|
The Potsdam Arrayed Waveguide Spectrograph (PAWS) is built upon an integrated photonic spectrograph designed for astronomy. Similar to integrated optic beam combiners for interferometry, PAWS is intended to demonstrate on sky how a traditional bulk optics spectrograph with R = 15.000 in the H band can be miniaturized to fit on a chip. The integrated photonic spectrograph is based on second-generation Arrayed Waveguide Gratings (AWG) with unprecedented performance in terms of spectral resolution and throughput. The fibre-coupled AWG serves as a first dispersive element. The pre-dispersed light is fed into a free-space optical system located in a cryostat. Here the overlapping spectral orders are separated by cross-dispersion. The resulting echellogram is recorded by a Teledyne 2kx2k H2RG near-infrared array. Locally controlled constant cryogenic temperatures are required for the operation of the cryostat. This was achieved by fine-tuning and optimizing the original cryostat design using experimental data from multiple cryogenic cycles. These steps included the optimization of thermal interfaces, gold coating of the radiation shield, and an appropriate cooling sequence using the constraints of the allowed cooling rate for H2RG focal plane arrays. Using the readout electronics and GEIRS software provided by the Max Planck Institute for Astronomy (MPIA), frames of the H2RG were obtained, allowing performance calculations and dark pre-characterization of the system. For the optimum alignment of the optical system, the coefficient of thermal expansion (CTE) was measured with an interferometric set-up that recorded the spatial displacements of two reflecting optical elements within the cryostat during a cryogenic cycle. An appropriate strategy was developed to adjust the cryogenic cross dispersion optics inside the vacuum chamber to the AWG coupling optics outside the chamber.
|
