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Table of Contents

1. Introduction

The Daniel K. Inouye Solar Telescope is pleased to solicit proposals for Cycle 4 observations. The proposal submission window will be open for one month starting April 29 and ending May 29, 2025.

The Cycle 4 observations are planned to take place during four Observing Windows (OW4.1, OW4.2, OW4.3, OW4.4) covering an approximately 9-month wide operations window opening in March and closing in November 2026. On average, each OW will span approximately 7 weeks of continuous observing (7 days per week if not announced otherwise) followed by 2-3 weeks of technical time before going back on sun for the next Observing Window. Technical time consists of planned facility preventative maintenance and engineering, testing and commissioning of new telescope or instrument capabilities, and coudé/FIDO (Facility Instrument Distribution Optics) re-configuration time.

DKIST Science Operations plans on continuing the testing of observations performed as a service to the community (Service Observing Time), for example topical coordinated observing campaigns or multi-messenger solar science and respective observing in conjunction with Solar Orbiter and Parker Solar Probe. Any data acquired in the context of this service observing time will be released directly to the community with no exclusive access period. The DKIST also may continue to dedicate some time for early and limited prototyping and test-exercising of Guaranteed Observing Time (GOT) and Directors Discretionary Time (DDT) as regulated by policy and obligations to selected partners. We roughly estimate that about 130-180 days could be available in Cycle 4 for the observing of accepted Cycle 4 Proposals, expansion of Service Observing Time, and the early GOT and DDT testing efforts. These 130-180 days do not account for bad weather (including seeing conditions that are not suitable for science observing) or unplanned technical outages.

1.1 Important Dates

 2. Proposal Call Summary

2.1 Overview

The table below summarizes the major Cycle 4 capabilities.

2.2. Priorities

In Cycle 4, we continue to prioritize those proposals that aim to study active regions (specifically their fine-structure throughout the sun’s atmosphere as observable by the DKIST) requesting on-disk AR targets (e.g. sunspots, pores, active region filaments) or aim to study the corona off-limb (with the Cryogenic Near-Infrared Spectro-Polarimeter - Cryo-NIRSP). The latter is specifically to further the understanding of the corona through the coronagraphic capabilities of the DKIST.

In addition, DKIST plans on prioritizing flare research. Individual Proposers requesting flare observations are welcome to do so through the normal process of regular observing time allocation in response to the Cycle 4 Proposal Call (as before in Cycle 1, 2, and 3). 

Furthermore, DKIST will dedicate additional time commensurate with up to the size of an individual observing window (about 7 weeks) within Cycle 4 for flare observing made available outside of the regular observing time allocation process. All data that is acquired within that dedicated time will be made public immediately after calibration by the DKIST data Center. During that dedicated time, the intent is to perform flare observations that are coordinated with interested space- and ground-based observatories. Observatories interested in joining the campaign are invited to contact the DKIST through a Google form https://docs.google.com/forms/d/1iHE3D-pi3UqdT4Nf7r88GGHORd7dekXGEksxW5kTtYI/edit.  In order to assure that the coordinated observations are scientifically optimized and to the benefit of the community while well aligned with science objectives and goals of all the coordinating assets, a working group will be formed in the coming months leading to the Cycle 4 observing window. Overall membership of the working group will be limited. An announcement in Solar News after the release of the Cycle 4 Proposal Call will provide more information and details.

2.3 Miscellaneous

For Cycle 4, Proposers should not forget and be aware of the following.

 3. Proposal Call Details

Please make sure to submit proposals that are compatible with the Proposal Call details and capabilities offered. Non-compliance with the capabilities offered in this Call can lead to the rejection of submitted proposals.

Particularly, Proposers should be diligent and verify whether the following are Proposal Call compliant:

• Requested spectral lines for the ViSP.

• Requested ViSP spectral lines in combination with the offered FIDO configurations.

• Requested slit width for the ViSP.

• Requested filters for the VBI.

• Requested VBI filters in combination with the offered FIDO configurations.

• Requested spectral lines for the DL-NIRSP.

• Requested DL-NIRSP spectral lines in combination with the offered FIDO configurations.

• Requested spectral lines and modes for the Cryo-NIRSP.

• Requested instrument combinations and their individual configurations (ViSP spectral lines combined with VBI filter choices).

• Requested pointing and research target in combination with the requested instruments (pointing restrictions may apply for specific instruments).

