A document discusses a broadband (white light) point source, located at the telescope Cassegrain focus, which generates a cone of light limited by the hole in the secondary mirror (SM). It propagates to the aspheric null-mirror, which is optimized to make all the reflected rays to be normal to the primary mirror (PM) upon reflection. PM retro-reflects the rays back through the system for wavefront analysis. The point source and the wavefront analysis subsystems are all located behind the PM. The PM phasing is absolute (white light) and does not involve the SM.
A relatively small, aspheric null-mirror located near the PM center of curvature has been designed to deliver the high level of optical wavefront correction. The phasing of the segments is absolute due to the use of a broadband source. The segmented PM is optically aligned independently and separately from the SM alignment. The separation of the PM segments alignment from the PM to the SM, and other telescope optics alignments, may be a significant advantage, eliminating the errors coupling. The “point source” of this concept is fully cooperative, unlike a star or laser-generated guide-star, providing the necessary brightness for the optimal S/N ratio, the spectral content, and the stable on-axis position. This concept can be implemented in the lab for the PM initial alignment, or made to be a permanent feature of the space-based or ground-based telescope.
This work was done by Mayer Rud of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com /tsp under the Physical Sciences category. NPO-47032
This Brief includes a Technical Support Package (TSP).
Large Telescope Segmented Primary Mirror Alignment
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Overview
The document titled "Large Telescope Segmented Primary Mirror Alignment" from NASA's Jet Propulsion Laboratory discusses the challenges and solutions related to the optical alignment of segmented primary mirrors (PM) in large telescopes, including those intended for space-based applications. As telescope dimensions increase, achieving precise alignment of the optical segments becomes increasingly complex.
Traditionally, alignment in laboratory settings has relied on auto-collimating flats (ACF), which pose significant logistical challenges due to their large size. In field applications, astronomers typically use distant point sources, such as stars or laser-generated guide stars, to align the PM segments. However, these sources must be bright and well-positioned within the telescope's field of view, which can be limiting.
To address these challenges, the document proposes a novel approach that replaces the ACF and distant point sources with a fully cooperative built-in source for optical alignment. This innovative concept aims to streamline the alignment process, making it more efficient and effective for large telescopes.
The technical details include the use of a broadband (white light) point source located at the telescope's Cassegrain focus. Light from this source is directed through a hole in the secondary mirror (SM) to an aspheric null-mirror, which is optimized to ensure that all reflected rays are normal to the primary mirror upon reflection. This setup allows for accurate wavefront evaluation as the primary mirror retro-reflects the rays back through the system.
The document also provides specific technical parameters, such as the double-pass wavefront measurement of 3.3 nm RMS and the characteristics of the aspheric surface of the null-mirror. The aspheric null-mirror is designed with precise coefficients to achieve the desired optical performance.
Overall, this technical support package outlines a significant advancement in the field of telescope design and alignment, emphasizing the importance of innovative solutions to meet the engineering challenges posed by increasingly large and complex astronomical instruments. The proposed methods not only enhance alignment accuracy but also have broader implications for future telescope developments and astronomical research.
