Cryogenic instrumentation

Besides the complexity of building a cryostat which allows the cooling of the instrument to the required temperature there are various engineering aspects to cryogenics that makes the design rather complex.

In order to be able to cool in instrument to these special temperatures (80 K and below) any convection within the instrument should be eliminated, also to avoid contamination because of freezing up onto the instrument. This means that the instrument needs to be placed in a vacuum environment (.. mbar) and dedicated designs are compulsary.

A big influence is the variation of the material properties at different temperatures, some examples:

  • Coefficient of Thermal Expansion (CTE), important for the opto-mechanical design; all materials shrink during cool-down e.g. the mounting of optics where at minimum two different materials are involved. The shrinkage rate varies between materials and even alloys and on top of that behavior is non-linear below ca. 100 K; the total shrinkage of for instance Aluminium (6061) is 0.42% when cooled from room temperature to 40 K. This means a 4.2 mm reduction on every meter of instrument.
  • Thermal conductivity (λ), important for the thermal design
  • Specific Heat Capacity, important for the thermal design, together with the thermal conductivity they determine themal behaviour during temperature change; e.g. a cooking pan grip, Bakelite compared to Aluminium. The first is much more effective in insulating the heat from the pan.
  • Index of Refraction (IoR), important for the design of optics; lenses, prisms etc.
  • Young's Modulus (E), important for the mechanical engineering

The general knowledge regarding the material properties at cryogenic temperatures is, unfortunately, very limited.

Developments

Through the years the group developed various dedicated engineering solutions for a variety of typical (cryogenic) instrumentation issues. These solutions vary from ready-made designs to special skills, methodologies and strategies. These special developments provide the means to realize the optimum instrument at reasonable time and costs.

  • Extreme light-weighting
  • Dedicated cryogenic mechanisms/guiding systems
    • Detector focus system
    • Tip/tilt focus unit
    • Tip/tilt mirror
    • Sliders
    • Cryogenic shutter
  • Cryogenic Mirror Mounts
    • VISIR mount
      • TPM
      • SPM
    • X-shooter mount
    • MIRI mount
  • Cryogenic Transmission optics Mounts
    • X-shooter prism mount
    • SPIFFI lens mount
    • KROMP mount
  • 5 axis simultaneous milling
  • On assembly manufacturing
  • Polishing of flat surfaces
  • Polishing of complex (double) curved surfaces

Design strategies

No adjustment strategy - In general what can move, will move, especially within a cryogenic environment. A much more stable instrument is realized through reduction of active alignment possibilities. This way the alignment accuracy depends strongly on the design, tolerance analysis and detailed knowledge of the material behavior and manufacturing process. This no alignment strategy is only possible when several important design drivers are taken into account.

Integral design strategy - Integral design is most important when the no adjustment strategy is used. By taking into account all the engineering aspects during the whole instrument development, an optimal instrument can be accomplished. This is not necessary the best solution for one of the disciplines involved, but a cost effective instrument is developed, where manufacturing and verification are taken into account by means of e.g. special mounting strategies and integrated verification features.

Modular design - By dividing the instrument in various logical modules the instrument a flexible instrument is created, where the best interfaces are chosen for maximum accuracy and additional interfaces are identified for e.g. verification tools. When late changes are demanded in one module only this module is affected and redesign is easier than on a whole instrument. Also concurrent engineering can take place on the various modules, this way largely reducing total lead time of the instrument.

Analysis -The no adjustment philosophy and modular design requires early error distribution and understanding. This is supported by extensive analyses of all effects within the instrument; optical analyses (Sensitivity, Monte Carlo), mechanical analyses (Tolerances, Finite Element FEA) and also thermal analysis (Transient, steady state, Finite Element FEA) and more.

Monolithic - Each bolted connection reduces stiffness from a structural point of view and increases insulation from a thermal point of view, unnecessary mounting needs to be avoided. Creating structures from a monolithic piece, the number of internal interfaces can be reduced as well as the effects of these interfaces on the structural integrity and favorable thermal behavior.

Homogeneity - Extensive research resulted in a number of "standard" solutions for engineering issues, like; stress free mounting of optics during all circumstances, stress free mounting of different materials without accuracy loss etc. You can use these specific solutions at various places for various problems. This simplifies the design which results in a logical low risk instrument, which is easy to handle and maintain.

Design: Kuenst.    Development: Dripl.    © 2020 ASTRON