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Visible Spectro-Polarimeter (ViSP)

Visible Spectro-Polarimeter (ViSP)

[ 1 Mission ] [ 2 Description ] [ 3 Technical Details at a Glance ] [ 4 Instrument Modes ] [ 5 Publications ]

Important note: Please refer to the latest DKIST Observing Cycle Proposal Call for the definition of available instrument modes. The information below is a summary of the instrument capabilities as designed and does not necessarily reflect the modes available.

Mission

The mission of the Visible Spectro-Polarimeter (ViSP) is to simultaneously record maps of the solar surface in up to three flexibly selectable spectral lines within the visible spectrum, with high spatial and spectral resolution, and high polarimetric precision whenever full-Stokes-polarimetry measurements are acquired.

Description

The Visible Spectro-Polarimeter (ViSP) is optically located behind the DKIST Adaptive Optics (AO) system and consists of a high-spectral-resolution single-slit diffraction-grating based spectrograph that can simultaneously observe up to three spectral lines in a wavelength range from 380-900 nm (the visible spectrum). The ViSP’s three camera arms can be moved and focused independently on an arched rail to access the selected spectral lines for a given diffraction grating angle and spectrograph order. The slit can be moved across the spectrograph’s entrance focal plane to scan maps of the solar surface. A modulator optic behind the slit assembly modulates the polarization state of the incoming light so that the full Stokes vector can be measured in the camera focal plane. Being dual beam polarimeters, each ViSP arm captures two orthogonal polarization states co-temporally on its detector to minimize crosstalk in the measurements.

ViSP can be run in parallel to all other DKIST instrumentation (VBI, VTF, DL-NISRP) apart from the Cryo-NIRSP, but only the start times of observations are synchronized.

visp_1.jpg

 

 

visp_2.jpg

ViSP instrument layout with all relevant optics and motorized stages, taken from de Wijn et al., 2022, SoPh, 297, 22.

Technical Details at a Glance

Observing Modes

  • Spectroscopic mode: acquisition of intensity spectra. The slit is moved continuously across the solar image at a speed that is matched to the slit width.

  • Polarimetric mode: acquisition of the full Stokes vector IQUV in a dual-beam configuration with a rotating retarder as polarization modulator. The spatial scanning of the solar surface is achieved by discrete stepping of the entrance slit across the solar image.

Spatial Sampling and Field of View

  • As a post-AO instrument, ViSP

    • is fully and routinely supported by the DKIST AO system, and

    • VISP can observe in parallel with VBI, VISP and DL-NIRSP.

      • NOTE: Available instrument combinations depend on the FIDO configuration.

  • The instrument was designed to perform at about twice the spatial diffraction limit of DKIST at most wavelengths.

Spatial (Pixel-)Sampling along slit

  • Camera arm 1: 0.0295 arcsec / pixel

  • Camera arm 2: 0.0236 arcsec / pixel

  • Camera arm 3: 0.0195 arcsec / pixel

Slits widths available

  • 0.0284 arcsec

  • 0.041 arcsec

  • 0.054 arcsec

  • 0.107 arcsec

  • 0.214 arcsec

Field of View

  • Maximum optical field: 2×1.25 arcmin2 (full post-AO DKIST 2×2 arcmin2 field of view, restricted by the camera format)

    • Each of the three ViSP arms has a different magnification due to the spectrograph characteristics:

      • Camera arm 1: ~75 arcsec using 2560 spatial pixels

      • Camera arm 2: ~60 arcsec using 2560 spatial pixels

      • Camera arm 3: ~50 arcsec using 2560 spatial pixels

    • Full 2 arcmin that the ViSP light feed telescope transmits is covered through lateral spatial scanning

Spectral Range and Resolution

Spectral range

  • 380 - 900 nm (designed range), with continuous coverage

    • i.e., for a ViSP configuration using just one single wavelength, any portion of the spectrum can be imaged in either of the 3 spectral arms

  • observe up to 3 spectral bands simultaneously

    • at λ = 630 nm: spectral bands are ~1 nm wide at ~1 pm / pixel spectral sampling, with 2×1000 px for dual beam polarimetry

  • NOTE: The actual wavelength range accessible by ViSP is potentially limited by the configuration of the Facility Instrument Distribution Optics (FIDO) in use.

