Near Infrared Spectro-polarimeter (NIRSP) Design Status August 2004

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Near Infrared Spectro-polarimeter (NIRSP) Design
Status August 2004

• Don Mickey
• Jeff Kuhn
• Haosheng Lin

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NIRSP Modules

• Two spectrograph modules, one on coudé platform
  and the other at Nasmyth.
• Common basic design common camera.
• Coudé high spatial resolution, multi-camera
  capable. Warm optics on optical bench. AO
  observations of limited field.
• Nasmyth Coronal observations and wide-field disk
  observations. Cold optics.

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NIRSP Requirements
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NIRSP Coudé Unit

• Feed at f40
• Spatial scale 0.776 mm / arc sec
• Field lens near entrance slit (if required)
• Makes beam telecentric
• Slit nominal 72 µm to match telescope spot size
• Spatial sampling 0.092 arc sec
• Grating is R2 (tan ? 2) echelle
• Angular dispersion 4/? µrad/pm (near-Littrow)
• Reflective Littrow layout, off-axis parabola
• Coma-corrected at any grating angle
• Minimal astigmatism
• Spectrograph magnification 1.0

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NIRSP Coudé, more

• Collimator focal length 4.0 m
• Geometric pupil diameter 100 mm
• Linear dispersion 10.2 µm/pm at ? 1.565 µm
• Grating width 300 mm (projected width 135 mm)
• Diffraction image width at focal plane is 76 µm
  (line width)
• Slit diffraction overfills grating 15 light
  loss at l 1.565
• Ruling frequency TBD
• Detector TBD
• Assume 24 µm pixels, 20 x 20 mm, array 4h x 1w
• Field 105 arc sec along slit
• Spectral sampling 7.0 pm
• Resolving power 222,000 at l 1.565
• Rmax is 340,000 can narrow slit and bin 2x,
  grating spill goes up to 23.

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Coudé layout
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Coudé Spot Diagram
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Multiple Slits

• At Coudé, the FPA width covers 2 nm at l 1.565.
  The optical quality is good over the entire
  field, so we might think of using a narrow-band
  blocker and two slits.
• This design is adaptable to different pixel
  sizes, and the focal length could be shortened if
  needed.

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NIRSP Nasmyth

• Feed at f13.08 (same as Gregorian image)
• Spatial scale 0.254 mm / arc sec
• Build spectrograph 2x faster, and compact focal
  length one meter, f6.5.
• Permits fiber-feed IFU
• Field lens near entrance slit
• Makes beam telecentric
• Slit nominal 96 µm for coronal observations, 24
  µm for disk observations
• Spatial sampling 0.38 arc sec to 0.09 arc sec
• Grating is R2 (tan ? 2) echelle
• Angular dispersion 4/? µrad/pm (near-Littrow)
• Reflective Littrow layout, off-axis parabola

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Nasmyth Layout, f6.5
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NIRSP Nasmyth, more

• Collimator focal length 1.0 m
• Geometric pupil diameter 76 mm
• Linear dispersion 2.56 µm/pm at ? 1.565 µm
• Grating width 300 mm (projected width 135 mm)
• Diffraction image width at focal plane is 15 µm
• Slit diffraction overfills grating 8 light loss
  at l 1.565 with 24 µm slit (2 loss with 96 µm
  slit)
• Ruling frequency TBD
• Detector 20 x 20 mm, array 1 x 4
• 24 µm pixels, bin 4 ? 6 (or more!) for corona
• Field 315 arc sec
• Spectral sampling 9.4 pm photosphere 26 pm
  corona 1.07 µm
• Resolving power 167,000 at l 1.565

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Nasmyth Encircled Energy, f6
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Nasmyth Encircled Energy, f13
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Scanning

• Scan the slit.
• Image moves on the detector, but the image is
  curved anyway so we have to locate the line in
  data reduction.
• Scan range is about 10 mm at coudé.
• At Nasmyth, dispersion is 2.56 mm/nm, so theres
  room for five or six slits if 1 nm filters are
  available. The scan range is also smaller, 2.5
  mm.
• Optical performance supports scan over entire
  field.

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Wish List

• More room for pre-slit activities. Can we modify
  the Nasmyth relay to get the image 500 mm beyond
  instrument attachment, instead of 210?
• Fast camera
• Fast modulator

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Polarimetry

• Polarizing beam-splitter at slit is possible. Can
  get 5 mm separation in a Savart plate. Takes a
  cylindrical lens (f62m) in f13. This would help
  if we need to do slow modulation.
• Nasmyth relay has modest polarization effect.
• Should we consider PEM phase-locked modulator?
  May have some hope. Spherical aberration from 2
  cm CaF2 in f13 is negligible.