wduncanboulder.nist.gov

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Wide field optical design for SCUBA-2
(wduncan_at_boulder.nist.gov)
W. D. Duncan (NIST) Eli Atad, Tully Peacock, John
Harris, Wayne Holland, Damian Audley Helen
McGregor, David Montgomery, David Gostick,
(UKATC),
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Problem areas

• adding wide field instruments to telescopes which
  are not designed for wide field operation
• Cassegrain full fields of view are physically
  large. For JCMT 600 mm dia. at F12 Cassegrain
  focus detectors are much smaller than this, are
  cooled to 100mK and are absorber coupled
• need re-imaging optics to allow stray light
  rejection and match field to array size (100 mm
  sq).
• Wide field instrument too large for JCMT
  instrument cabin
• Nasmyth focus chosen does not co-rotate
  with cabin - relay optics must shear against each
  other.

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SCUBA 2 Optics
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Problem areas and Criteria

• Telescope is not perfect Strehl ratio 75.
• needs optics to have Strehls gt 90 in order to
  not degrade this still further
• Low field distortion needed to maximize image
  quality
• goal of 2 or less
• Diffraction of the beam is not negligible at near
  mm wavelengths
• need to size optics appropriately to avoid
  spillover
• Elevation bearing tube is nominally too small to
  pass field with simple focus in tube
• Low transmission loss in optics
• need all reflecting design hard to
  control aberrations

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Problem areas and Criteria

• Alignment and control of shape and position of
  cold mirrors
• good mechanical design, stress relief and
  highly polished zones on mirrors to check
  alignment

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Location on telescope
Elevation bearing
Tertiary mirror
Cryostat
Antenna floor
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Optical path
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Field distortion at 60 degrees
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Field distortion variation
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Cold stop image quality
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Performance as designed

• Used CODE V to do major part of optics design
  after initial concept in Zemax
• Optimized over 80 to 20 degree elevation range
• Strehls gt98 over that range
• Field distortion lt2 and very small change (few
  percent with elevation angle)
• Good cold stop imaging for control of stray light
  (cold stop at 1K)

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Performance as designed

• CODE V reverse ray trace and field calculation at
  each mirror to set spillover a lt 20 db for beams
  at the edge of the field.

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The mirrors

• 9 mirrors general aspheres and large

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The mirrors

• lt1 micron surface roughness
• lt20 microns p-v adherence to surface form over
  length scales of 100 500 mm.
• Surfaces accurately placed relative to mounting
  points
• Surfaces measured after machining and polishing
  and profiles used to evaluate
• lots of cash to TPD in Holland

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Key Features Optics
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N5 cold mirror
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1K Box Assembly

• Covers Removed for Clarity

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1K Box Service

• Service Items
• Array Module
• Cold Stop Filter
• Dichrioc

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FPU MODULE

• Array Module
• Band Pass Filter

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