Aberration-corrected cryogenic objective mirror with a 0.93 numerical aperture

We developed a cryogenic objective mirror [Toratani-Fujiwara (TORA-FUJI) mirror] with a 36-mu m field of view (FOV) radius and a 0.93 numerical aperture. The latest reported cryogenic objective mirror (INAGAWA mirror) under a superfluid-helium immersion condition had a nearly maximum numerical aperture (0.99) and was perfectly achromatic. However, its FOV radius was restricted to 1.5 mu m, mainly due to coma aberration. In the TORA-FUJI mirror, correcting coma aberration realized the 36-mu m FOV radius. In addition, the remaining four Seidel aberrations and the chromatic aberrations were sufficiently corrected. To evaluate the optical performance, the cryogenic fluorescence imaging of individual dyes was performed with the TORA-FUJI mirror at a 685-nm excitation wavelength. This result shows that the TORA-FUJI mirror in superfluid helium at 1.8K exhibits nearly diffraction-limited performance in the FOV region.