6. Mechanics and Cryogenics

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.

• Mechanical design

  The mechanical design includes several interacting units, that are
  
  
  • vacuum vessel;
  • radiation shield;
  • internal support structure for cold optics and detector;
  • cold optics, focal masks and filter wheels;
  • step motors with reduction gear for slide and wheels driving.
  
  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.

  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° .





• 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.

  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.

  Figure 6: Cryo-mechanical drawing.

  

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