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The Atacama Cosmology Telescope (ACT) is a 6 m telescope designed to map the Cosmic Microwave Background
(CMB) simultaneously at 145 GHz, 220 GHz and 280GHz. The receiver in ACT, the Millimeter Bolometer Array
Camera, features 1000 TES bolometers in each band. The detector performance depends critically on the total
optical loading, requiring the spillover contributions from the optics to be minimal. This inspired the use of
a cold Lyot stop to limit the illumination of the primary and the use of guard rings surrounding the primary
and secondary reflectors. Here, we present a direct measurement of the illumination aperture for both reflectors
and of the attenuation level outside the main optical path. We used a 145 GHz, 1mW source and a chopper
wheel to produce a time-varying signal with a broad beam profile. We sampled the response of the camera for
different locations of the source, placed in front and beside the primary and secondary mirrors. The aperture
of the primary was measured to be 5.72 ± 0.17m in diameter (95 ± 3% of its geometrical size), while the
aperture of the secondary yielded 2 ± 0.12m in diameter. Both apertures are consistent with the optical design.
Comparing to previous measurements of the beam solid angle from planet observations, we estimate an optical
efficiency of 72.3 ± 4.8%. We found that the attenuation outside the primary aperture was −16 ± 2 dB, which
is below the theoretical expectations, and −22 ± 1 dB outside the secondary aperture, which is consistent with
simulations. These results motivated the extension of the baffles surrounding the secondary mirror, with the
following reduction in detector optical loading from 2.24pW to 1.88 pW.
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