Next: Stability and radial velocities
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Nearly all of the light losses within ISIS are due to reflective and air-glass surfaces. There is a small loss of light (<10%) in each camera due to optical vignetting; and for the highest dispersions there is some loss due to overfilling of the grating (see Appendix D). The throughput of ISIS has been measured by C.R. Jenkins and P. Terry, and their results are discussed in detail in ING La Palma Technical Note no. 88. Briefly the throughput of the red channel without the grating is measured to be 51%, and that of the blue channel without the grating to be 42%. These measurements were made with a HeNe laser at 6300 Å, and thus for the blue arm are slightly beyond the wavelength range over which it is optimised. The values for the red arm are also slightly low as the red fold mirror in use at the time was below its specification.
The efficiencies of the ISIS gratings have been measured in the laboratory and are presented in Figure 15. The holographic grating has a lower efficiency than the ruled gratings, and is also used at a grating angle such that the beam overfills the grating, resulting in some light loss.
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The calculated peak efficiency for FOS-II is 70% in first order at 7300 Å, and 50% in second order at 4100 Å. The diagrams below show the calculated efficiency as a function of wavelength, and the measured efficiency of the entire system (including telescope, atmosphere and detector) as a function of wavelength.
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The efficiency of the overall system is dependent upon the throughput of atmosphere, telescope and instrument, and the efficiency of the detector. Measured values of the number of photons//s detected for objects of accurately known magnitude are presented in Section 6.