May 21, 2024: DKIST Cycle 3 Proposal Call: CryoNIRSP Status

A Message for Current and Prospective PIs

DKIST Science Operations and the CryoNIRSP instrument team wish to provide additional information to prospective Cycle 3 investigators, as well as Cycle 2 investigators, regarding the status of the CryoNIRSP observations, and in particular, coronal observations.

First and foremost, we wish to encourage Cycle 2 users and prospective users to submit and/or resubmit science proposals for CryoNIRSP for execution in Cycle 3 to take advantage of recent upgrades that have significantly improved the quality of CryoNIRSP data.

First and foremost, we wish to encourage Cycle 2 users and prospective users to submit and/or resubmit science proposals for CryoNIRSP for execution in Cycle 3 to take advantage of recent upgrades that have significantly improved the quality of CryoNIRSP data.

A recent presentation given by the Cryo-NIRSP instrument team to the NSO User’s Committee can be accessed at this link: It provides a high-level overview of the ongoing CryoNIRSP data calibration and characterization efforts, much of which has taken place during the Operations Commissioning Phase. It also provides an early glimpse at the first major advancement of DKIST/CryoNIRSP pertaining to measurements of the Zeeman effect in the off-limb corona. More context on the calibration and performance enhancement effort is provided below.

The CryoNIRSP Spectrograph: Instrument Performance Assessment and Recent Upgrades

One of the primary science targets for CryoNIRSP is to measure the circularly polarized Stokes V spectrum of coronal emission lines between 1 and 3.9 um. Such signals have been detected only a few times in history and were last reported 20+ years ago. The relative amplitude of Stokes V is approximately 10^-4 per Gauss, which corresponds to parts per billion of the solar disk radiance. This is a very challenging, technically demanding, measurement. Since first instrument commissioning, a number of instrument systematics within the CryoNIRSP instrument have been mitigated that paved the way to achieving accurate full Stokes measurements. While these issues most severely impacted achieving accurate polarimetry; there have been associated challenges pertaining to spectroscopic observations. The calibration effort continues; yet, we happily report that the instrument performance at the 1 micron lines of Fe XIII 1074 nm, Fe XII 1079 nm, and He I 1083 nm has significantly matured since commissioning, and we anticipate improved scientific output in Cycle 2 and 3 data.

Implementation of the new 1078.8 nm (13.8 nm FWHM) bandpass filter: The first major improvement in the 1 micron performance was thanks to the fabrication and installation of a new bandpass filter for all three science lines near 1 micron. The installation occurred near the beginning of Cycle 2 in March of 2023. This new filter helped eliminate out-of-band leakage (particularly at short wavelengths <500 nm) that otherwise compromised the spectral purity of the instrument, and consequently prevented accurate polarimetry. The leakage also had a secondary affect on spectral photometric accuracy. We note that achieving good spectral isolation is a common challenge for advanced astronomical instruments. For CryoNIRSP, similar issues are currently being investigated at long wavelengths, e.g. CO 4.6 micron. The solutions employed involve custom built optics that--once designed, built, and tested---must be installed within the CryoNIRSP large cryostat. Warming up, opening, closing, and re-cooling the spectrograph cryostat is a 10+ day process, which can only happen during longer engineering/maintenance periods planned by DKIST operations.

Mitigation of internal ghosts and scattered light: After the installation of the new 1078.8 nm filter, we found at least an order of magnitude improvement in polarization accuracy (as quantified via fit residuals of calibration measurements). Still, issues remained that had been previously hidden by the filter issues. In May 2023, we identified a back-reflection from the polarimetric modulator that was then further reflected by the upstream attenuation filter in the direction of the spectrograph. On 16 May 2023, the attenuation filters were tilted to eliminate this ghost. In August 2023, we learned that chord cuts at the edge of the MgF2 crystal modulator, which were outside of the designed clear aperture, were inducing a large degree of polarized scattered light. This was mitigated with masking placed in front of the modulator in September 2023. These changes significantly improved the baseline polarimetric accuracy of the instrument.

