  We fabricated a 302 mm diameter low-pass filter made of alumina that has an
anti-reflection coating (ARC) made with laser-ablated sub-wavelength structures
(SWS). The filter has been integrated into and is operating with the MUSTANG2
instrument, which is coupled to the Green Bank Telescope. The average
transmittance of the filter in the MUSTANG2 operating band between 75 and 105
GHz is 98%. Reflective loss due to the ARC is 1%. The difference in
transmission between the s- and p-polarization states is less than 1%. To
within 1% accuracy we observe no variance in these results when transmission is
measured in six independent filter spatial locations. The alumina filter
replaced a prior MUSTANG2 Teflon filter. Data taken with the filter heat sunk
to its nominal 40 K stage show performance consistent with expectations: a
reduction of about 50% in filters-induced optical power load on the 300 mK
stage, and in in-band optical loading on the detectors. It has taken less than
4 days to laser-ablate the SWS on both sides of the alumina disk. This is the
first report of an alumina filter with SWS ARC deployed with an operating
instrument, and the first demonstration of a large area fabrication of SWS with
laser ablation.