Proposers should also make sure that the following information is consistent between the information provided in the observing strategy file and the Proposal Architects web interface:

• Requested spectral lines and mode(s) for the ViSP.

• Requested filters for the VBI.

• Requested spectral lines and mode(s) for the DL-NIRSP.

• Requested spectral lines and mode(s) for the Cryo-NIRSP.

• Requested instrument combinations and their individual configurations (ViSP spectral lines combined with VBI filter choices).

• Requested pointing and research target in combination with the requested instruments (pointing restrictions may apply for specific instruments).

• Optimum and minimum requested observing time.

For assistance with any questions about the technical feasibility of the proposal or with its compliance with the Call, we strongly recommend that Proposers utilize the DKIST Help Desk (available through the following link: Jira Service Management) when preparing the proposal prior to submission.

3.1 Instruments

3.1.1 Visible Broadband Imager (VBI)

The VBI (Blue and Red channel) is offered on its nominal central (physical) FOV or its full (optical) FOV of 2 x 2 arcmin² (through the field-sampling mode) with all available filters. Which VBI filters receive light and as such can be used depends on the choice of the observing configurations (a.k.a. FIDO or coudé configuration) offered in this Call.

Additionally, the VBI allows the Proposer to select the following acquisition modes:

• With VBI receiving all light within a certain filter/bandpass:

  • Speckle reconstruction enabled with 80 images (fixed and not free to choose), or

  • Speckle reconstruction and frame selection enabled with 80 out of YY images (YY > 80, and defined by the Proposer)

• With VBI only receiving very limited photon flux within a certain filter/bandpass:

  • Speckle reconstruction disabled with or without frame selection with one image delivered every ~1 second.

The VBI will be operated whenever possible at 30 Hz for any of the above choices. It is however to the discretion of science operations to run the instrument at a lower rate.

The VBI will not support binning or ROI selection within this Call.

In Cycle 4, Proposers are not required to submit a VBI Instrument Performance Calculator (IPC) file with their proposal.

However, Proposers should follow the instructions in the VBI IPC Manual to determine the instrument configuration for their specific choice of filters, FOV sampling, mode, number of repeats and other settings, and provide the information with their Observing Strategy.

It should be noted that settings could be subject to change based on technical needs. We will work with accepted Proposers on the details of the VBI configuration during experiment generation when the observing programs and their respective scripts are created.

Proposers requesting the VBI should familiarize themselves with known general issues of the instrument as reported on https://nso.atlassian.net/wiki/spaces/DDCHD/pages/1959985377.

Reference Publications

Instrument Summary Article:      

• Woeger, F. et al (2021): The Daniel K. Inouye Solar Telescope (DKIST)/Visible Broadband Imager (VBI), Solar Physics, Volume 296, Issue 10, id.145

Other Publications:

• Kuridze, D. et al. (2024): Insight into the solar plage chromosphere with DKIST, The Astrophysical Journal, Volume 965, Issue 1, id.15

3.1.2 Visible Spectro-Polarimeter (ViSP)

The ViSP is offered in its polarimetric (full Stokes vector, stepped scanning) and intensity mode (Stokes I only, continuous scanning) over its full spatial scanning range (i.e. up to 130 arcsec). In Cycle 4 the ViSP can be used with the slit widths of 0.218 arcsec, 0.107 arcsec, and 0.0536 arcsec and the following spectral lines (and the lines falling into the respective passband of about 1 nm):

• Ca II K 393.4 nm

• Ca II H 396.8 nm (includes H I 397 H-Epsilon in the red wing of Ca II H)

• H I 410.1 nm (H-Delta) New

• H I 434.0 nm (H-Gamma) New

• Sr I 460.7 nm

• H I 486.1 nm (H-Beta)

• Mg I b2 517.3 nm

• Na I D2 589.0 nm, Na I D1 589.6 nm (either individually or both lines combined when centered at 589.3 nm)

• Fe I 525.0 nm

• Fe I 557.6 nm (no Zeeman splitting, Lande factor g = 0) New

• Fe I 617.3 nm New

• Fe I 630.2 nm

• H I 656.3 nm (H-Alpha)

• Fe I 709.0 nm (no Zeeman splitting, Lande factor g = 0) New

• Ca II 849.8 nm New

• Ca II 854.2 nm

• Ca II 866.2 nm New

It should be also noted that requesting the Ca II H or K lines will significantly slow down the instrument because the exposure times have to be comparably high even for spectroscopic applications only.

