Large aspherical mirror testing with CGH and auxiliary lenses

A method combining CGH (computer-generated hologram) and auxiliary lens to realize zero-position compensation interferometry through hybrid compensation measurement can solve the problem of high-precision surface detection of large-aperture and long-focal length optical reflectors.

 

At present, the radius of curvature of the primary mirror in some large-aperture optical telescopes has reached the order of tens of meters. If the surface of the mirror is tested simply by CGH, the length of the testing optical path is not lower than the length of its radius of curvature. Due to factors such as site size and ambient airflow disturbance, it is difficult to achieve high-precision measurement of the mirror surface under these conditions. In order to solve the problem of high-precision surface testing of large-aperture long focal length optical mirror, a hybrid compensation method is proposed.

This article proposes a high-precision detection scheme for the surface measurement of large curvature radius aspheical surfaces. The scheme combines CGH (computer generated hologram) and auxiliary lenses to achieve zero position compensation interference measurement through hybrid compensation measurement, while shortening the detection optical path length and improving measurement accuracy.

 

Hybrid Compensation Model of CGH and Auxiliary Lenses
In the optical path design, an auxiliary lens is used to shorten the detection optical path length, making the curvature radius of the rear surface of the lens close to the central curvature radius of the non-spherical surface. The CGH is designed at the focal plane of the interferometer, utilizing its flexibility to compensate for residual aberrations, thereby achieving zero position compensation measurement for long focal length non-spherical mirrors. This method reduces the detection optical path length to within 1/8 of the non-spherical curvature radius, with a measurement accuracy better than RMS λ/100 (λ=632.8nm).

 

Hybrid compensation optical path with CGH and auxiliary lenses

 

Optical System Precise Alignment Model
To ensure measurement accuracy, it is necessary to precisely align each optical component in the optical path (interferometer, CGH, auxiliary lens, and the non-spherical mirror to be measured).
The CGH is designed with five areas to assist in alignment:

Non-spherical detection area: for measuring the non-spherical mirror.

CGH and auxiliary lens alignment area: for aligning the auxiliary lens.

CGH and interferometer alignment area: for aligning the interferometer.

Mirror reference area: for projecting crosshairs onto the edge of the non-spherical surface.

Lens reference area: for projecting crosshairs onto the edge of the auxiliary lens.

This passage is selected from “Long focal length aspherical mirror testing with CGH and auxiliary lenses (invited)” Journal Infrared and Laser Engineering