Improved methods for polarimetric calibration: Another important finding occurred last October 2023. While the calibration optics at the Gregorian Optical Station (GOS) have known (that is, metrologized) polarimetric non-uniformities, the consequences of these variations had not been realized by any DKIST instrument. Due to the larger field-of-view of CryoNIRSP (i.e. 4 arcminutes along the slit), the impact of these non-uniformities become more evident. We have developed a new calibration approach that use the GOS optics with a set of modified orientation angles that help up constrain these aperture dependent responses of the GOS optics. The method has been quite successful and has since been adopted as the standard approach for CryoNIRSP polarization calibration; although, incorporation of this method in the Data Center pipeline is pending.

Implementation of a fringe-free polarimetric modulator at 1 micron: Even given the above improvements, achieving high accuracy polarimetry can still be limited by the presence of interference fringes. Between July 2022 and Feb 2024, the CryoNIRSP used a polarimetric modulator that consisted of two optically contacted MgF2 crystal pairs co-mounted with an air-spaced gap. This optic is broadband, allowing for use from 1 to 4.6 micron, but it is not optimized for interference fringe reduction. At 1 micron, seven dominant fringe frequencies were introduced by this optic with amplitudes as large of ~ 0.7%. Preliminary methods for removing these fringes and residual crosstalk have been developed, but these currently require hands-on treatment during calibration processing, and they have further limitations and drawbacks. In late 2023, a new modulator was built that pairs polycarbonate retarders with BK7 windows that have anti-reflection coatings highly optimized for 1 and 1.43 micron. This optic was installed within CryoNIRSP this past February (2024) and can boast essentially complete elimination of the polarized interference fringes. This has led to significantly improved data acquired starting in March 2024. This modulator is planned to be used temporarily while another crystal modulator is being developed with optimized performance at all key CryoNIRSP wavelengths.

Additional visible blocking filter: As a refinement to the above upgrades, we have also introduced an additional warm filter within the CryoNIRSP feed optics. This filter is coated with a dichroic longpass coating (similar to the FIDO coatings) that provides additional visible band blocking for the 1078.8 nm bandpass filter. While the new filter removed the majority of the out-of-band leakage, very narrow spectral transmission leaks remained and required further suppression.

A note regarding the CryoNIRSP Context Imager

The CryoNIRSP instrument consists of two arms: the Spectrograph (SP) and the Context Imager (CI). The context imager has a field of view of ~ 100'' x 100'' and is optically fed by the Fresnel reflection of a wedged optical beam-splitter between the polarimetric modulator and the spectrograph. At present, the context imager is impacted by a ghost image that is introduced by the 2nd surface of this wedged beam splitter. Given both the 1st and 2nd reflections are Fresnel reflections, they are of similar magnitudes. The 2nd reflection is deflected by the optical wedge by 100'' at the context imager focal plane; however, this is not enough to fully separate the two images of the 2.8 and 5 arcmin telescopic field-of-views. During the motion of the steering mirror, changes in the parasitic light are evident, which relate to this issue. This issue will need to be addressed optically in the future. For now, any scientific use of the context imager should be aware that there is a 2nd defocused image contaminating the recorded images. This channel is currently best used for context only.

CryoNIRSP Pipeline Status at the Data Center

The implementation of the CryoNIRSP data calibration pipeline within the Data Center has been impacted by the optical issues outlined above. We can report that significant progress has been made on the pipeline, and full end-to-end trial runs of that pipeline have been accomplished successfully within the past few weeks. These runs convert the raw data into the full Level 1 ASDF-compliant FITS data served by the Data Center. We anticipate an initial pipeline and data release shortly; however, this pipeline will be unable to incorporate all the methods that have been developed to deal with the discussed systematics. Thankfully, the most recent data (i.e. starting March 2024) very significantly reduces these challenges. Some of the most recent data include non-embargoed data sets acquired under Director’s Discretionary Time. Existing Cycle 2 data, i.e. prior to March 2024, will be calibrated by the pipeline and distributed by the Data Center with additional caveats.

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