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

1
Near Infrared Spectro-polarimeter (NIRSP) Design
Status May 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 36 µm
• Spatial sampling 0.046 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 2.25 m
• Geometric pupil diameter 56 mm
• Linear dispersion 5.75 µm/pm at ? 1.565 µm
• Grating width 300 mm (projected width 135 mm)
• Diffraction image width at focal plane is 26 µm
• Slit diffraction overfills grating 12 light
  loss at l 1.565
• Ruling frequency 87 mm-1, or TBD
• Detector like Hawaii-2 RG, 1 ? 2 mosaic
• 18 µm pixels, bin 2 ? 2
• Field 95 arc sec
• Spectral sampling 6.3 pm
• Resolving power 200,000 at l 1.565
• Increase f to 3m and w to 400 mm gives R270,000

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Multiple Slits

• At Coudé, the FPA width covers 6.4 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 up to six slits.

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NIRSP Nasmyth

• Feed at f13.08 (same as Gregorian image)
• Spatial scale 0.254 mm / arc sec
• Field lens near entrance slit
• Makes beam telecentric
• Slit nominal 108 µm for coronal observations, 18
  µm for disk observations
• Spatial sampling 0.425 arc sec to 0.07 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 Nasmyth, more

• Collimator focal length 1.3 m
• Geometric pupil diameter 99 mm
• Linear dispersion 3.32 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 20 light
  loss at l 1.565 with 18 µm slit (3 loss with
  108 µm slit)
• Ruling frequency 87 mm-1, or TBD
• Detector like Hawaii-2 RG, 1 ? 2 mosaic
• 18 µm pixels, bin 6 ? 6 (or more!) for corona
• Field 290 arc sec
• Spectral sampling 5.4 pm photosphere 22 pm
  corona 1.07 µm
• Resolving power 225,000 at l 1.565

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Nasmyth Scanning

• How do we scan the image at Nasmyth?
• Scan the telescope?
• Scan the spectrograph?
• Add a scanning mirror to the relay optics?

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Tip-tilt at Nasmyth

• Nasmyth observations need tip-tilt correction
• T-T options
• Active secondary
• Active second relay mirror
• Image moves 2.5 mm for 10 arc sec pupil moves
  1.2 mm.
• Add a pupil mirror to the relay optics

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Nasmyth Context Image

• How do we get a context image?
• Multiple slits make slit viewer difficult.
• Dichroic or bandpass ahead of slit?
• Infrared, or visible, context image?