To get the data shown below we took 2 exposures of 1000 second on Polaris and then subtracted one from the other. We moved the telescope in between the two exposures so that Polaris moved from one part of the IFU to another hence the positive and negative spectra. With the 5.5 mm telescope we get EXACTLY the same amount of light from the sky as we would on an 8-meter because the IFU lenses are each 8 arcmins across. On the other hand, as Polaris is a point source, it gives us as much light as an H~17 star would give on an 8-meter telescope. The data is the therefore the same as we will get on an 8-meter in terms of photon arrival rates, readnoise, instrument background level, cosmic rays, hot pixels, etc. The inter-OH sky brightness will of course be lower at a site better than Cambridge.
The total throughput (from above the Earth's atmosphere to disk file) measured from this observation is 8%.
One of the two 1000 sec exposures is shown below. The wavelength range is approx 14700 to 16900 Angstroms (short H). The faint spectrum of Polaris can be seen about 40% of the way down from the top.
The result of subtracting the second exposure from the first is shown below. Note the excellent sky subtraction. We consistently find that we can achieve Poisson-limited sky subtraction with fibres by using this beam-switching technique.