The Visible Tunable Filter (VTF) for the Daniel K. Inouye Solar Telescope is an imaging spectro-polarimeter for the wavelength range between 520 and 860 nm. It operates at the highest spatial resolution possible for the DKIST, 20 km on the Sun, equivalent to an angular resolution of 0.028 arcseconds. It will perform 2D Stokes imaging polarimetry and 2D imaging spectroscopy at a high spectral and temporal resolution.

Quick Facts

  • Instrument type: Two-dimensional filter spectrometer based on tunable Fabry Perots in telecentric configuration with two imaging channels (broad band and spectroscopic)

  • Polarimeter type: Dual beam polarimeter
  • Polarization Modulation: Ferroelectric Liquid Crystal Modulators
  • Wavelength range: 520 to 860 nm

  • Field of View: 60 arcsec x 60 arcsec, simultaneous

  • Spatial resolution: 0.028 arcsec (20 km on the Sun) at 520 nm

  • Spectral resolution: 100,000 (6 pm at 600 nm)

  • Polarimetric sensitivity: 3 x 10-3

  • Magnetic field sensitivity: 20 G (longitudinal)

  • Doppler sensitivity: 90 m/s
  • Observing modes:

    • Spectropolarimetric imaging ("Full Stokes measurements"),
    • Doppler imaging,
    • Monochromatic intensity imaging

  • Temporal resolution (cadence for single-line observations):

    • 13 s (Spectropolarimetric imaging)
    • 4 s (Doppler imaging)
    • 0.8 s (Monochromatic imaging)

  • Multi-line observations: Add 2 s for each pre-filter change. Add up to 2 s delay between filter changes, if cadence is shorter than 2 s.
  • Spectral lines: limited by availability of suitable narrow-band pre-filters (0.6 - 1.2 nm, dependent on wavelength)
  • First-light set of pre-filters: Na D 589.0 nm, Fe I 630.25 nm, H I 656.3 nm, Ca II 854.2 nm
  • Post-facto image reconstruction: Speckle reconstruction of broad-band images, deconvolution of narrow-band data