In order to search for ultra-dense gas condensations as possible sites of future star formation, observations with highest possible angular resolution in high-gas density probing molecular species are required. Here we report first results from two-telescope (IRAM 30-m & CSO 10.4-m) multi-transition excitation studies of the linear molecules CS and HCN, both with critical densities n c ∼107 cm−3. We obtained maps towards a number of galactic cloud cores in the CS(J=5−4;7−6) and the HCN(3-2;4-3) mm/submm transitions and those of their optically thin isotopic species (C34S; H13CN) accessible from ground. For a proper excitation analysis, the gas temperature has been determined independently from the symmetric top molecules NH3 or CH3CN. Here we present first results obtained towards the NGC 2024 molecular cloud core (Schulz et al. 1990).