The Immersion Micro-Objective

A special immersion micro objective was developed for operation under liquid helium conditions [43]. It is a homocentric mirror objective and consists of two suprasil components (see Fig. 22, inset c) The smaller element was fabricated from a full sphere. One of its planar surfaces was polished and served as specimen holder. The distance between this plane and the center of the sphere determines the magnification of the objective. On the other side a second planar surface produced a separation from the reflective coating, which was deposited on the second element. This component forms a homocentric meniscus shell with a convex surface and a small planar central output area. The convex and concave surface were mirror-coated. Both elements are held together by adhesion. 

This setup has the following advantages (i) It is insensitive to vibrations, which occur in the cryostat (ii) the full fight path passes through suprasil and is not affected by immersion in liquid helium (iii) Suprasil has a small expansion coefficient, which does not change the optical properties of the objective at low and high temperatures and (iv) the shrinking of the optical components leaves the ratios of all dimensions invariant and does not alter its functionality. 

Assuming that the sample has the same refraction index as suprasil (about 1.455), the objective has a magnification of 100 in lateral and a 8000 in axial direction. The numerical aperture is NA = 0.722, but a central part with an aperture of NA = 0.329 is lost due to the Cassegrain mirror design. Its theoretical resolution is 0.52 pm. Small spherical and chromatic aberrations reduce the resolution to 1.0 pm. 

---