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The Visible Spectro-Polarimeter (ViSP)

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 Kn ölker (P.I.).

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The Visible Spectro-Polarimeter (ViSP)

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  1. 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.)

  2. 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 ∆β 2) Beware of “Anamorphic” Magnification: R = R0·cos(α)/cos(β) 3) Mirrors & lenses = flexibility! Grating (1 of 3)

  3. Wavelength range: 380-1083**nm, three lines covered simultaneously Spatial resolution: 2x ATST diffraction limit (0.04 arc seconds @ 600nm) Spatial FOV: 2 arc minutes (goal of 2.8) Spectral Resolution: 2.5pm @ 600nm→R=170,000 (goal of 240,000) Polarimetric Capability: 1x10-4 IC sensitivity, 5x10-4 accuracy Temporal Cadence: 1x10-3 Ic polarimetry in 10 seconds @ full spectral/spatial res. Simultaneous operation with: Near-IR Spectro-Polarimeter (NIRSP) Visible Tunable Filter (VTF) Visible Broadband Imager (VBI) ViSP Specifications 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!

  4. The Grating Equation: nλ/d = sin(α) + sin(β) (n = ‘order’ of diffraction) Echelle Grating (Φ>45˚) Φ≈80˚ d When α+β = 2·Φ Efficiency is maximized Φ≈20˚ Diffraction Gratings:Terminology and Conventions Substrate (glass, <λ/10) Figure from Gray: “Observation and Analysis Of Stellar Photospheres”

  5. Ruling and ‘n’ don’t appear!! 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 R=170,000 @ 600nm with a 30cm grating @ 60˚, W<0.16 arc seconds • Prominence use case @ 1 arc second → R=27,200 ! • Sin(β)→use echelle gratings

  6. Gratings as InterferometricFilters

  7. Examples of real ViSP configurations: 255 lpmm – photosphere/chromosphere 404 lpmm - Prominence 5876 5173 6302 6563 10830 8542 255 lpmm – photosphere/chromosphere 295 lpmm - Filament 5173 8542 8542 6563 6302 10830

  8. 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 Conclusions:

  9. 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 telescope’s polarizing properties

  10. A “Casini-Tomczyk” Modulator:A stack of 4 ‘ordinary’ retarders in series

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