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Diffraction-Limited Near-Infrared Spectropolarimeter (DL-NIRSP)

Diffraction-Limited Near-Infrared Spectropolarimeter (DL-NIRSP)

[ 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 DL-NIRSP is to provide unprecedented high cadence spectropolarimetry over various domains of solar remote sensing diagnostics for a wide range of solar targets and applications.

Description

The Diffraction-Limited Near-Infrared Spectropolarimeter (DL-NIRSP) is a diffraction grating based integral field spectrograph developed by the University of Hawai`i. Its purpose is to study solar magnetic fields at high spatial resolution with high spectral resolution and polarimetric accuracy. Unlike any other instrument currently available to the solar community, DL-NIRSP prioritizes the simultaneity of spatial and spectral coverage. It is able to provide this capability via the use of a novel machined image slicer for integral field spectroscopy. With this capability, observing schemes need only balance the scientific requirements for spatial resolution, temporal resolution, and field-of-view coverage.

The DL-NIRSP consists of a configurable feed optics telescope that feeds the DKIST beam into an integral field unit (IFU) after passing it through a polarization state modulator optic. The IFU uses an image slicer optical design that result in 36 μm size pixels for the high- and medium resolution mode of the DL-NIRSP. The IFU individual slit images are subsequently transferred into a multi-armed near-Littrow spectrograph. The 3 arms enable the instrument to capture 3 spectral windows simultaneously. Being dual beam polarimeters, each DL-NIRSP arm captures two orthogonal polarization states co-temporally on its detector to minimize crosstalk in the measurements.

Technical Details at a Glance

Spatial Sampling and Field of View

  • As a post-AO instrument, DL-NIRSP:

    1. is fully and routinely supported by the DKIST AO system and

    2. can observe in parallel with VBI, ViSP, and VTF.

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

Field of View

  • Optical: 2x2 arcmin2 (the full single telescope pointing, post-AO DKIST field of view)

  • Physical: Depends upon choice of instrument feed-optics and IFU. Larger maps made by stepping physical FOV.

    • High Res mode: 1.68″  x 1.80″, with 56 x 60 spatial pixels (36 μm)

    • Mid Res mode: 4.31″  x 4.62″, with 56 x 60 spatial pixels (36 μm)

Spatial Sampling

  • High Res mode: 0.030 arcsec

  • Mid Res mode: 0.077 arcsec

  • NOTE: The theoretical spatial resolution of DKIST @ λ = 1 μm is 0.063 arcsec (Rayleigh criterion). The effective spatial resolution in DL-NIRSP images is affected by the selected sampling mode, and furthermore can be limited by atmospheric turbulence that impacts performance of adaptive optics system.

Spectral Range and Resolution

 Spectral Range

  • 500 – 1800 nm (designed range)

  • The design spectral range is covered by the individual arms in spectral bands:

    • Arm 1: 500 nm – 900 nm

    • Arm 2: 900 nm – 1350 nm

    • Arm 3: 1350 nm – 1800 nm

  • One spectral channel is observed in each of the three spectral bands

    • spectral channels correspond to one (sometimes multiple) specific wavelength diagnostic

  • Channel bandwidth

    • Bandpass filters isolate narrow spectral regions for each wavelength diagnostic (spectrum line).

    • +/- 120 km/sec Doppler coverage in each channel (0.72 nm wide @ 900 nm)

Spectral Resolution

  • Maximum resolving power is approximately R ~ 125000 @ λ = 900 nm

Currently available channel wavelengths

Diagnostic

Wavelength [nm]

Arm

 

Diagnostic

Wavelength [nm]

Arm

 

Ca II

854

1

 

He I / Si I

1083

2

 

Fe I

1565

3

 

Temporal Cadence

  • Must be calculated using the DL-NIRSP instrument performance calculator.

  • Single field temporal cadence is limited by camera frame time, and is determined by instrument mode (e.g. full Stokes polarimetry or Stokes-I only mode) and the depth of integration (i.e. target SNR).

  • Stepping the IFU field to an adjacent tile in a map introduces, at minimum, a two frame time delay of time between positions.

  • See example modes below.

Polarimetric Capabilities

  • Each arm is a dual beam polarimeter.

  • 5 x 10-4 P/Icont polarimetric sensitivity (depending on instrument configuration requested)

Photometric Capabilities

  • 1% photometric precision or better for on-disk intensity observations.

  • Photon-limited precision for Q/I, U/I, and V/I observations.

Instrument Modes

  • Three different spatial resolution modes (see above).

  • Full Stokes polarimetry and spectroscopic-only modes.

  • Field stepping for larger field of views (lowers cadence).

  • On-disk and at-limb observations.

Example Modes of Operation

Example 1: High resolution, small field-of-view with polarimetry

  • 3 x 3 Mosaic, Ca II 854 / He I 1083 / Fe I 1565 nm.

  • 5.03” x 5.39” field-of-view.

    • 15 sec/mosaic (2 accumulations), with Stokes parameter sensitivities (per spectral channel, respectively):

      • ~0.3%/~0.08%/~0.07%
        (on-disk measurement, 1 sigma values).

    • 25 sec/mosaic (4 accumulations), with Stokes parameter sensitivities (per spectral channel, respectively):

      • ~0.2%/~0.06%/~0.05%
        (on-disk measurement, 1 sigma values).

    • For more information on polarimetric sensitivity with these and other settings, please see Anan et al. (SPIE, 2024).

Example 2: Mid resolution, medium field-of-view with polarimetry

  • 25 x 25 Mosaic, Ca II 854 / He I 1083 / Fe I 1565 nm.

  • 108” x 116” field-of-view, 13 min/mosaic (fastest).

Publications

Principal Investigator

Dr. Haosheng Lin
Institute for Astronomy, University of Hawai`i

Instrument Scientist

Dr. Sarah A. Jaeggli
National Solar Observatory

Instrument Support Scientist

Dr. Tetsu Anan
National Solar Observatory

Instrument Performance Calculator

DKIST Instruments