Adaptive Optics: How the TROIA System Corrects Atmospheric Turbulence

Light from a star travels as a smooth wavefront until it enters Earth's atmosphere. There, pockets of air with different temperatures and densities bend the light by different amounts and the wavefront becomes wrinkled. At the telescope's focus the result is a star image that dances around and is far wider than it should be. Astronomers call this broadening seeing. For the Eastern Anatolia Observatory (DAG) the institution quotes 0.93 arcseconds — a normal value for a good site, yet far from what a 4-metre mirror could deliver in principle.

The diffraction limit and turbulence

The sharpest image a telescope can form is set by the mirror diameter and the observed wavelength. As a rough estimate, a 4-metre mirror resolves about 0.14 arcseconds at a wavelength of 2.2 micrometres, so without the atmosphere images could be six to seven times sharper than 0.93-arcsecond seeing allows. Adaptive optics aims to close this gap: it is a technique that offsets the rapidly changing distortion caused by turbulence in real time.

How the loop works

An adaptive optics system relies on three main parts working together in a closed loop:

• A wavefront sensor uses light from a bright source to measure how the wavefront is distorted.
• A control computer turns that measurement into the correction to be applied.
• A deformable mirror bends its surface, through many actuators behind it, into the opposite shape of the incoming distortion.

Because the atmosphere never stops changing, this measure–compute–correct cycle repeats hundreds to thousands of times per second. The correction is never perfect, and its quality is usually expressed as the Strehl ratio: the peak brightness of the observed star image divided by the peak expected from a flawless optical system. The closer it gets to 1 (100 per cent), the closer the image is to the diffraction limit.

TROIA's components

The adaptive optics system of the DAG400 telescope is called TROIA (Turkish Adaptive Optics System for Infrared Astronomy). According to the technical table published by the institution, its deformable mirror is made by ALPAO and carries 468 actuators. The wavefront is measured with a pyramid sensor, in which a four-sided prism at the focal plane splits the light into four separate pupil images; the brightness differences between them reveal the slope of the wavefront. The sensor camera is listed with a 128 × 128-pixel field, a quantum efficiency above 90 per cent at 600 nanometres and a read noise below 0.1 electrons — properties that help fast measurements even on faint sources.

The table also lists a dichroic mirror specified to reflect more than 95 per cent of light between 400 and 900 nanometres and to transmit more than 90 per cent between 1,000 and 2,500 nanometres. Such a split is consistent with a layout in which visible light feeds the wavefront measurement and near-infrared light goes to the science instruments. Indeed, DAG400's infrared camera DIRAC is designed to be fed by the adaptive optics module or by the coronagraph.

Performance targets

The institution's page states that, with adaptive optics correction, telescope-induced aberrations are at the 25-nanometre level and the residual wavefront error at about 10 nanometres. The same table gives Strehl ratios of 83, 90 and 94 per cent in the J, H and K bands. The rise with wavelength is expected: turbulence matters less at longer wavelengths, so the same correction yields a better image in the infrared. This is also why adaptive optics is decisive above all for near-infrared observations.

Measuring the turbulence: DAGTPS

How well adaptive optics performs on the sky depends on how strong the turbulence is and at which heights it concentrates. DAGTPS, on the DAG site, is a seeing and turbulence-profiling system made of two telescopes of 30 centimetres each. The institution notes that it matters for monitoring atmospheric conditions, for site selection and for testing the performance of telescope optics. Two methods are used. GDIMM derives the seeing by comparing images formed by light passing through openings in a mask. MASS-DIMM combines a fast camera with four photomultiplier tubes in a single instrument and measures turbulence using the telescope's full aperture. DIMM-type methods obtain the overall seeing from how the images jitter relative to each other, while MASS derives the height distribution of turbulence from the twinkling of starlight.

Why it matters

Adaptive optics turns a large ground-based telescope from a light collector limited by the atmosphere into an instrument that can form images as sharp as its mirror size allows. On DAG400, TROIA works together with the infrared camera DIRAC and the coronagraph PLACID to underpin observations that demand high resolution and high contrast.

References

• DAG400 Telescope · Türkiye National Observatories (trgozlemevleri.gov.tr)
• DAGTPS Telescope · Türkiye National Observatories (trgozlemevleri.gov.tr)
• Erzurum DAG Campus · Türkiye National Observatories (trgozlemevleri.gov.tr)

Last accessed: 24 September 2026.