Alignment-invariant mirror holder for cryogenic environment and its application to GIANO-TNG

There are many ways to achieve the positioning accuracy established from tolerance in a cryogenic environment. One method is an opto-mechanical design which stays aligned at room temperature and at liquid nitrogen temperature without modifications. This could be achieved using aluminium mirrors with thermally matched aluminium holders and optical bench, that isotropically contract at cryogenic temperatures. The design of holders should allow some adjustment of the optics positions to correct possible errors in the manufacturing of the optical bench and/or optical holders themselves, without precluding the isotropic contraction during the cooling to the working temperature. This work illustrates the last prototype of cryogenic mirror-holder developed in our laboratory. The mirror mounting is based on a set of six forces which pull the mirror against the tree orthogonal faces of a reference corner. Adjustments to the mirror alignment can be achieved by means of six aluminium micrometers. Here we describe the device and illustrate the test results.