  This paper presents the effects of radio frequency interference (RFI)
mitigation on a radio telescope's sensitivity and beam pattern. It specifically
explores the impact of subspace-projection mitigation on the phased array feed
(PAF) beams of the Australian SKA Pathfinder (ASKAP) telescope. The goal is to
demonstrate ASKAP's ability to make science observations during active RFI
mitigation. The target interfering signal is a self-generated clock signal from
the digital receivers of ASKAP's PAFs. This signal is stationary, so we apply
the mitigation projection to the beamformer weights at the beginning of the
observation and hold them fixed. We suppressed the unwanted narrowband signal
by 31dB, to the noise floor of an 880s integration on one antenna, with a
typical degradation in sensitivity of just 1.5%. Sensitivity degradation over
the whole 36 antenna array of 3.1% was then measured via interferometric
assessment of system equivalent flux density (SEFD). These measurements are in
line with theoretical calculation of noise increase using the correlation of
the beam weights and RFI spatial signature. Further, degradation to the main
beam's gain is 0.4% on average at the half-power point, with no significant
change to the gain in the first sidelobe and no variation during extended
observations; also consistent with our modelling. In summary, we present the
first demonstration of mitigation via spatial nulling with PAFs on a large
aperture synthesis array telescope and assess impact on sensitivity and beam
shape via SEFD and holography measurements. The mitigation introduces smaller
changes to sensitivity than intrinsic sensitivity differences between beams,
does not preclude high dynamic range imaging and, in continuum 1MHz mode,
recovers an otherwise corrupted holography beam map and usable astronomical
source correlations in the RFI-affected channel.
