         &            &                  &        &               &            &                &  (arcsec) \\
\\
R150V  & B+L   &  150    & 5250    & 66    & 5500     & 132.2           & 0.51  \\
R300V  & NPL   &  300    & 5460    & 72    & 5500     &  66.1           & 0.52 \\
R400B  & B+L   &  400    & 3900    & 50    & 4000     &  49.6           & 0.52 \\
R400V  & B+L   &  400    & 5100    & 72    & 5500     &  49.7           & 0.52 \\
R400R  & B+L   &  400    & 7250    & 59    & 8000     &  49.9           & 0.54 \\
R600R  & B+L   &  600    & 6700    & 65    & 7000     &  33.3           & 0.56 \\
R600IR & B+L   &  600    & 10000   & 70    & 8500     &  33.4           & 0.58 \\
R632V  & NPL   &  632    & 5460    & 72    & 5000     &  31.6           & 0.54 \\
R831R  & B+L   &  831    & 7500    & 65    & 8000     &  23.9           & 0.59 \\
R900V  & B+L   &  900    & 5100    & 69    & 5000     &  22.1           & 0.56 \\
R1200U & J-Y   &  1200   & 3500    & 68    & 3500     &  16.7           & 0.55 \\
R1200B & J-Y   &  1200   & 4000    & 76    & 3500     &  16.7           & 0.55 \\
R1200Y & B+L   &  1200   & 6000    & 68    & 6500     &  16.4           & 0.61 \\
R1200R & B+L   &  1200   & 8000    & 66    & 8000     &  15.5           & 0.63 \\
H1800V & PTR/NPL & 1800  & 5300    & 64    & 5500$^6$  &  10.5           & 0.65 \\
       &       &         &         &       & 7000     &   9.8           & 0.73 \\
H2400B & J-Y   &  2400   & 4000    & 60    & 3500$^{5,7}$ &  8.1           & 0.62 \\
       &       &         &         &       & 6000     &   6.8           & 0.82
\end{tabular}
\vskip 0.5cm
1) Prefix R for ruled grating; prefix H for holographic grating. \\*
2) Key: NPL = National Physical Laboratory; B+L = Bausch and Lomb; \\*
\hspace*{0.5cm} J-Y = Jobin-Yvon; PTR/NPL = PTR replica from NPL master. \\*
3) Absolute efficiency values measured at RGO. \\*
4) At low and intermediate dispersions the dispersion changes little with central wavelength. \\*
5) Near maximum useful angle of incidence \\*
6) Optimized for 5000-7000 \AA. \\*
7) Optimized for 3500-5500 \AA. \\
\end{table}

\item {\bf Gratings.} The gratings have a 
    ruled area of 102 mm $\times$ 128 mm. A wide selection is available,
    and some of the important characteristics of each grating are summarised
    in Tables 3.1 and 3.2. The efficiency of the gratings as a function of 
    wavelength is shown in Figure 2.4. Grating changes can be made during 
    the night by support staff. 

\item {\bf Cross-disperser.} A transmission grism, R60B with 60 lines~mm$^{-1}$
    and 
    blazed at 4800~\AA, can be inserted into the collimated beam in the 
    235 mm camera to provide a cross-dispersion option. This is 
   used in conjunction with an R150 echelle grating, which is blazed at 
   2.7~$\mu$m. The FOS-1 dekker mask is used to limit the slitlength to 25
   arcsec in order to avoid order confusion.
   The wavelength range for the CCD and dispersion provided by this grating in 
each order, for a grating angle of 60\DEG, are summarised in Table 3.3. There
are no gaps in wavelength coverage with the IPCS.
   The CCD is not normally considered the detector best suited to use with the 
   cross-disperser, because part of the wavelength coverage is lost between
   the orders due to the physical size of the chip. However, it is usually
possible to adjust the grating angle in the range 59.5\DEG - 60.5\DEG \ so as to
ensure that all interesting spectral features are placed in the chip.
Besides the 
   advantages typical of CCDs, the combination of GEC chip and cross-disperser, 
   seems to have a higher sensitivity than the combination of
   IPCS and cross-disperser.  13 counts/sec/\AA\ were obtained for a 
   m$_V$=11.9 star at an airmass of 1.32 with the CCD. 1 count/sec/\AA\ 
therefore, can be expected for a m$_V$=14.8 at an airmass of 1.0. This count
rate corresponds to a m$_V$=13.9 with the IPCS, but of course the IPCS's 
quantum efficiency is much lower at 5500 \AA\ than that of the CCD.
The central wavelength and wavelength coverage in \AA\ can be obtained for 
each order $n$ as
$${\Delta\lambda = 3586/n}$$
and
$${\lambda_c = {[26364 - 2166.5*(Angle-60\DEG)]\over n}}$$
\begin{table}[htp]
\begin{center}
\caption{Characteristics of the IDS R150 echelle grating}
\begin{tabular}{cccc}
    Order &  Central  &   Wavelength  &  Dispersion \\
          & wavelength &      range   &  (\AA\ mm$^{-1}$) \\
          &   (\AA)  &       (\AA)  \\
\\ 
      3   &   8790    &  8190 - 9385  &       94.0 \\      
      4   &   6590    &  6145 - 7035  &       70.5 \\
      5   &   5275    &  4920 - 5630  &       56.4 \\
      6   &   4395    &  4100 - 4690  &       47.0 \\
      7   &   3766    &  3510 - 4020  &       40.3 \\
      8   &   3295    &  3070 - 3520  &       35.2 \\
      9   &   2930    &  up to  3130  &       31.3 
\end{tabular}
\end{center}
\end{table}
\newpage
 
