Active Optics

Relevant syllabus point:

  • outline methods by which the resolution and/or sensitivity of ground-based systems can be improved, including:
    • active optics

No mirror can be made perfectly smooth. Distortions in the reflecting surface reduce the quality of any image formed. Prior to the 1980s, primary mirrors were rigid with a thickness normally about one-sixth the diameter. This ensured that they did not flex when pointing to different regions of the sky. The weight of the primary required a strong mounting system which also added to the overall mass of the system.

Thick mirrors also take longer to reach thermal equilibrium each night, reducing the achievable resolution and adding to seeing effects. Modern primaries, however, can be made very thin. Those for the Gemini telescopes are only 20-cm thick. The mounts for these therefore can be much lighter.

The primary mirror for Gemini North. Note the person in the center. The 8.1m primary is only 20 cm thick. Image: Gemini Observatory.

The problem is that the mirrors sag under their own weight as they point to different parts of the sky. To produce any worthwhile image the primary must be actively corrected by continuous computer control. By measuring a reference star within the field of view, corrections are sent to electromechanical actuators on the back of the primary. These push or pull on a section of the primary to change its shape. Active optic systems correct the primary shape about once per minute. The photo below shows the 150 actuators for one of the VLT primary mirrors. Each VLT primary is 8.2m diameter, 17cm thick and weighs 22 tonnes.

VLT actuators for active optics of 8.2m primary mirror.