Distribution of continuum point sources within the area of FAST sky coverage (limits shown with orange), which covers a declination range of −14.°35 to 65.°65. The red circles represent 372 sources with flux densities greater than 2.5 Jy in the NVSS survey. In initial observation periods, FAST will adopt a drift scan mode. The threshold of 2.5 Jy corresponds to a 3σ detection in OH absorption for gas with an optical depth of 0.01, in a drift scan of 12 s, at a velocity resolution of 0.25 km s⁻¹, with a system temperature of 25 K. The blue filled circles represent 1071 sources with flux densities greater than 1.25 Jy in the NVSS survey. The threshold of 1.25 Jy allows for a 3σ detection for OH having an optical depth of 5.5 × 10⁻³ in a total observing time of 10 minutes (ON+OFF) in tracking mode. The gray background is the integrated H I intensity map from the LAB H I survey (Hartmann & Burton 1997; Arnal et al. 2000; Bajaja et al. 2005). The limits of the coverage of Arecibo are shown with green solid lines. The positions of the 44 point sources used in this paper are plotted with yellow filled squares.