A superfluid helium insert was developed for cryogenic microscopy of millimeter-sized specimens. An optical-interferometric position sensor, cryogenic objective mirror, and piezo-driven cryogenic stage were fixed to an insert holder that was immersed in superfluid helium. The single-component design stabilized the three-dimensional position of the sample, with root-mean-square deviations of ( x, lateral) 0.33 nm, ( y, lateral) 0.29 nm, and ( z, axial) 0.25 nm. Because of the millimeter working

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... ange of the optical sensor, the working range of the sample under the active stabilization was ( x, y) 5 mm and ( z) 3 mm in superfluid helium at 1.8 K. The insert was used to obtain the millimeter-sized fluorescence image of cell nuclei at 1.8 K without a sample exchange.