Abstract
We explore causes and possible techniques to minimize shape deformation of high-precision monocrystalline calcium fluoride optical surface formed during thermal annealing. Due to the anisotropic mechanical properties of calcium fluoride crystals, machining of lens shapes introduces defects into the crystal lattice. The thermal annealing thus leads to activation of the processes of recovery, resulting in the formation of characteristic surface structures causing both shape error and increased microroughness. The surface deformation can be gradually minimized by thermal treatment of the optical element and subsequent polishing steps to the order of units of nanometers, so they do not represent a fundamental problem for optical performance.
Highlights
In recent times, crystalline materials have been increasingly used in the field of ultraprecision optics for high-performance optical systems, including systems for space
One of the cases realized in the TOPTEC Centre is the optical telescope of the FLORIS satellite fluorescence spectrometer designed for observing photosynthetic activity from orbit within the ESA FLEX mission.[1]
Due to the high spectral resolution demand, the fundamental requirement is stray light, which is limited to avoid pollution of the fluorescence signal; the important contribution is the spectral stray light, which comes from the light scattered by the telescope and spectrometer optical elements and which defines very tight requirements for the optical elements surface microroughness and mid-spatial frequencies (MSF) content.[1]
Summary
Crystalline materials have been increasingly used in the field of ultraprecision optics for high-performance optical systems, including systems for space. FLORIS is a push-broom hyperspectral imager, flying on a medium size platform, that measures the vegetation fluorescence in the spectral range between 500 and 780 nm at medium spatial resolution (300 m) and over a swath of 150 km. It accommodates an imaging spectrometer with a very high spectral resolution (0.3 nm) to measure the fluorescence spectrum within two oxygen absorption bands. The FLORIS telescope is the Petzval type objective with four spherical (two from monocrystalline calcium fluoride: CaF2) and one aspheric lens It has a real entrance pupil located 75 mm in front of the first lens, where the instrument aperture stop is accommodated, and it is telecentric in the image focal plane. Due to the high spectral resolution demand, the fundamental requirement is stray light, which is limited to avoid pollution of the fluorescence signal; the important contribution is the spectral stray light, which comes from the light scattered by the telescope and spectrometer optical elements and which defines very tight requirements for the optical elements surface microroughness and mid-spatial frequencies (MSF) content.[1]
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