Transmission of bandpass filter in Hertz at room temperature. We assume that the bandpass when the filter is at ~4 K is essentially the same. The peak transmission is 78% at 353 μm. The transmission drops to 10% of that at 320 and 382 μm. The blocking at longer wavelengths is excellent. Any leaks in the filter at shorter wavelengths are blocked by other low‐pass filters in the instrument. Also shown is the zenith atmospheric transmission measured from the CSO during good weather, characterized by approximately 0.8 mm of precipitable water vapor and τ_{225 GHz} ≈ 0.05 (Serabyn et al. 1998).