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Magnitudes are in the Johnson system. All grating efficiencies assume
they are used at the most efficient angle for that band, except 580V
and 385R which assume a blaze of 4750A and 7000A respectively. The
SPIRAL lens unit substrate does not transmit in the UV, limiting U
band performance.
Sky subtraction can be either with dedicated sky fibres from outer
parts of the IFU (few % sky subtraction accuracy), or in nod & shuffle
beam-switching mode (~0.1% sky subtraction accuracy, only half time on
source, read and sky noise doubled), or, (for sources small enough to
fit on half the detector and with adjacent clean sky on both sides) in
cross beam-switching mode (~0.1% sky subtraction accuracy, all time on
source,read and sky noise doubled).
Total time assumes readout through a single amplifier on each CCD, and
a single arc and a single fibre-flat exposure.
The calculator assumes sources are of uniform surface brightness
within the aperture. No account is taken of the effect of optimal
(variance weighted) combination of output spectra, to reduce sky and
read noise for centrally condensed sources.
This version of the calculator includes the shortfalls in throughput
measured at commissioning, as compared with the original predictions.
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