  Recording the fluorescence of a magneto-optical trap (MOT) is a standard tool
for measuring atom numbers in experiments with ultracold atoms. When trapping
few atoms in a small MOT, the emitted fluorescence increases with the atom
number in discrete steps, which allows to measure the atom number with
single-particle resolution. Achieving such single particle resolution requires
stringent minimization of stray light from the MOT beams, which is very
difficult to achieve in experimental setups that require in-vacuum components
close to the atoms. Here, we present a modified scheme that addresses this
issue: Instead of collecting the fluorescence on the MOT (D2) transition, we
scatter light on an additional probing (D1) transition and collect this
fluorescence with a high-resolution microscope while filtering out the intense
MOT light. Using this scheme, we are able to reliably distinguish up to 17
$^{40}$K atoms with an average classification fidelity of 95 \%.
