The Visible SpectroPolarimeter

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The Visible Spectro-Polarimeter (ViSP) A
First-Light Instrument for the Advanced
Technology Solar Telescope (ATST) Pete
Nelson David Elmore (now NSO), Roberto Casini,
Steven Tomczyk, Alfred de Wijn, Rebecca Centeno
Elliot Greg Card, Rich Summers, Alice Lecinski,
Michael Knölker (P.I.)
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Basic ViSP Configuration
Fold mirrors (3)
Scan direction
Camera Lenses (3)
Entrance Slit (telescope Focus)
(2m F.L.)
Camera 1
Collimating Lens (moves with Slit)
Camera 2
Camera 3
1) Fold beams to get small angles ?ß 2) Beware of
Anamorphic Magnification R
R0cos(a)/cos(ß) 3) Mirrors lenses
flexibility!
Grating (1 of 3)
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ViSP Specifications

• Wavelength range 380-1083nm, three
  lines covered simultaneously
• Spatial resolution 2x ATST diffraction
  limit (0.04 arc seconds _at_ 600nm)
• Spatial FOV 2 arc minutes
  (goal of 2.8)
• Spectral Resolution 2.5pm _at_
  600nm?R170,000 (goal of 240,000)
• Polarimetric Capability 1x10-4 IC sensitivity,
  5x10-4 accuracy
• Temporal Cadence 1x10-3 Ic polarimetry in
  10 seconds _at_ full spectral/spatial res.
• Simultaneous operation with
• Near-IR Spectro-Polarimeter (NIRSP)
• Visible Tunable Filter (VTF)
• Visible Broadband Imager (VBI)

Not all specifications can be met simultaneously!
Example (2 arc minutes / 0.04 arc seconds)
3,200 slit positions. x 10s integration 8.8
hours!
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Diffraction GratingsTerminology and Conventions
Substrate (glass, lt?/10)
Figure from Gray Observation and Analysis Of
Stellar Photospheres
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The Problem with Spectrographson Large
Telescopes.
2 x ( arc seconds/radian)

• Same spectrograph at ATST will have 6x lower
  resolution than at the Dunn.

• Ex for R170,000 _at_ 600nm with a 30cm grating _at_
  60, Wlt0.16 arc seconds

• Prominence use case _at_ 1 arc second ? R27,200 !

• Sin(ß)?use echelle gratings

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Gratings as InterferometricFilters
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Examples of real ViSP configurations
404 lpmm - Prominence
255 lpmm photosphere/chromosphere
5876
5173
6563
6302
10830
8542
255 lpmm photosphere/chromosphere
295 lpmm - Filament
5173
8542
8542
6563
6302
10830
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Conclusions

• ViSP will make the highest spatial resolution
  spectro-polarimetric maps ever made of the Sun
• Capable of measuring 3 lines simultaneously
• Meets all requirements defined by the ATST
  Science Working Group
• Versatile optical table layout allows rapid
  reconfiguration

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ViSP Polarimetryin a Nutshell

• ViSP will use a rotating waveplate modulator at
  Gregorian focus
• Cameras will take images at 100Hz
• Polarizing beam splitters at cameras will be used
  as analyzers. Both beams will make an image of
  the spectrum on the camera (dual-beam
  polarimetry).

(100fps)
(1600fps)
(10 second integration time) Calculation by Phil
Judge
ViSP will require careful calibration of
telescopes polarizing properties
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A Casini-Tomczyk ModulatorA stack of 4
ordinary retarders in series