Ring Mirror Range Finder for Telescope Mirror Radius Measurement
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Solution Overview
Problem
Existing methods for measuring the radius of curvature of primary mirror segments in telescopes are impractical, especially in cryogenic environments, due to limitations with spacer rods, laser finders, and center-hole fixtures, which face challenges with calibration, vibration errors, and accessibility.
Innovation Solution
A system using a range finder with a ring mirror and multi-surfaced prism to steer and reflect a laser beam, combined with a movable mask and nulling device, allows for precise measurement of mirror segments by providing a zero reference point and sequential addressing of optical surfaces, enabling accurate radius of curvature determination.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a spacer rod is used to mechanically set the radius of curvature, then the radius can be precisely controlled, but the device becomes impractical for large spacings and requires calibration at cryogenic temperatures
Solution Approach 1:
The patent replaces the mechanical spacer rod system with an optical measurement system using a range finder and ring mirror. Instead of physically setting the radius with a mechanical rod, the system uses laser ranging to measure the distance from the range finder to the mirror surface, combined with the ring mirror's geometric properties to determine radius of curvature. This substitution eliminates the need for long, calibrated mechanical rods while maintaining measurement precision.
2Manufacturing precision
If a spacer rod is used to measure radius of curvature, then mechanical positioning is achieved, but the rod tip may damage the coating or glass of the PM surface
Solution Approach 1:
The patent eliminates the mechanical contact between the measurement tool and the mirror surface by replacing the spacer rod with an optical range finder. The range finder measures distance non-contactually using laser time-of-flight, and the ring mirror provides a reference surface without requiring physical contact. This substitution completely removes the risk of mechanical damage to the delicate mirror coating or glass.
3Stability of the object's composition
If laser finders are mounted on different isolation platforms, then vibration isolation is achieved, but vibration errors affect the measurements
Solution Approach 1:
The patent merges the range finder and ring mirror into a single integrated measurement system. The ring mirror is positioned such that both the range finder beam and the reference features are measured from the same platform and reference frame. This integration ensures that any platform vibrations affect both measurements equally, allowing the differential measurement to cancel out vibration errors while maintaining high precision.
4Measurement precision
If the center-hole fixture is used with spherical mirror, then indirect determination of RoC is achieved, but extensive calibration of metal fixture and glass spherical mirror is needed
Solution Approach 1:
The patent extracts the calibration complexity from the measurement system by using a ring mirror with geometrically defined reference features rather than a spherical mirror requiring extensive calibration. The ring mirror's references are defined by its geometry (inner and outer diameters) which can be manufactured with high precision and do not require the same level of individual calibration as spherical mirror surfaces. This extraction of calibration requirements simplifies the overall system setup and maintenance.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This system enables quick and accurate measurement of the radius of curvature of multiple mirror segments, reducing calibration needs and avoiding vibration errors, while being insensitive to optical path length changes, thus improving alignment and diagnostic capabilities in cryogenic conditions.
Implementation Method 1
The laser rangefinder generally measures the distance to the object under test by computing a time of arrival of a transmitted pulsed laser beam
Implementation Method 2
The ring mirror has (a) a circumferential reflecting surface for reflecting a portion of the transmitted beam back to the range finder, and (b) a central aperture for passing another portion of the transmitted beam toward the optical surface
Data Source
AI summary
A system for measuring distance to an optical surface includes a range finder for transmitting a beam toward the optical surface, and a ring mirror disposed between the range finder and the optical surface. The ring mirror has (a) a circumferential reflecting surface for reflecting a portion of the transmitted beam back to the range finder, and (b) a central aperture for passing another portion of the transmitted beam toward the optical surface. The circumferential reflecting surface is effective in providing a zero reference point for the range finder. The central aperture is effective in passing the transmitted beam to the optical surface for sequentially addressing the optical surface at different locations on the optical surface. The transmitted beam may be steerable for (a) providing the zero reference point circumferentially about the reflecting surface, and (b) providing sequential addresses to locations on the optical surface.