Mounting and cooling a large BaF2 lens

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Mounting and cooling a large BaF2 lens
A. Montané, R. Probst, E. Pearson, R.
Galvez NOAO-La Serena and NOAO-Tucson

• ISPI is an infrared camera that will operate in a
  sidelooking position on the 4-m Blanco telescope
  at CTIO. We present our design solution for
  cryogenic mounting of the large first
  collimator lens (right hand lens in cutaway
  drawing below).

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Lens parameters and design goals

• 190 mm diameter, 40 mm center thickness, 6 mm
  edge thickness, weight 3.2 kg
• Assemble and align optical system warm
  operating temperature of first lens is 150 K
• Thermal cycle in 24 hours or less
• Use spring finger cell for advantage of design
  heritage
• BaF2 is highly sensitive to mechanical and
  thermal shock

Must avoid local thermal and mechanical stress
during cooldown and warmup cycles
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Modeling lens/cell interaction with FEA

• Lab setup glass dummy lens with temperature
  sensors in cryocooled test dewar
• Attempt to match lab data with mechanical and
  thermal FEA models, extend results to BaF2
• Unreliable for local stress predictions at lens
  edge and lens/cell interface

• Conductive cooling through the thin edge judged
  to have high risk
• Radiative cooling model shows low stress and
  acceptable cooldown time
• Cell shall provide mechanical support, thermal
  isolation

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Cell Components
Spring
Insulation ring
Spring clamp ring
Slotted ring or spring fingers
Insulation at lens/cell interfaces
Spacer
Lens
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Spring finger cell
Insulation ring
Clamp ring
Spring
Spacer
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Mechanical support Spring parameters

• Warm fingers support lens under gravity but
  exert no additional force spring ring seats lens
• Cooling with cell cold, lens warm (worst case)
  uniform pressure around rim 46 gm/mm2 high
  frictional force
• Cold intermediate case
• Spring ring provides axial restoring force when
  warm. This prevents cumulative lens creep.
• FEA graphics show finger deflection warm and
  cold, in inches

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• Thermal isolation design verification in situ

Measure thermal behavior of cell dummy lens in
ISPI dewar
Center temperature sensor
Edge sensor
Frame (cell) sensor
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Results and present status

• Radiative cooling dominates for lens
• Still have some conductive cooling to cell
• Local thermal stress at cell/lens interface
  within acceptable limits (delta-temperature per
  unit time)
• Time behavior roughly as predicted by radiative
  model
• Cells for remaining lenses follow same general
  design
• Ready to test in situ with actual lens
• Delayed by apparent degradation of BaF2 lens in
  storage
• Lens under evaluation by NOAO, vendor

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Image of BaF2 lens in present state
The apparent crazing is internal , not a
surface phenomenon.