Basic principle of curved slit spectroscopic observations with ARGOS plus LUCI. Top panel: K_s-band image of a gravitationally lensed object (the 8 o’clock arc) with the mask design overlaid. From the design, a custom, laser cut mask is made and inserted and stored cryogenically in the LUCI instruments. During the observation only the light from the desired object enters the spectrometer. The ARGOS system allows tiny slits to be used with all light from the object passing through. As a result the spectral resolution is high and the skylines are well resolved. This allows for much better object flux extraction.