Cryogenic Near-Infrared Spectro-Polarimeter (Cryo-NIRSP)

Mission

The mission of the Cryo-NIRSP is to take advantage of the full coronagraphic capabilities of DKIST to observe both the near-limb and off-limb corona out to 1.5 Solar Radii, and, additionally, leverage the DKIST ability to scientifically observe the solar atmosphere at infrared wavelengths up to 5 μm.

Description

The Cryo-NIRSP consists of a long-slit based spectropolarimeter and a context imager that measure the polarization state of spectral lines between 1000 and 5000 nm. To reduce the thermal IR background, it operates at cryogenic temperatures. It can also target on-disk targets, e.g. using the fundamental CO band at 4651 nm. The spectrograph and the Context imager use a custom infrared camera system with an 18-micron pitch, 2048×2048 pixel H2RG detector.

Cryo-NIRSP is not supported by the DKIST adaptive optics and cannot co-observe with any of the other instruments. The time to switch between Cryo-NIRSP observations and the post-AO instruments is ~> 30 minutes.

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Cryo-NIRSP in the coudé laboratory. The spectrograph (SP), the context imager (CI), and the Cryo-NIRSP warm optics are mounted on a common steel frame. The light from the telescope is relayed by the mirrors M7, M8 (not in the picture), and M9 before it is picked off by M9a and directed towards Cryo-NIRSP. From there the beam propagates to the field steering mirror FM1 and the external imaging mirror FM2. Also in the picture the DL-NIRSP instrument and the National Solar Observatory Coudé Laboratory Spectropolarimeter (NCSP) which uses the Cryo-NIRSP feed mirrors.

Technical Details at a Glance

Spatial Sampling and Field of View

The spectrograph and context imager share a common steering mirror. When the solar image is scanned across the slit, the context image follows and is always centered on the slit.

• Image stability to better than 1 arcsecond rms (Cryo-NIRSP is not supported by DKIST adaptive optics).

• The scanning mirror simultaneously scans the solar image across the slit and context imager.

• The solar image can be scanned both parallel and perpendicular to the spectrograph slit allowing full field coverage.

• The slit orientation for a single scanned observation can be freely rotated with respect to the solar image.

Spectrograph

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Panoramic view of the Cryo-NIRSP spectrograph. The spectrograph filter/slit wheel (SPW) is located immediately downstream of the cryostat entrance window. The flat fold mirrors SM2 and SM4 help to keep the cryostat as small as possible. SM3 is the collimator, GT is the grating, and SM5 is the camera mirror that directs the light into the polarization analyzer assembly, which also holds the infrared camera.

Maximum accessible optical field

• 5′ (arcmin) round for off-limb targets with no solar limb coverage in 5’ field

• 4′ x 3′ for near-limb targets using limb occulter at Gregorian telescope focus

• 2′ square for on-disk targets

Slits

• 0.12” per pixel sampling spatial sampling along the slit

• 0.15” wide 117” long nominally used for on-disk

• 0.5” wide 225” long nominally used for off-limb and near-limb

• The 0.15” slit critically samples diffraction-limited resolution at 4770 nm (1.22 λ /D -> 0.3”)

• The 225’' long slit covers 1875 pixels in the spatial dimension, and the dual beam spectra are located on separate halves of the array.

• The 117’’ slit spectra extends across 1034 pixels along the slit.

Context imager

Optical field

• 100” x 100” (centered on spectrograph slit)

• 0.052” per pixel sampling spatial sampling

Spectral Range and Resolution

Spectral range

• 1000 to 5000 nm (designed range)

• only observe one spectral bandpass at a time

• not all filter passbands have been commissioned

Spectral resolution

• spectra extend over 1024 detector pixels (one beam covers half of the detector)

• Resolving power better than R ~ 30000 using the 0.5” slit

• Resolving power better than R~100000 for the 0.15” slit

Spectrograph Filters

Context Imager Filters

Temporal Cadence

• Cadence given by offered programs in call or must be calculated based on required S/N and resolution using the Cryo-NIRSP Instrument Performance Calculator.

• Dual-beam polarization modulation with up to 3.6 Hz camera frame rate.

• 70 seconds required to change spectrograph wavelength.

Polarimetric Capabilities

• Spectrograph: Full Stokes vector (dual-beam) polarimetry

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

• Intensity-only spectroscopy is also available.

Photometric Capabilities

•  Low background thermal emission: 10 millionths of disk brightness at Si IX 3934 nm coronal line.

• Total efficiency greater than or equal to 10%.

