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We noted above that the geometrical efficency of the instrument without the focal modifier lens is 80 per cent of that with the lens. In other words, inserting the focal modifier lenses increases the geometrical efficency of the instrument by 25 per cent. At first sight it would therefore seem best to operate the instrument with the lens in the beam.
However, removing the lens increases the input f/ratio of UES from f/11.1 to f/13.9, and makes it possible to increase both the slit length and the slit width by 13.9/11.1, a factor of 1.25, whilst leaving the resolution at the detector constant. The gain this provides in spectrograph efficiency depends on the type of object being observed. For a spatially extended source, where the amount of emission detected is limited both by the slit length and the slitwidth, the improvement in slit-losses is 1.25, or an extra 50 per cent. For stellar work, slit-losses are due to broadening of the image by seeing, and depend on the ratio of the seeing FWHM to the slitwidth; in this case we can expect gains of up to 25 per cent, depending on the seeing.
The other reason for for removing the lens is that the transmission of the lens is low. The transmission was measured by illuminating the slit with a diffuse source of light. Spectra were taken with the focal modifer lenses both in and out of the beam, adjusting the slitwidth and length as described above. Under these circumstances we expect the spectrum taken without the focal modifier lenses in the beam to be 25 per cent brighter than the spectrum with the focal modifier lenses in the beam (a 50 per cent increase due to the larger slit, partially offset by a 20 per cent reduction due to overfilling of the grating)
In practice we found that at all wavelengths the spectra taken without the focal modifier lenses in the beam were over 50 per cent brighter than the spectra with the focal modifier lenses in the beam. For the red focal modifier lenses, between 5000 and 7000 Å, the factor was even larger.
This result implies losses due to the focal modifiers themselves of at least 20 per cent. Taken together with the existence of slit losses, this more than offsets the slight gain in efficency at the grating obtained by using the focal modifiers. There are therefore no circumstances under which it makes sense to use the focal modifier lenses.