1. HIGH ALTITUDE OBSERVATORY NCAR – Boulder, CO Boulder Solar Days – Friday, March 20th, 2009 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. HIGH ALTITUDE OBSERVATORY NCAR – Boulder, CO Boulder Solar Days – Friday, March 20th, 2009 Basic ViSP Configuration: Camera Lenses (3) Fold mirrors (3) Entrance Slit (telescope Focus) Camera 1 Camera 3 Camera 2 Grating (1 of 3) Collimating Lens (moves with Slit) 1) Fold beams to get small angles ∆β 2) Beware of “Anamorphic” Magnification: R = R 0 ·cos(α)/cos(β) 3) Mirrors & lenses = flexibility! Scan direction (2m F.L.)

3. HIGH ALTITUDE OBSERVATORY NCAR – Boulder, CO Boulder Solar Days – Friday, March 20th, 2009 ViSP Specifications 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 I C sensitivity, 5x10 -4 accuracy Temporal Cadence: 1x10 -3 I c polarimetry in 10 seconds @ 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!

4. HIGH ALTITUDE OBSERVATORY NCAR – Boulder, CO Boulder Solar Days – Friday, March 20th, 2009 Diffraction Gratings: Terminology and Conventions Figure from Gray: “Observation and Analysis Of Stellar Photospheres” Substrate (glass, <λ/10) Φ≈20˚ When α+β = 2 · Φ Efficiency is maximized Echelle Grating (Φ>45˚) Φ≈80˚ The Grating Equation: nλ/d = sin(α) + sin(β) (n = ‘order’ of diffraction) d

5. HIGH ALTITUDE OBSERVATORY NCAR – Boulder, CO Boulder Solar Days – Friday, March 20th, 2009 The Problem with Spectrographs on Large Telescopes…. 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 Sin(β)→use echelle gratings 2 x (# arc seconds/radian) Ruling and ‘n’ don’t appear!! Prominence use case @ 1 arc second → R=27,200 !

6. HIGH ALTITUDE OBSERVATORY NCAR – Boulder, CO Boulder Solar Days – Friday, March 20th, 2009 Gratings as Interferometric Filters

7. HIGH ALTITUDE OBSERVATORY NCAR – Boulder, CO Boulder Solar Days – Friday, March 20th, 2009 Examples of real ViSP configurations: 5173 5876 6563 8542 6563 6302 5173 8542 6302 8542 10830 255 lpmm – photosphere/chromosphere295 lpmm - Filament 404 lpmm - Prominence255 lpmm – photosphere/chromosphere

8. HIGH ALTITUDE OBSERVATORY NCAR – Boulder, CO Boulder Solar Days – Friday, March 20th, 2009 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

9. HIGH ALTITUDE OBSERVATORY NCAR – Boulder, CO Boulder Solar Days – Friday, March 20th, 2009 ViSP Polarimetry in 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). (10 second integration time) Calculation by Phil Judge (1600fps) (100fps) ViSP will require careful calibration of telescope’s polarizing properties

10. HIGH ALTITUDE OBSERVATORY NCAR – Boulder, CO Boulder Solar Days – Friday, March 20th, 2009 A “Casini-Tomczyk” Modulator: A stack of 4 ‘ordinary’ retarders in series