Extreme adaptive optics

The sky-background limit

At a dark site the optical sky background is about g = 21 mag per square arcsecond. Most spectrographs on today’s 8–10 m telescopes, such as X-Shooter on the VLT, are designed for 1″ seeing. This limits them in two ways: for fainter targets the sky dominates the signal-to-noise ratio, and sources closer together than about 1″ cannot be separated.

Imaging surveys such as the Vera C. Rubin Observatory and Euclid already chart the sky beyond g = 25, with resolutions well below 1″. Yet no current facility routinely obtains spectra deeper than g ≈ 23, or resolves sources in crowded regions without giving up spectral coverage. Instruments designed for seeing-limited images are also, by necessity, bulky and expensive.

Going to the diffraction limit

Extreme adaptive optics in the optical blue (400 nm) breaks this deadlock: for point sources the diffraction limit, not the seeing, sets the image size.

• For a 2 m telescope the image shrinks from about 1″ to about 0.1″.
• The sky background in the aperture drops by a factor of 100.
• The instruments shrink too: going from a seeing-limited 8 m telescope to a diffraction-limited 2 m telescope reduces their linear size by a factor of about 40.

What the TDT needs

The TDT aims for an enclosed energy of at least 80% within about 0.12″ at 400 nm. Today’s systems reach Strehl ratios of about 15% at 400 nm, so this needs a development path:

• adaptive-optics systems with more actuators, running at higher frequencies;
• more powerful laser guide stars, because the return flux is more limited;
• robust, fully robotic operation for 100 telescopes on one site, a step beyond today’s robotic systems in the red such as Robo-AO.

The adaptive optics also shapes the unit telescope itself. Larger units would need many more actuators in the blue, and the return flux of the natural and laser guide stars is a key factor in choosing the 2 m aperture. With many lasers on one mountain, and the VLT and the ELT nearby, laser avoidance and collision are part of the design: the first TDT units, proto-TDT/FAROS on the VLT Auxiliary Telescopes, will test this at Paranal, and the core array may be spread over several sites.