Instrumentation for Extremely Large Telescopes

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Instrumentation for Extremely Large Telescopes
Hubert of Liege 656-727

• Patron Saint of
• Hunters
• Archers
• Liege, Belgium
• Forest Workers
• Precision Instrument Makers
• Machinists
• Mad Dogs

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Production and Use of VPH Gratings

• Purpose To relate our experiences with VPH
  gratings in the Goodman Laboratory to
  builders of instrumentation
• for ELTs.
• VPH Grating Design
• VPH Grating Production
• VPH Gratings in the Goodman Spectrograph

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Volume Phase Holographic (VPH) Gratings
Under the geometry pictured, the Bragg condition
is   ,   where m is the integer order,
l is the wavelength, L is the grating period, n
is the refractive index, and a is the incident
angle
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More Generally
Courtesy Sam Barden
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Designing a Grating
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Goodman Laboaratory Design Software
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VPH Manufacturing Facilities
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Multiplex VPH Design
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VPHs in useThe Goodman Spectrograph
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Standard Features

• All-refracting imaging spectrograph operating
  from 320 to 850 nm
• 5' x 5' imaging field with 0.146"/pixel sampling
• R 900 to 9,000 (3 pixel sampling) at full
  throughput
• Maximum R 14,000 (2 pixel sampling) at lower
  throughput
• Multi-slit mask and atmospheric dispersion
  corrector option for multi-object spectroscopy of
  100 targets in 3' x 5' field

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Challenging Specifications

• Articulated camera (90 deg) to use transmission
  VPH gratings
• Flexure limit of 1/10 pixel for 360 rotation at
  Nasmyth
• Image degradation lt 0.16, broadband, at field
  corners
• Imaging throughput maximum of 75, inclusive of
  CCD (68 at 350 nm).
• Spectroscopic throughput maximum of 65,
  inclusive of CCD (50 at 350 nm)

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Construction Photos
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Delivered June 2004
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Flexure Tests
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First Lab Image
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Conclusion

• VPH gratings offer advantages in size,
  efficiency, scattered light, but not without
  introducing some new difficulties

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• Extra mechanical or optical complexity
• Mitigated by dual purpose articulation
• Vendors Limited
• Kaiser Optical Systems Inc. (75 mm max)
• Ralcon (now Wasatch Photonics)
• Centre Spatial de Liege (CSL)
• Manufacturing Problems
• Curved substrates due to glue shrinkage (KOSI)
• Wrong Line Density (KOSI)
• Coatings Removed (RALCON)
• Bandwidth-efficiency variations across pupil
  (everybody)
• Who knows?
• Film Limitations
• Maximum index modulation 0.1
• Absorption in UV/IR

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The Goodman Spectrograph Deployment
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Optical Components in Chapel Hill
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Mechanical Components in Chapel Hill
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Shipping and Receiving
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In the Gemini Labs
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Into the SOAR Nasmyth Cage
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To the Mountain
La Salchicha Grande
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Waiting..
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First Light
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Problems
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