Proposers are required to submit a ViSP Instrument Performance Calculator (IPC) file for each of the ViSP configurations requested with their proposal.

Proposers should follow the instructions in the ViSP IPC Guide to determine the instrument wavelength configuration for their specific choice of spectral lines, i.e. the grating angle, the wavelength distribution on the arms, the angle position of the arms, and spectral order. Once the wavelength configuration is determined, the ViSP IPC should be used to define all remaining details of the observation(s).

Those details are:

• Choice of instrument mode: intensity or polarimetry.

• Choice of slit width: there are three slit widths to choose from (be aware that the IPC will allow to select from five slits, only three of which are supported by this Call).

• Choice of slit step size (if polarimetry mode) or slit velocity (if intensity mode).

• Choice of number of slit positions (if polarimetry mode) or number number of exposures (if intensity mode).

• Choice of integration time per slit position (if polarimetry mode) or frame rate (if intensity mode).

• Choice of map repeats.

What lines can be used in combination with each other is constrained by the choice of the selected spectral lines used in the VBI-Blue and VBI-Red, i.e. by the choice of the Observing Configuration (a.k.a Facility Instrument Distribution Optics (FIDO) or coude configuration).

The ViSP can be requested scanning a Field-of-View up to 130 arcsec on an equidistant stepping grid or for stationary slit (sit-and-stare) observations. In spectropolarimetric mode the ViSP will use 10 modulation states. When using the IPC, Proposers need to set the number of modulation states equal to 10 and not use any other number. Proposers should also be aware of that the number of modulation cycles (a.k.a accumulations) must be an even number and Proposers need to take that into account when defining the integration time per slit position (or dwell time).

When using the ViSP IPC tool Proposers should not expect that the tool is 100% accurate in the computation of the map time, map cadence time, total time, and signal-to-noise ratios. Proposers will not be penalized in the technical review for inaccuracies resulting from the IPC.

It is required to upload at least one ViSP IPC output file with the proposal to capture the spectrograph configuration (grating angle, arm positions).

Proposers should be aware that settings could be subject to change based on technical needs. We will work with accepted Proposers on the details of the ViSP configuration and on the timing aspects of their Proposal (i.e. map time, map cadence and total time of the observation) during experiment generation when the observing programs and their respective scripts are created.

Proposers requesting the ViSP should familiarize themselves with known general issues of the instrument as reported on https://nso.atlassian.net/wiki/spaces/DDCHD/pages/1959985377.

Reference Publications

Instrument Summary Article:      

• de Wijn, A. G. et al. (2022): The Visible Spectro-Polarimeter of the Daniel K. Inouye Solar Telescope, Solar Physics, Volume 297, Issue 2, id.22

Other Publications:

• Kuridze, D. et al. (2024): Insight into the solar plage chromosphere with DKIST, The Astrophysical Journal, Volume 965, Issue 1, id.15

• Judge, P. et al. (2024): Magnetic Fields and Plasma Heating in the Sun's Atmosphere, The Astrophysical Journal, Volume 960, Issue 2, id.129

• Campbell, R. et al. (2023): DKIST Unveils the Serpentine Topology of Quiet Sun Magnetism in the Photosphere, The Astrophysical Journal Letters, Volume 955, Issue 2, id.L36

• da Silva Santos, J. M. et al. (2023): Magnetic Fields in Solar Plage Regions: Insights from High-sensitivity Spectropolarimetry, The Astrophysical Journal Letters, Volume 954, Issue 2, id.L35

• French, R. J. et al. (2023): First Observation of Chromospheric Waves in a Sunspot by DKIST/ViSP: The Anatomy of an Umbral Flash, The Astrophysical Journal Letters, Volume 945, Issue 2, id.L27

3.1.3 Cryogenic Near-InfraRed SpectroPolarimeter (Cryo-NIRSP)

The Cryo-NIRSP spectrograph (SP) and context imager (CI) are offered during Cycle 4 for off-limb observations only. Observations by the spectrograph will be limited to Fe XIII 1074.7 nm, Fe XIII 1079.8 nm, He I 1083 nm, and Si X 1430 nm spectral lines. The ~4.5 nm bandwidth of the SP allows only one line to be observed at a single time. Observations of multiple lines can be acquired sequentially, but not at a fast cadence. A combination of spectroscopy and spectropolarimetry can be used within a single experiment for the selected lines. The context imager is offered in the He I 1083 nm bandpass only.

