In the infrared system, cooling down the optic components' temperature is a
better choice to decrease the background radiation and maximize the sensitivity. This
paper presented a 100K cryogenic optical system, for which an integrated designation
of mechanical cooler, flexible thermal link and optical bench was developed. The
whole infrared optic components which were assembled in a vacuum box were cooled
down to 100K by two mechanical coolers. Low thermal conductivity supports and
low emissivity multi-layers were used to reduce the cryogenic optical system's heat
loss. The experiment results showed that in about eight hours, the temperature of the
optical components reached 100K from room temperature, and the vibration from the
mechanical coolers nearly have no affection to the imaging process by using of
thermal links. Some experimental results of this cryogenic system will be discussed in
this paper.
The imaging spectrometer supplies spectral images in one spectral dimension and two spatial dimensions simultaneously. The Offner spectral imaging system was outstanding because of its small volume, light weight, free spectral smile and little keystone. However, the manufacture of the convex grating is a challenge and the cost is high. Here, an optical design of a compact 400-1000nm spectral imaging system using a planar grating based on a single freeform mirror was proposed. The spectrograph was similar with the Offner structure, only the grating is planar. The multi-spectra was split by the planar diffraction grating, and the collimating mirror and the focusing mirror were the same freeform surface by using it twice. The freeform surface was non-rotational symmetry. Its large degree of freedom can correct kinds of aberration, such as astigmatism, smile and keystone. The system has a compact volume as 120×100×100mm3. The entrance slit was 6mm, and the object NA(numerical aperture) was 0.12. The pixel size of the detector was 16μm×16μm, and its resolution was 375(spatial)×400(spectral). The design result showed that image quality close to the diffraction limit has been obtained. The maximums of the keystone and the smile at all working wavelengths in all fields were respectively 1.6μm and 7.5μm, which were both less than half of the pixel size. At last, the tolerance analysis considering manufacture and alignment of the system was done, the result showed that the manufacturability of the existing diamond turning machining technology can satisfy the accuracy need of the freeform mirror.
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