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Дата изменения: Thu Jul 28 18:24:56 2005 Дата индексирования: Thu Jan 15 11:28:13 2009 Кодировка: Поисковые слова: saturn |
The drawing presented in Figure 6
suggests a cryo-mechanical
design based on a cylindrical vacuum shell, with total size dictated by
total length of the optical system and wheels diameter. The
operations at the derotated Nasmyth focus implies the rotation of the
instrument itself, making unpractical the use of liquid cryogenics for
cooling and mantaining the operative temperature. Instead, we based the
cryogenic system on a closed cycle cooler. A number of external motors
take care of wheels and slides positioning, depending on the requirements
of the specific observing mode.
The first wheel, the closest to the entrance wheel,
hosts the fiels stops, several slits for long slit spectroscopy, and the
small slits for high resolution spectroscopy. Very close to this wheel,
a movable holder allows the insertion of the mask for polarimetric
measurements. On the pupil plane, a second motor-driven holder puts in place
the pupil stop when needed. Two wheels host the filters, the grisms and
and other optical analyzers. The
fourth wheel mounts the array of cameras. We decided to have a degree of
freedom also on the position of the array detector along the optical
axis; an external motor allows very fine adjustments, with a total run
of the order of 0.2 mm.
The motors are micro-stepping devices, which can sustain the required low
torque with speed of several thousand turn per minute without stalling.
This allows fast homing when performing the initialization procedure,
preserving the necessary resolution and positioning accuracy, which is
of the order of 6 x 104 on 360° .
The detector, in thermal contact with the cold plate, will have an active
temperature control, with an RDT sensor and a heater, both connected to
an independent temperature controller. An activated charcoal
cryo-adsorption pump will be installed on the cold work surface to
guarantee a good vacuum in the presence of outgassing or small leaks for
periods greater than 180 days (the classic zeolite molecular sieve is
working fine in our IR instrumentation). For maintenance purposes, the
molecular sieve should be equipped with a suitable heating element and
thermal switch. As a further safety consideration, it is possible to
connect a turbomolecular pump, which can be used also during
observations.
Electrical heaters on the cold plate will allow to warm up
the cryostat in a period of few hours. A set of heaters
is preferred to a large one, both for minimizing hot spots and for
reliability purposes. The total heating power should be of the order of
300-400 W; an external temperature controller will be provided as a part
of the control system.
A set of vacuum gauge will monitor internal pressure, one that covers
the atmospheric to 10-3 torr pressure range; a second ionization
gauge will monitor the high vacuum range. All the measurements of
temperature and pressure will be transmitted to the control computer,
where the software can control the regularity of operational parameters
on a short timescale.
Mechanical design
The mechanical design includes several interacting units,
that are
The vacuum vessel is a cylindrical tank with two end plates, having its
simmetry axis orthogonal to the TNG exit beam. This shape has the
advantage of low weight and high stiffness, associated to reasonable
cost. The cylindrical case is rigidly connected to the Nasmyth adapter
flange with short beams and, in turn, supports the internal cooled
structure via thermally isolating, rigid elements. The radiation shield
is connected to the cold structure; the volume between the pupil plane
and the detector is protected from stray light by means of a second
radiation shield. The internal structure supports all the optical
functions, that is the four wheels (mounted on ball bearings), the
collimator, and the detector, keeping them at the required relative
positions.
Cryogenic system
In normal operation, the working temperature of about ~ 80 K will be
maintained by one closed cycle cooler. Because of the relatively large
mass of the instrument, cooling down to the operating temperature is
best achieved by means of a continuous flow of liquid nitrogen inside a
pipe welded to the optical bench; during normal operation
the liquid nitrogen is not necessary. Figure 6: Cryo-mechanical drawing.