Cophasing

Lore & Background

Cophasing implies precise, active control of three degrees of freedom of each individual segment mirror: translation along the optical axis (piston) and rotation about two axes perpendicular to the optical axis (tip-tilt). Each segment of a segmented telescope is a solid body having 6 degrees of freedom exposed to gravitational force, wind, and other mechanical forces. If the position of each segment is not controlled, the resolution of the whole telescope will be the same as if the telescope had the diameter equal to the size of one segment. To achieve a resolution commensurable with that of a monolithic telescope of the same diameter, the segmented surface must be controlled with a precision better than λ/40 surface rms. Projects for future extremely large telescopes (ELTs) generally depend on the use of a segmented primary mirror. While the basic technologies required for segmented telescopes have been demonstrated for the 10m Keck telescope or GTC telescope, ELTs of diameters from 50 to 100 m represent a qualitative change with respect to wave front control related to segmentation in comparison with the current 10 meter technology.

Reader's Guide

Cophasing is a critical technology for segmented telescopes, as it directly determines whether the telescope can achieve the resolution of a monolithic mirror of equivalent diameter. The article states that without cophasing, resolution is limited to that of a single segment. The required precision is better than λ/40 surface rms. The basic technologies have been demonstrated on the 10m Keck telescope and GTC telescope. However, the article notes that extremely large telescopes (ELTs) with diameters from 50 to 100 m represent a qualitative change in wave front control related to segmentation compared to current 10 meter technology. This indicates that while the foundational principles are established, scaling up introduces new challenges that are not merely incremental. The legacy of cophasing is thus tied to the feasibility of future ELTs, which depend on segmented primary mirrors and the ability to control them with sufficient precision.

Did You Know?

• Cophasing controls three degrees of freedom per segment: piston, tip, and tilt.
• Each segment has six degrees of freedom exposed to gravitational and wind forces.
• Without cophasing, the telescope's resolution equals that of a single segment.
• Future ELTs of 50 to 100 m diameter represent a qualitative change in wave front control compared to 10 m technology.