FLASH $^{+}$ —A Dual-Channel Wide-Band Spectrometer for APEX

We report on the design and on-site performance of the recently upgraded FLASH ⁺ instrument operated at the APEX telescope in the Chilean Atacama desert in 5100 m altitude. The dual-channel receiver allows parallel observations in the atmospheric submillimeter windows between 268 and 516 GHz. A signal of in total 16 GHz is processed simultaneously. Equipped with state-of-the-art sideband separating mixers (spin-off developments from the ALMA bands 7 and 8), its outstanding on-sky performance makes FLASH ⁺ a most efficient spectral line mapping machine. Operation is fully automated and allows flexible remote observations from the APEX base in San Pedro de Atacama.