The Utrecht Echelle Spectrograph (UES), on the drive side Nasmyth platform
of the WHT, has a dedicated A&G box. This has been
upgraded to allow guiding on objects down to V=17 in good seeing.
The features of the A&G system are:-
- Acquisition.
Two field viewing options are provided: autoguide fibre and TV slit viewing.
- Typically the pointing of the WHT is better than 1.5 arcsec rms.
Hence, with good target coordinates, normal practice is to acquire
the target directly onto the slitviewing fibre, which has a field of
18 arsec.
- Alternatively it is possible to initially acquire the target using the TV slitviewer
option. Here the entire 5 arcmin field (at a scale of about
1 arcsec/pixel) can be viewed off the slit jaws
onto a Westinghouse ISEC TV. This operation is relatively slow --- the
A&G mirror must be moved so that the TV pickoff mirror views the slit,
the slitangle must be set to zero degrees, all main beam filter
wheels must be set to clear postion, and the dekker must be removed.
The field observed will even then be partially obstructed by the
slit-viewing probe, in particular the central part of the TV field will
be obstructed; once the object has been identified, a standard
aperture offset will be needed to bring it onto the centre of the
slit.
This slit viewing option cannot be used during the exposure.
- Slit viewing during an observation. The image on the slit jaws
can be monitored during an exposure using the slit viewing probe. This
fibre-optic light guide relays the image to the CCD autoguider camera
(as used for acquisition). The field is 18 arcsec square.
It is possible to autoguide on the slit image,
and this is the recommended method if no
off-axis guide star is available or if the derotation optics are not in
use.
- Off Axis Autoguiding An alternative method of autoguiding is to use an
offset guide star. This option is implemented using a pierced autoguider
mirror and movable guide probe. The beam emerging from the telescope
(via the derotation optics) is intercepted by the autoguider mirror. In
the resulting reflected focal plane there is a fibre-optic guide probe,
which is used to relay stellar images to a CCD autoguider. The
autoguider mirror can be moved transversely in order to place different
parts of the mirror in the beam, allowing for different modes of
operation. Three of these modes use holes cut in the autoguider mirror
to allow for unvignetted fields of view at the slit of approximately 15
arcsec (point source observations, no sky), 70 arcsec
(shortslit --- allows for good sky subtraction and is the normal option
when observing in echelle mode) and 160 arcsec (longslit ---
monochromatic imaging observations).
The field remaining for offset guide stars in these three cases is
approximately 16, 13 and 8 arcmin
respectively, allowing 50 per
cent vignetting of the autoguider beam.
For very bright objects, it is possible to attenuate the light with
one of the 11 autoguider filters, which include a range of neutral density
(ND0.5, ND2.0, ND3.0, ND5.0) and colour filters (RG9, RG610,
GG455, VG9, BG28, UG11, BK7).
- Comparison lamps. The A&G unit provides for up to 6 different
calibration lamps. However as of 1994 the only lamps provided
are the thorium-argon lamp for wavelength calibration, and quartz-halogen
for flat-fielding. To achieve high brightness,
the system uses a diffuser rather than an integrating sphere.
Correct pupil imaging is provided. Each lamp has a shutter,
allowing it to be left on to stabilise prior to a calibration
exposure. The calibration lamp unit is detachable and can be used
for laboratory tests of other WHT instruments when the UES is
not in use.
The calibration unit has two filter wheels, one containing a selection of
colour filters (FG15, FG13, RG6, UG3+GG475, RG780, KG5, LB120, FG3, F512),
the other a range of neutral density filters (ND0.5--ND4.0).
However, the use of the
ND filters is not recommended because they introduce some
fringing to the calibration spectra.
- Main Beam Filters. There are separate filter wheels for ND
and colour filters. The wheels have 10 positions and take filters up to
80 mm in diameter (5 arcmin) and with a thickness of 10 mm (ND) or 25
mm (colour). The normally mounted set of colour filters is: UG11,
UG1, BG28, BG38, GG385, GG420, GG495, OG590, RG715, RG830.
Also, any of the TAURUS narrow band interference
filters can be used with UES. (These would be needed to isolate
individual orders when observing with UES in long slit mode.) Note:
Advance notice is needed to use TAURUS filters, as
the correct mount may have to be manufactured. Each
wheel also has a clear position which is used when viewing the slit
directly with the CCD camera.
It has been found that the ND filters introduce an unacceptable
degree of fringing, hence their use is discouraged. It should be noted
that it is unlikely that ND filters would be needed when observing
astronomical sources.
- Polarisation calibration. There is a filter wheel for polarisers
or other auxiliaries.
This has 10 positions and takes polarisers up to 80mm in diameter
and 15mm in thickness. To date (1994) no spectropolarimetry
facility is planned for UES.
The GSS software may be used to locate suitable guide stars for the UES
It is quite likely that targets at high
galactic latitude will not have suitable guide stars in the catalogue
due to the small
size of the unvignetted field. In this case guide stars can sometimes
be found by searching.
arcsecs.
. The derotation optics
give a full field of 5 arcmin diameter.
The following functions are provided:
