  Scanning Tunneling Spectroscopy (STS) is a unique technique to probe the
local density of states (LDOS) at the atomic scale by measuring the tunneling
conductance between a sharp tip and a sample surface. However, the technique
suffers of well-known limitations, the so-called set-point effect, which can
potentially introduce artifacts in the measurements. We compare several STS
imaging schemes applied to the LDOS modulations of the charge density wave
state on atomically flat surfaces, and demonstrate that only constant-height
STS is capable of mapping the intrinsic LDOS. In the constant-current STS,
commonly used and easier-to-implement, the tip-sample distance variations
imposed by the feedback loop result in set-point-dependent STS images and
possibly mislead the identification of the CDW gap edges.
