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HIGH ALTITUDE OBSERVATORY
NCAR – Boulder, CO
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.)
Boulder Solar Days – Friday, March 20th, 2009
Basic ViSP Configuration:
HIGH ALTITUDE OBSERVATORY
NCAR – Boulder, CO
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!
Boulder Solar Days – Friday, March 20th, 2009
Grating
(1 of 3)
HIGH ALTITUDE OBSERVATORY
NCAR – Boulder, CO
ViSP Specifications
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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!
Boulder Solar Days – Friday, March 20th, 2009
HIGH ALTITUDE OBSERVATORY
NCAR – Boulder, CO
Diffraction Gratings:
Terminology and Conventions
Substrate
(glass, <λ/10)
Echelle Grating
(Φ>45˚)
The Grating Equation:
nλ/d = sin(α) + sin(β)
(n = ‘order’ of diffraction)
Φ≈80˚
d
Φ≈20˚
When α+β = 2·Φ
Efficiency is maximized
Figure from Gray: “Observation and Analysis
Of Stellar Photospheres”
Boulder Solar Days – Friday, March 20th, 2009
HIGH ALTITUDE OBSERVATORY
NCAR – Boulder, CO
The Problem with Spectrographs
on Large Telescopes….
Ruling and ‘n’
don’t appear!!
2 x (# arc seconds/radian)
RSLIT
412530" sin(  ) WGRATING

W "SLIT DTEL
•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
Boulder Solar Days – Friday, March 20th, 2009
Gratings as Interferometric
Filters
Boulder Solar Days – Friday, March 20th, 2009
HIGH ALTITUDE OBSERVATORY
NCAR – Boulder, CO
HIGH ALTITUDE OBSERVATORY
NCAR – Boulder, CO
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
Boulder Solar Days – Friday, March 20th, 2009
Conclusions:
HIGH ALTITUDE OBSERVATORY
NCAR – Boulder, CO
• ViSP will make the highest spatial resolution spectropolarimetric 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
Boulder Solar Days – Friday, March 20th, 2009
ViSP Polarimetry
in a Nutshell:
HIGH ALTITUDE OBSERVATORY
NCAR – Boulder, CO
•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
Boulder Solar Days – Friday, March 20th, 2009
A “Casini-Tomczyk” Modulator:
A stack of 4 ‘ordinary’ retarders in series
Boulder Solar Days – Friday, March 20th, 2009
HIGH ALTITUDE OBSERVATORY
NCAR – Boulder, CO