The inclusion of the Si X 1430 nm line began in Cycle 3. Unlike the other offerings, the Si X 1430 bandpass is limited to spectroscopy-only. The peak of the Si X 1430 nm contribution function occurs near 1.4 MK, cooler in comparison to Fe XIII at 1.8 MK. In the future, this line may be offered in polarimetric mode. Users should be aware; however, that this bandpass contains significant telluric absorption, which adds to the observational and calibration risk of using this line. For more details, we refer users to Schad et al. (2024), which discusses the diagnostic potential and calibration methods developed for this line. We advise users to only include Si X 1430 nm in their experiment if there is a compelling scientific reason to do so; in general, the use of the Fe XIII 1074 and 1079 nm lines should be prioritized wherever possible.

In Cycle 4, Proposers are not required to use the Cryo-NIRSP Instrument Performance Calculator (IPC) and upload an output file with the proposal. Instead, two separate programs for spectroscopic or spectropolarimetric observations are offered that Proposers can select from and use to build a custom experiment:

• Coronal (off-limb) program.

• Prominence (and/or coronal rain) program.

These programs have most settings predefined leaving only some specific settings for the Proposer to define in the Observing Strategy form in the respective Cryo-NIRSP section. These (available) settings are:

• Choice of program: corona off-limb -or- prominence (and/or coronal rain)

• Choice of polarimetric modulation.

• Choice of spectral line for the coronal program.

• Choice of scan width for the spectrograph.

• Choice of slit step size.

• Choice of map repeats.

For the details of these programs see the Tables below. In addition to these programs, we provide recommended sequences that address some common use cases. See below for more details.

An individual experiment may incorporate one or a mix of these programs depending on the science use case. However, we encourage Proposers to minimize wavelength switches for the spectrograph as this introduces operational and calibration complexity.  All settings are subject to change based on technical needs or guidance during experiment generation.

The entire 3 arcmin scanning range in the direction perpendicular to the slit axis will be available for image rastering. While the slit may be oriented at an arbitrary angle by rotation of the Coude table, it is recommended that either a radial or limb-tangential slit orientation is used whenever possible. Stationary slit observations (sit-and-stare) are supported by selecting a scan width and step size equal to the slit width.

The coronal and prominence programs may be requested with or without limb occulting using the GOS occulters (for near-limb pointing). We will work with accepted Proposers on which choice will be better for the observation.

Important notes about spatial resolution:

• Cryo-NIRSP does not benefit from the use of adaptive optics, and all observations are seeing-limited. While Cryo-NIRSP uses 0.15 arcsec and 0.5 arcsec slit widths, it should not be expected that <1 arcsec resolutions are achievable. Generally, 1 to 2 arcseconds is a good target for CryoNIRSP off-limb spatial resolution. However, this cannot be guaranteed at the time of program execution.

Important notes about data quality:

• Data quality continues to increase at the instrument matures; though challenges remain. For example, interference fringing and temporal instabilities are common challenges and require additional calibration efforts that may not be supported by the first versions of the calibration pipeline.

• The context imager currently suffers from an optical issue where the science image is overlapped by an out-of-focus ghost image of nearly the same magnitude. Until this issue is resolved, the CI images are to be used primarily for context purposes; any further science investigations must consider the full impact of this optical issues, which are not addressed by the calibration pipeline at this time.

Reference Publications

Instrument Summary Article:         

• Fehlmann, A. et al. (2023): The Daniel K. Inouye Solar Telescope (DKIST) Cryogenic Near-Infrared Spectro-Polarimeter, Solar Physics, Volume 298, Issue 1, id.5

Other Publications:

• Morton, R.J. et al. (2025) High-frequency coronal Alfvenic waves observed with DKIST/Cryo-NIRSP, Astrophysical Journal, Volume 982, Issue 2, id.104

• Schad, T.A. et al. (2024): Mapping the Sun’s coronal magnetic field using the Zeeman effect, Science Advances, Volume 10, Issue 37

• Schad, T.A. et al. (2024): Coronagraphic Observations of Si X 1430 nm Acquired by DKIST/Cryo-NIRSP with Methods for Telluric Absorption Correction, The Astrophysical Journal, Volume 965, Issue 1, id.40

• Schad, T.A. et al. (2023): First Infrared Coronal Spectra from DKIST/Cryo-NIRSP: Comparisons with Global MHD Models, The Astrophysical Journal, Volume 943, Issue 1, id.59

Available instrument observing modes