Primary Spectral Diagnostics Table

Instrument Modes

Targeting modes

• On-disk

• Near-limb with limb occulted at telescope’s Gregorian focus (+/- 5” occulting)

• Near-limb without limb occulter

• Off-limb (solar disk entirely inverse occulted at prime telescope focus. Max. pointing at 1.5 R_sun)

Pick-off mirror modes

• Beam Splitter: Simultaneous spectrograph and context image observations

Polarimetry

• Full-stokes spectropolarimetry

• Stokes-I spectroscopy

Example Modes of Operation

It is important to note that the Cryo-NIRSP instrument is designed for operational flexibility to meet a range of research needs, both those currently known and well-understood and many unknown or only poorly understood. The instrument thus aims to serve a wide range of exploratory science, and the use cases below are only examples.

Example 1: Disk/limb Observation

Scan 90×90 arcsec field of view (70 arcsec on limb, 20 arcsec off-limb) at CO wavelength of 4.65 um with a cadence of 90 seconds. This yields 4 maps of polarized (IQUV) line profiles with a resolution of 100,000 and seeing limited spatial resolution of one arcsec.

Example 2: Near limb Observation

Scan of 4×3 arcmin with 1 arcsec spatial and R=30,000 spectral resolution tangent to limb that includes prominence with a cadence of 2 hours. Obtain context image after each full spectral scan and then switch wavelength. This yields 4 maps of polarized (IQUV) line profiles for each wavelength.

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Example of a near-limb pointing overplotted on a representative SDO/AIA 171Å image (inverted color scale). The 5 arcminute optical field of view (orange) is defined by the telescope’s field stop. The black box represents the 100’’ x 100’’ context imager FOV while the blue line represents the 231-arcsecond long slit (0.5’’ slit width). The solar image can be scanned across the slit/context imager in 2 different directions (parallel and perpendicular to the slit).

Example 2: Off-limb Observation

Scan 4×3 arcmin region 10arcsec above limb active region with 1 arcsec resolution and 30 minutes cadence. Obtain Stokes Q context image once per field scan. This yields 4 maps of polarized (IQUV) line profiles and a context image.  

Open

Example of an off-limp pointing overplotted on a representative SDO/AIA 171Å image (inverted color scale). The 5 arcminute optical field of view (orange) is defined by the telescope’s field stop. The black box represents the 100’’ x 100’’ context imager FOV while the blue line represents the 231-arcsecond long slit (0.5’’ slit width). The solar image can be scanned across the slit/context imager in 2 different directions (parallel and perpendicular to the slit).

Publications

• Morton, R.J., Molnar, M., Cranmer, S.R., et al. “High-frequency Coronal Alfvénic Waves Observed with DKIST/Cryo-NIRSP“, Astrophysical Journal, 982, 2 (2025)

• Schad, T.A., Fehlmann, A., Dima, G.I., et al. “Coronagraphic Observations of Si X 1430 nm Acquired by DKIST/Cryo-NIRSP with Methods for Telluric Absorption Correction“, Astrophysical Journal, 965, 1 (2024)

• Fehlmann, A., Kuhn, J.R., Schad, T.A. et al. “The Daniel K. Inouye Solar Telescope (DKIST) Cryogenic Near-Infrared Spectro-Polarimeter“, Solar Physics, 298, 5 (2023)

• Schad T.A., Kuhn J.R., Fehlmann, A. et al. “First Infrared Coronal Spectra from DKIST/Cryo-NIRSP: Comparisons with Global MHD Models”, Astrophysical Journal, 943, 59 (2023)

• Rimmele, T.R., Warner, M., Keil, S.L. et al., “The Daniel K. Inouye Solar Telescope – Observatory Overview“, Solar Physics 295, 172 (2020)

• Fehlmann, A., Giebink, C., Kuhn, J. R., et al., “Cryogenic near infrared spectropolarimeter for the Daniel K. Inouye Solar Telescope“, Instrumentation for Astronomy VI, Proc SPIE 9908: 99084D (2016)

• Elmore, D. F., Rimmele, T. R., Casini, R., et al., “The DKIST First Light Instruments and Critical Science Program“, Ground-based and Airborne Instrumentation for Astronomy V, Proc SPIE 9147: 914707 (2014)

• Berger, T., and ATST Science Team, “The ATST Instrumentation suite: capabilities, synergies, and science goals“, American Astronomical Society, SPD meeting #44, #400.02 (2013)

• Kuhn, J. R., Scholl, I., and Mickey, D., “Solar Dark Matter and Dark Energy: How can CryoNIRSP Help?“, 2nd ATST-EAST Workshop in Solar Physics: Magnetic Fields from the Photosphere to the Corona, ASP Conf Ser 463: 207 (2012)

Principal Investigator

Dr. Jeff Kuhn
Institute for Astronomy, University of Hawai’i

Instrument Scientist

Dr. Andre Fehlmann
National Solar Observatory

Instrument Performance Calculator

• Currently under revision - not needed for the current call

DKIST Instruments

• Instrument overview 

• VBI

• ViSP

• Cryo-NIRSP

• DL-NIRSP