the visible spectro-polarimeter (visp)

HIGH ALTITUDE OBSERVATORYNCAR – 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 NelsonDavid Elmore (now NSO), Roberto Casini, Steven Tomczyk, Alfred de Wijn, Rebecca Centeno Elliot

Greg Card, Rich Summers, Alice Lecinski, Michael Knölker (P.I.)



Basic ViSP Configuration:

CameraLenses (3)

Fold mirrors (3)

Entrance Slit(telescope Focus)

Camera 1

Camera 3

Camera 2

Grating(1 of 3)

CollimatingLens

(moves with Slit)

1) Fold beams to get small angles ∆β2) Beware of “Anamorphic” Magnification: R = R0·cos(α)/cos(β)3) Mirrors & lenses = flexibility!

Scan direction

(2m F.L.)



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 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)

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!



Diffraction Gratings:Terminology and Conventions

Figure from Gray: “Observation and AnalysisOf Stellar Photospheres”

Substrate(glass, <λ/10)

Φ≈20˚

When α+β = 2·ΦEfficiency is maximized

Echelle Grating(Φ>45˚)

Φ≈80˚

The Grating Equation:nλ/d = sin(α) + sin(β)

(n = ‘order’ of diffraction)

d



The Problem with Spectrographson Large Telescopes….

TELSLIT

GRATINGSLIT DW

WR

"

)sin("412530

•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 !



Gratings as InterferometricFilters



Examples of real ViSP configurations:

51735876

6563

8542

6563

6302

51738542

6302

8542

10830

10830

255 lpmm – photosphere/chromosphere 295 lpmm - Filament

404 lpmm - Prominence255 lpmm – photosphere/chromosphere



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



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).

(10 second integration time)Calculation by Phil Judge

(1600fps)(100fps)

ViSP will require careful calibration of telescope’s polarizing properties



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

the visible spectro-polarimeter (visp)

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