We describe a new aerogel threshold Cherenkov detector installed in the HMS spectrometer in Hall C at Jefferson Lab. The Hall C experimental program in 2003 required an improved particle identification system for better identification of π / K / p , which was achieved by installing an additional threshold Cherenkov counter. Two types of aerogel with n = 1.030 and n = 1.015 allow one to reach ∼ 10 - 3 proton and 10 - 2 kaon rejection in the 1–5 GeV/ c momentum range with pion detection efficiency better than 99% (97%). The detector response shows no significant position dependence due to a diffuse light collection technique. The diffusion box was equipped with 16 Photonis XP4572 photo multiplier tubes (PMTs). The mean number of photo-electrons in saturation was ∼ 16 and ∼ 8 , respectively. This allows even for separation of slower particles slightly above threshold.