  We present the results of commissioning observations for a new digital
beamforming back end for the Focal plane L-band Array for the Robert C. Byrd
Green Bank Telescope (FLAG), a cryogenically cooled Phased Array Feed (PAF)
with the lowest measured T_sys/eta of any PAF outfitted on a radio telescope to
date. We describe the custom software used to apply beamforming weights to the
raw element covariances to create research quality spectral line images for the
new fine-channel mode, study the stability of the beam weights over time,
characterize FLAG's sensitivity over a frequency range of 150 MHz, and compare
the measured noise properties and observed distribution of neutral hydrogen
emission from several extragalactic and Galactic sources with data obtained
with the current single-pixel L-band receiver. These commissioning runs
establish FLAG as the preeminent PAF receiver currently available for spectral
line observations on the world's major radio telescopes.
