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The A&G unit is part of the Wide-Field Photographic Camera (WFC), and
can only be used with that instrument. See the ``1-m camera users manual''
(La Palma Users' Manual V) for
more details.
The following facilities are provided:
- Acquisition. There is no acquisition TV camera for viewing
the field direct. Instead, a guiding probe views a
quadrant of the field about 0.6 square degrees in size
(the area of the sky recorded on the plate is approximately 1.77
square degrees). The quadrant sampled is determined by the
orientation of the instrument turntable. The probe is mounted on
(x,y) slides which have a maximum travel of 120 mm. Each axis is
encoded in units corresponding to a linear motion of 0.1 mm
(2.6 arcsec). For visual acquisition and guidance an eyepiece
with double crosswires and red illumination is available to view the
field through the guiding probe. The eyepiece can be rotated with
a gooseneck to different
positions for ease of viewing. It is also possible to view the full 90
arcmin field through a field eyepiece which can be mounted in place of the
plate-holder.
- Autoguider. Once a suitable guide star has been acquired,
its image can be
directed onto the autoguider by means of a movable prism. The autoguider is
equipped with two filter wheels, each having three positions plus clear, for
colour (U, J, F) and neutral density (ND = 0.5, 1.0, 2.0) filters
respectively. The detector is an FW-130 image dissector
with an S-20 photocathode.
It is possible to find guide stars at the telescope
using an eyepiece as described above. However, this is a
very inefficient use of observing time, and
probe positions should therefore always be computed in advance,
preferably with GSS.
- Comparison lamps. A 25 step spot-sensitometer is used for calibration
purposes. During an exposure light is projected continuously through
the calibration wedge and the filter onto one corner of the plate.
A series of apertures is available to optimize the intensity of the
calibration source according to the exposure time and plate/filter
combination in use. Each plate is identified by projecting onto an
unexposed corner the information carried in the top left-hand corner
of the corresponding observing card, consisting of: (i) telescope
identification; (ii) plate number; (iii) plate--centre coordinates.
- Filters. The following broad-band filters are available: UG1, GG 385,
GG 395, GG 455, GG 495, RG 630 and RG 715. The filters are 250
200 4 mm
and are coated with single-layer broad-band anti-reflection coating.
Relevant transmission curves, supplied by the manufacturers for uncoated
filters, are collected in Appendix . Additionally, sub-frame assemblies
are provided to hold 100 mm 100 mm interference filters. Two filters
centred at 5007 Å and 6563 Å are currently available.
Next: The Guide Star
Up: Acquisition and guiding
Previous: Acquisition unit for
Tue Aug 15 16:42:46 BST 1995