Spectral resolution

  • Maximum resolving power better than R ~ 180000 @ λ = 630 nm

    • put differently: better than 3.5 pm @ λ = 630 nm (when sampling < 1.75 pm/px)

    • NOTE: resolving power depends on wavelength and spectrograph arm configuration

Spectrograph Lines

Diagnostic

Wavelength [nm]

 

Diagnostic

Wavelength [nm]

 

Ca II K

393.4

 

Ca II H

396.8

includes H I 397 nm H-Epsilon in the red wing of Ca II H

H I

410.1

H-Delta

H I

434.0

H-Gamma

Sr I

460.7

 

H I

486.1

H-Beta

Mg I b2

517.3

 

Fe I

525.0

 

Fe I

557.6

no Zeeman splitting, Lande factor g = 0

Na I D2

589.0

either individually or both lines combined when centered at 589.3 nm

Na I D1

589.6

Fe I

617.3

 

Fe I

630.2

 

H I

656.3

H-Alpha

Fe I

709.0

no Zeeman splitting, Lande factor g = 0

Ca II

849.8

 

Ca II

854.2

 

Ca II

866.2

 

  • NOTE: the available line list for a Cycle is announced in the Proposal Calls

Temporal Cadence

  • reconfiguration of arms: ~20 minutes (including order sorting filter change)

  • temporal cadences depend critically on the science use case, and resulting spectrograph configuration

    • cadence examples

      • spectroscopy: lateral FOV of 2.2×1 arcmin2 maps can be observed at about 62 seconds cadence
        (λ = 630 nm: 0.054 arcsec slit width, 33 Hz exposure rate, 2.132 arcsec/px slit velocity)

      • polarimetry: lateral FOV of 0.5×1 arcmin2 maps can be observed at about 25 minute cadence
        (λ = 630 nm: 0.107 arcsec slit width, 33 Hz exposure rate, 0.0663 arcsec/slit step, 450 slit steps)

        • NOTE: Modulation rate is driven by the slowest of the 3 camera arms

Polarimetric Capabilities

  • Each arm is a dual beam polarimeter

  • Full Stokes vector polarimetry, or Stokes-I only

  • polarimetric sensitivity per slit position: 10-3 Icont at λ > 450 nm (depending on instrument configuration requested)

Photometric Capabilities

  • Flat fields repeatable to the 2% level

Instrument Modes

Example Modes of Operation

Example 1: Spectroscopic mode with 3 wavelength channels (arms)

  • 0.054 arcsec slit width

  • 2.132 arcsec/px slit velocity, acquired at ~33 Hz camera exposure rate

  • 4 maps/λ, with sizes 2.2×[1.25, 1, 0.83] arcmin2, cadence: 62 seconds / map

  • Arm configurations:

    • Arm 1: λ = 589.59 nm, ∆λ = 0.0009569 nm

    • Arm 2: λ = 630.15 nm, ∆λ = 0.0011478 nm

    • Arm 3: λ = 517.27 nm, ∆λ = 0.0011808 nm

Example 2: Spectro-Polarimetric Mode with 2 wavelength channels (arms)

  • 0.1071 arcsec slit width

  • 10 modulation states to retrieve full Stokes vector, acquired at ~33 Hz camera exposure rate

  • 450 slit positions, 0.10625 arcs slit step width

  • 1 map/λ, with sizes 0.8×[1, 0.83] arcmin2, cadence: ~25 minutes / map

  • Arm configurations:

    • Arm 2: λ = 630.15 nm, ∆λ = 0.0011478 nm

    • Arm 3: λ = 517.27 nm, ∆λ = 0.0011808 nm

Publications

 

 

Contact

If you have any specific question about ViSP, please use the DKIST Help Desk to post it.

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