HARMONI & ELT: prototyping and testing the spectrograph TMA collimator alignment procedure

HARMONI is the first light, adaptive optics assisted, visible and near-infrared integral field spectrograph for the European Southern Observatory’s Extremely Large Telescope (ELT). It covers a large spectral range from 450 nm to 2450 nm with spectral resolving powers R ≡ / from 3300 to 17000 and spatial sampling from 60 mas to 4 mas. It contains four identical spectrograph modules that collimate, disperse, and image the long slit at the exit of the integral field unit onto the science detectors. The collimation optics comprises of a three mirror anastigmat, using off-axis, aspheric mirrors. Previously we simulated an iterative alignment procedure that uses interferometric wavefront measurements together with a numerical optical model to minimise the wavefront error by adjusting a set of compensators. In this paper we demonstrate the feasibility of this procedure with an off-axis parabolic mirror as a prototype collimator, that has the same number of compensators as the HARMONI collimators. We show that we achieve a sufficiently low wavefront error within a few iterations of alignment. We cooled the test setup in a cryostat and obtained preliminary wavefront measurements at a cryogenic temperature of 130 ˚K, demonstrating the athermal design of the prototype.