Adaptive Optics Upgrade Plan to Give Inouye Solar Telescope a Wider View of the Sun

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December 6, 2022

By Dirk Schmidt with contributions by Evan Pascual

World’s most powerful telescope is on a path to implement GLAO, MCAO systems

A team at the National Solar Observatory is working to upgrade the adaptive optics system of the Nation Science Foundation’s Daniel K. Inouye Solar Telescope. The upgraded system will expand its capabilities to correct atmospheric disturbances and deliver a clearer picture of the Sun over a much larger area than previously recorded.

The Role of Adaptive Optics in Solar Astronomy & Inouye Solar Telescope Operations

Earth’s atmosphere naturally hinders our observations of the fine details on the Sun. Turbulent airflows blur the solar image in the same optical effect that can be seen at the exhaust of a jet engine or above hot pavement. The Inouye Solar Telescope sports an optical system that can partially correct for this effect by adapting quickly to counteract the optical error in our atmosphere in real-time during a solar observation!

This is called an adaptive optics system and is commonly used in modern astronomy, particularly in solar astronomy. Adaptive optics enable telescopes like the Inouye Solar Telescope to reveal details of the Sun that have not been seen before, exploiting the telescope’s 4-meter aperture at the diffraction limit.

The Inouye Solar Telescope currently uses a classical single conjugate adaptive optics system that consists of a sensor and one corrector. The measuring device is called a wavefront sensor which takes in light and measures the optical error in the atmosphere, in a particular direction, while a deformable mirror corrects the error by quickly changing its surface figure to counteract the effect. This is done by a computer that analyzes the measurements from the wavefront sensor and adjusts the mirror figure accordingly.

DKIST Coude Lab Adaptive Optics with Deformable Mirror

The Inouye Solar Telescope instrument laboratory, known as the Coudé Lab, houses the facility’s adaptive optics infrastructure. The deformable mirror, pictured right, is seen with a protective cover on.

A classical adaptive optics system with one sensor and one deformable mirror can correct the solar image best in a field of view of around 10 arc seconds. This limitation comes from two characteristics of a classical adaptive optics system:

1. The wavefront sensor can only measure the optical error over a field of view of about 10 arc seconds (that includes several solar granules). The error in the atmosphere, however, decorrelates on a similar spatial scale such that two viewing directions that are for example 15 arc seconds apart suffer from completely different errors.

2. The deformable mirror is located in an image plane of the telescope aperture (a pupil image). Thus, the same “correction” gets applied to the entire field of view, eventually applying the wrong correction for directions that are far from the direction where the wavefront sensor is pointed, i.e., farther than 10 arc seconds.

Upgrading the Adaptive Optics System

The team’s plan to upgrade the classical adaptive optics system will add 8 more wavefront sensors and 2 more deformable mirrors to the telescope. With a total of nine wavefront sensors that can span a field of view of up to 2.8 arc minutes, the system will be able to measure the optical error in multiple directions, rather than a single direction, in the telescope’s field of view.

Ground Layer Adaptive Optics

In the first phase of deployment, the additional wavefront sensing capability will optimize the correction for the entire field of view rather than a narrow patch. This is called ground-layer adaptive optics (GLAO). Since GLAO averages the optical error over multiple directions, the correction applied is not optimal for any direction and cannot reveal the very finest detail. The benefit, however, is that the entire field of view gets equally well corrected – albeit less effectively.

 

Solidworks rendering of the new wavefront sensor system.

Multi-Conjugate Adaptive Optics

To boost the correction over an expanded field of view of up to 60 arc seconds, the team will deploy the additional deformable mirrors in subsequent phases. These mirrors will not be located in a pupil image, but will be placed such that they correct for optical errors that occur higher up in the atmosphere, and thus, for the portion of optical error that varies across the field of view. Such a configuration is called multi-conjugate adaptive optics (MCAO) since the deformable mirrors are placed (“conjugated”) to correct the optical error in several different altitudes in the atmosphere. The additional wavefront sensors are needed to adequately sample the optical errors across the field of view while additional deformable mirrors are chosen to correct most of the optical error along the optical path through the atmosphere in a compromise of correction performance and complexity of the system.

A solar image comparison between three adaptive optic systems. From left to right: classical adaptive optics, ground layer adaptive optics, and multi-conjugate adaptive optics. Images made with the MCAO pathfinder system, “Clear”, on the Goode Solar Telescope at the Big Bear Solar Observatory, California, in an international collaboration between NSO, the New Jersey Institute of Technology (NJIT), and the Leibniz Institute for Solar Physics (KIS, Germany).

In short, these upgrades will add new potential to the wavefront correction system and boost the scientific output of the telescope! The GLAO and MCAO upgrade plan was presented at the Adaptive Optics Systems conference at the SPIE Astronomical Telescopes + Instrumentation meeting in Montreal and is outlined in the conference proceedings: click here to learn more.

What’s Next?

The team is finalizing the design of the new wavefront sensors for production and implementing the computer system that will analyze the data from the nine wavefront sensors and control the three deformable mirrors. Each wavefront sensor will have a camera that takes 2,000 images per second, each of which is 1,000×1,000 pixels large. This is a data rate of 4 gigabytes per second per camera, or 36 gigabytes per second for the entire system – way too much for a single computer to process! The new MCAO system will feature ten accurately synchronized computers powered by 704 microprocessor cores to adjust each deformable mirror 2,000 times per second!

The real-time computer system for the MCAO system in the lab at the NSO headquarters in Boulder, CO.

Deployment is expected to begin in 2024. Once completed, Inouye Solar Telescope will feature both the most powerful and the most versatile adaptive optics system in solar astronomy. Scientists will be able to choose between classical, ground-layer, and multi-conjugate adaptive optics for what best fits the solar effects they want to study, shedding more light on the mysteries of our Sun.

Full text:
On the upgrade path to GLAO and MCAO on the Daniel K. Inouye Solar Telescope
Authors: Dirk Schmidt, Andrew Beard, Andrew Ferayorni, Bret Goodrich,  Scott Gregory, Luke Johnson, Lukas Rimmele, Thomas Rimmele, Erik Starman, Friedrich Wöger

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