In astronomy, the term cophasing or phasing describes the process of controlling the individual segments in a segmented mirror or a telescope so that the segments form a larger composite mirroring surface. Cophasing implies precise, active control of three

degrees of freedom Degrees of freedom (often abbreviated df or DOF) refers to the number of independent variables or parameters of a thermodynamic system. In various scientific fields, the word "freedom" is used to describe the limits to which physical movement or ...

of each individual segment mirror: translation along the

optical axis An optical axis is a line along which there is some degree of rotational symmetry in an optical system such as a camera lens, microscope or telescopic sight. The optical axis is an imaginary line that defines the path along which light propagat ...

(piston) and rotation about two axes perpendicular to the optical axis (tip-tilt). Each segment of the segmented

telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...

is a solid body having 6 degrees of freedom exposed to the gravitation force, wind blowing, and other mechanical forces. If the position of each segment is not controlled the

resolution Resolution(s) may refer to: Common meanings * Resolution (debate), the statement which is debated in policy debate * Resolution (law), a written motion adopted by a deliberative body * New Year's resolution, a commitment that an individual mak ...

of the whole telescope will be the same as if 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

\lambda/40

surface rms. Projects for future

extremely large telescope The Extremely Large Telescope (ELT) is an astronomical observatory currently under construction. When completed, it is planned to be the world's largest optical/near-infrared extremely large telescope. Part of the European Southern Observator ...

s (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 The W. M. Keck Observatory is an astronomical observatory with two telescopes at an elevation of 4,145 meters (13,600 ft) near the summit of Mauna Kea in the U.S. state In the United States, a state is a constituent political entit ...

or GTC telescope, ELTs of diameters form 50 to 100 m represent a qualitative change with respect to wave front control related to segmentation in comparison with the current 10 meters technology.

References

{{reflist, refs= {{cite journal , title=Fizeau interferometric cophasing of segmented mirrors , last1=Cheetham , first1=Anthony C. , last2=Tuthill , first2=Peter G. , last3=Sivaramakrishnan , first3=Anand , last4=Lloyd , first4=James P. , display-authors=1 , journal=Optics Express , volume=20 , issue=28 , date=December 2012 , pages=29457–29471 , doi=10.1364/OE.20.029457 , pmid=23388772 , bibcode=2012OExpr..2029457C , doi-access=free Telescopes