\item {\bf Detectors.}  The IDS can be used with two detectors: the IPCS or
    a coated GEC P8603 CCD.
    Changing the detector on a particular camera can only be carried out
    during the day. However, it is possible to have a CCD set
    up on one camera and the IPCS on the other, and operate the two systems
    consecutively during the same night. Switching from one camera to the other
    only requires the grating to be rotated by 180\DEG\ around the optical
axis (or exchanged), and the control software to be reinitialized. Note that if
you only specify a detector to be on one camera in your proposal, you cannot 
guarantee what will be on the other (e.g.~it might be in use on another 
telescope). If you need both cameras, you have to specify that well before
your run.

\end{itemize}

\subsection{Performance}

\subsubsection{Wavelength range and resolution} 

    Table 3.4 summarises the wavelength
   range and resolution for the IDS gratings using a coated
   GEC CCD and either the 235 mm or the 500 mm cameras. With the exception of 
the 1800 and 2400 
   lines~mm$^{-1}$ gratings, the wavelength coverage does not significantly 
vary with the central wavelength. 

   The tables give for each configuration the wavelength range, the
   dispersion in \AA~pixel$^{-1}$, and the slit width projecting
   to one pixel at the detector.  For proper sampling, the wavelength 
resolution cannot be less than about 2 pixels. To match the slit to the 
resolution ot the spectrograph, a slit width should 
   be used which projects to about 2 pixels at the detector. If a 
   wider slit is used to increase the total light collected by the instrument, 
   this will degrade the wavelength resolution. This might be a good strategy 
in bad seeing conditions or for extended objects.

    One point worth noting is that if a grating were to be mounted with the wrong 
    orientation, (i.e.~the blaze
    directed towards the other camera) it would result in a dramatic decrease
    of throughput. The
    dispersion will be affected too, and, in fact, a resolution improvement
    of about 20\% can be obtained at high anamorphic magnification, but only 
    at the cost of a drastic loss of efficiency (see J.V.~Wall 1984, Gemini
    \# 12).


\begin{table}
\begin{center}
\caption{IDS Wavelength range and resolution}
\vskip 0.5cm
\begin{tabular}{lccccccc}
 &   & \multicolumn{6}{c}{GEC CCD} \\
 &   & \multicolumn{6}{c}{578 22 $\mu$m pixels} \\

