A cryogenic steering mirror for source selection in multi-integral-field spectrographs

Multi-integral-field spectrographs for near-infrared observations require a large number of complex cryogenic mechanisms to select source images in the telescope field of view which are then re-format on the spectrograph entrance slit. Source selection can be achieved in several ways, but the two methods most adequate for large fields and cryogenic environment are positioning of an optical element in the telescope field to pick off the source image, or steering a mirror located in a pupil image to deflect the light from a source into relay optics. The first solution permits high flexibility in source selection at the cost of large mechanical travels. The second solution limits the source selection to one per pre-defined sub-field, but gets by with small mirror tilts. Higher flexibility can be regained for the second solution by assigning different sub-field sizes to the steering mirrors in the central and in the peripheral areas of the field of view. We present a solution for a cryogenic steering mirror unit with a mirror diameter of about 20 mm and tilt angles of a few degrees, appropriate for source selection in a 1 arc minute field of an 8 m class telescope. The gimbaled mirror can be tilted about two perpendicular axes in the tangential plane of the mirror apex. The mirror is driven by two Nanomotors, and the motor strokes are measured by LVDTs. Motors and sensors are specified for operation at LHe temperatures.