Assembling the Cryogenic Front-end for the ALPACA Phased Array Feed

The Advanced L-band Phased Array Camera for Astronomy (ALPACA) is a 69-element, fully cryogenic, phased array feed operating from 1.3-1.7 GHz proposed to be deployed at the prime focus of the 100-m Green Bank Telescope. Here we report on the progress towards assembling the front-end including a large RF-transparent vacuum window, the receiving elements, and several solutions we have deployed to significantly reduce the complexity of scaling the number of elements in an array. All the cryogenic low-noise amplifiers (LNAs) have been thermally cycled and rigorously tested. LNA bias and readout is shared on multi-channel flexible striplines that route from outside the cryostat through a vacuum interface and terminate at the cold stage where the antenna elements are deployed. ALPACA will act as a ‘Radio Camera’ by digitally forming 40 simultaneous dual polarization beams enabling large surveys for various radio astronomical applications.