 &   & \multicolumn{3}{c}{235 mm camera} & \multicolumn{3}{c}{500 mm camera} \\
 Grating  & $\lambda_c$ & (1)   & (2)  & (3) & (1)   & (2)  & (3) \\
\\
 R150V   & 5500 & 3430 & 5.97 & 0.76   & 1680 & 2.91 & 0.37 \\
 R300V   & 5500   & 1760 & 3.05 & 0.77 & 840 & 1.45 & 0.38 \\
 R400B   & 4000   & 1320 & 2.30 & 0.78 & 625 & 1.09 & 0.37 \\ 
 R400V   & 5500   & 1320 & 2.30 & 0.79 & 625 & 1.09 & 0.38 \\
 R400R   & 8000   & 1320 & 2.30 & 0.81 & 630 & 1.10 & 0.40 \\
 R600R   & 7000   &  880 & 1.54 & 0.85 & 420 & 0.73 & 0.41 \\
 R600IR  & 8500   &  885 & 1.54 & 0.87 & 420 & 0.73 & 0.43 \\
 R632V   & 5500   &  840 & 1.46 & 0.81 & 400 & 0.70 & 0.40 \\
 R831R   & 8000   &  640 & 1.12 & 0.93 & 300 & 0.53 & 0.43 \\
 R900V   & 5000   &  585 & 1.02 & 0.86 & 280 & 0.49 & 0.41 \\
 R1200U  & 3500   &  445 & 0.78 & 0.84 & 210 & 0.37 & 0.40 \\
 R1200B  & 4000   &  445 & 0.78 & 0.84 & 210 & 0.37 & 0.40 \\
 R1200Y  & 6500   &  445 & 0.77 & 0.98 & 205 & 0.36 & 0.45 \\
 R1200R  & 7000   &  440 & 0.77 & 1.00 & 195 & 0.34 & 0.46 \\
 H1800V  & 5500   &  290 & 0.51 & 1.03 & 130 & 0.23 & 0.48 \\
         & 7000   &      &      &      & 120 & 0.22 & 0.54 \\
 H2400B  & 3500   &  220 & 0.38 & 1.00 & 100 & 0.18 & 0.45 \\
         & 6000   &      &      &      & $\;$85 & 0.15 & 0.60
\end{tabular}
\end{center}
\vskip 0.5cm
\hspace*{2.0cm} 1) Wavelength range (\AA) \\*
\hspace*{2.0cm} 2) Dispersion (\AA\ pixel$^{-1}$) \\*
\hspace*{2.0cm} 3) Slit width projecting to 1 detector pixel (arcsec) \\
\end{table}


\subsubsection{Throughput} 


    The efficiency of the cameras can be split 
    into two components, one due to the geometry of the cameras (e.g.
    size of the central obstruction) and one due to losses at each
    coated surface. For small grating angles, the geometric efficiencies
    of the 235 mm and 500 mm cameras are about 67 \% and 73 \% 
    respectively. At large grating angles (greater than about 44\DEG\ and 
    60\DEG\
    for the 235 and 500 mm cameras respectively), the geometric efficiency drops off
    rapidly due to overfilling of the grating. This will for example occur 
    with the 2400B grating when used in the red.

     Each camera contains two aluminium mirrors, with
    a reflectivity of about 85 \%, and four air/glass surfaces
    with single layer anti-reflection magnesium fluoride coating, each 
    of which has a reflectivity of about 2 \% at 5500 \AA.  We therefore
    expect the 235 mm and 500 mm cameras to have total effective 
    efficiencies of 45 \% and 49 \% respectively.


\subsubsection{Performance of cross-dispersion option}

   The wavelength coverage provided by the cross dispersion option
   depends on the physical size of the detector; it is given in Table~3.3.
   The current generation of CCD detectors are too small to
   cover the wavelength range at once, though it is usually possible to cover 
   most important emission lines by a careful choice of grating angle.

    The measured efficiency of echelle and the cross-disperser is discussed 
    in  Section 3.1.1.


\subsection{Calibration facilities}

Comparison lamps for the IDS are mounted in the INT Cassegrain A\&G box, 
which are described in some detail in the next Section (and Appendix C). 

\section{The Cassegrain acquisition and guiding (A\&G) box}

At the rear of the INT primary mirror cell is the Cassegrain turntable which
can be rotated by 365 degrees, and onto which the A\&G
unit is mounted.  The Cassegrain A\&G Unit of the INT has been 
designed primarily for use with the IDS and FOS-1.  Nevertheless, 
the overall philosophy of the
design has been to incorporate a range of facilities which
would be required by most instruments operating at the Cassegrain
focus.  The full field is 20 arcmin in diameter, but the 
comparison lamps and filters are only useful over a more restricted field 
of 4 arcmin, matching that of the IDS.

\begin{figure}[p]
\vskip 20cm
\caption[The INT Cassegrain field viewing system]
{The optical system for viewing the acquisition field at the Cassegrain
focus of the INT}
\end{figure}

The following functions are provided:

\begin{itemize}
\item {\bf Acquisition:} Figure 3.2 shows the details of the optical 
      system for viewing the acquisition field.  A Westinghouse extended 
      S20 TV camera views a nominal 1.8  $\times$ 1.8
      arcmin element of the 20 arcmin diameter unvignetted Cassegrain field (on-
      or off- axis).  The TV display includes labels giving 
      orientation and scale. 

      A useful facility available is the option to store the TV images,
      so as to take advantage of periods of exceptionally good
      seeing to record, for later use, high spatial resolution images of the
     objects under investigation; the images (called ``TV archive images'' 
     can also serve as a reference to show the slit position for extended objects.
     At the user's request, they can be obtained by the Telescope Operator.

