Null correctors for 6.5-m f/1.25 paraboloidal mirrors

Abstract

The instruments used to interferometrically measure the optical surfaces of the 6.5-m /1.25 primary mirrors for the MMT conversion and the Magellan telescope projects must compensate over 800 µm surface departure from the best fitting sphere. The errors in the optical test must not contribute more than 0.04 arc seconds FWHM to the final image and the conic constant must be held to 0.01%. This paper presents an overview of the instruments that were built to measure these giant mirrors to such high accuracy.

Architecture for in-space robotic assembly of a modular space telescope

“…This architecture is similar to the interferometric test tower used in the manufacture of monolithic 8.4 m mirrors. 38,62 Three technologies using this centralized laser source provide a possible metrology architecture that satisfies the requirements for telescope assembly and operation. These include an absolute metrology technique using a modulated dual-frequency laser, dynamic Zernike wavefront sensing for coarse mirror phasing, and array heterodyne interferometry (AHI) for fine phasing.…”

Laser-Induced Local Oxidation of Thin Metal Films: Physical Fundamentals and Applications

“…Another modern requirement is that a DOE with a specific ring structure should be generated. For example, the DOE designed to test and certify the aspheric wave front of a primary mirror in advanced telescopes must have circular diffractive structures with an accuracy of 10 nm at a minimal spacing less than 1 µm and overall sizes of several hundred millimeters [27]. These elements can only be fabricated with the desired accuracy and quality based on thermochemical method by means of a polar coordinate laser pattern generator or circular laser writing systems (CLWS).…”

“…To inspect the primary mirrors of telescopes with diameters of 6.5 and 8.4 m (MMT, Magellan, and LBT projects), DOEs have been developed and fabricated to control the null lenses for the visible (633 nm) and IR (10.6 µm) spectral ranges [26,27]. The reflective DOEs were fabricated using CLWS by the thermochemical technology on a chromium film (about 60 nm thick) deposited on high-quality astro-glass-ceramic substrates.…”

<title>Application of diffractive optical elements in laser metrology</title>

“…This 136 mm DOE has about 32000 diffractive zones and was written onto a λ/20 flat quartz substrate. The measurement of the null lens [23] depicted in Fig.9c shows its errors of 17 nm rms spherical aberration plus 12 nm rms surface irregularity (with spherical aberration removed). This agrees with the null lens tolerance analysis, which indicates 22 nm rms spherical aberration and 21 nm rms irregularity.…”

“…Our diffractive elements were applied for certification of 6.5-m and 8.4-m main mirrors of telescopes made in the Steward Observatory Mirror Lab of Arizona University (USA.) [5]. Our efforts were also directed on the development of the new methods of manufacturing continuous-relief DOEs [6].…”