      A filter wheel is available for the TV camera allowing up to four 
      circular filters to be loaded.  These filters are not
      immediately accessible, as it is envisaged that most users would not
      require them to be replaced. The filters currently available are blue
      (BG28), green (BG38), red (RG630) and clear (UBK7). (See Figure 2.5).


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

\item  {\bf Autoguider.} An autoguider is available which allows offset guiding
      using stars anywhere in a 90 arcmin$^2$ field. The area available
      for guide stars is shown in Figure 3.3. Guide stars should have
      magnitudes in the range 7 $<$ m$_v$ $<$ 13. 

\begin{figure}[p]
\vskip 18cm
\caption[Allowed region for guide stars at the INT Cassegrain focus]
{The area within which guide stars may be chosen for use with 
the autoguider at the Cassegrain focus of the INT.   Stars
may be selected in the shaded area.   If the autoguider probe 
is moved closer to the centre of the field, then the slit is
obscured.   Partial obscuration occurs in the area shaded with
dashed lines.   The field outside the circle
is heavily vignetted, but bright stars may 
be usable.   The orientation of the diagram on the sky is
given for the instrument rotator in PA = 0\DEG. At other PA's,
the diagram must be rotated anticlockwise by the PA.}
\end{figure}

      For blind setting of the telescope, for locating the spectrograph slit 
      relative to a guide star, or for controlling small telescope offsets to 
make a spectroscopic map of an extended object, guide stars  should be 
selected in advance of the observing
      run using Figure 3.3. If this is done, and the ($\alpha$, $\delta$) of the
guide stars is known, then it is possible to save considerable amounts of 
observing time. Alternatively, an overlay showing the IDS 
      slit and the area within
      which guide stars may be selected can be superimposed on the TV image
      of the finder field. A guide star can then be selected visually, 
      and the A\&G Unit TV and/or autoguider probes can be directed to 
      that position.  
      Care must be taken to avoid selecting a guide star
      so positioned in the field that the autoguider probe obstructs the on-axis
      star-beam. The Telescope Operator will normally set up the autoguider.

\item {\bf Comparison lamps.} The A\&G Unit includes reference sources for 
     calibrating the wavelength scale and the photometric response of 
     the attached instrument. The lamps are housed in
     an integrating sphere which can contain up to two hollow-cathode lamps and
     one tungsten lamp; more than one source at a time can
     illuminate the slit. 
     Illumination of the slit by the comparison lamps is
     achieved by movement of a dedicated mirror.

     The standard set-up consists of a tungsten lamp 
     as continuum source, and copper-argon and copper-neon discharge 
     lamps for wavelength calibration. A further selection of 
     the following discharge lamps is offered: Th-Ar, deuterium,
     Fe-Ne, Fe-Ar, Al/Ca/Mg-Ne, Na/K-Ne, Cu-He, and helium. Not all the lamps
are well documented. Arc maps are 
     available for the Cu-Ar (La Palma Technical Note 52), Cu-Ne (La Palma 
     Technical Note 35) and Th-Ar lamps (AAO User Manual 4; ESO Scientific 
     Report 6). See Appendix C.

     The comparison lamps have two dedicated ND filter wheels,
     providing a wide range of ND filtering (La Palma
     Technical Note 22). These are rarely needed when observing with the CCD.
 
\item {\bf Filters. }

     Two main trays are available for ND and colour filters common
     to the star and lamp beams. The trays can be
     loaded with up to 5 filters each (plus a clear position).
     The filters are 45 mm $\times$ 60 mm $\times$ 3 mm in size.
     The two main filter trays are easily
     accessible, allowing rapid replacement of the trays'
     contents.  Care must be exercised when removing the filter trays, since 
     the two trays (colour and ND) are {\em not } interchangeable. This is only
to be performed by the support staff.

     The colour filters normally available are UG1, BG28, BG38, GG385, WG360,
     GG495, RG630, RG695 and RG830. The wavelength dependence of transmission
     of these filters is given in Figure~2.5. The following ND filters
     are available; ND = 0, 0.3, 0.6, 0.9, 1.0, 1.2, 1.5, 1.8, 2.0 and 3.0.
\end{itemize}
 


%\begin{figure}[p]
%\vskip 2cm
%\caption{Efficiency curves for the IDS gratings}
%\end{figure}

%\begin{figure}[hp]
%\vskip 2cm
%\caption[A typical spectrum from the IDS cross-disperser]
%{A Cu-Ar arc exposure using the IDS cross-disperser. Orders 5 to 9
% can be seen.}
%\end{figure}




\vfill

