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The Faint Object Red Spectrograph (FORS)

FORS is a fixed-format low dispersion spectrograph using a collimatorless optical design with a transmission grating and a GEC P8600 series CCD mounted within an evacuated camera. It has a high overall efficiency because of its small number of optical elements, minimal vignetting in its optics, and surface coatings optimised for its designed spectral range. It was designed by Charles Wynne and, apart from not having cross-dispersion, is virtually a twin of the Faint Object Spectrograph (FOS) in use at La Palma Observatory.

The GEC CCD detector has a format of 576 x 385 22um square pixels covering 12.7 by 8.5mm with the longest dimension aligned in the dispersion direction, and in combination with the FORS optics gives the spectrographic parameters listed in Table 5.2.


Table 5.2: FORS System Parameters
Wavelength Scale 455Å/mm 10.1Å / CCD pixel
Anglular Scale 45.5"/mm 1" / CCD pixel
Wavelength Range 5200-11000Å
Spectral Resolution 20Å

FORS can be operated in three modes:

 
Figure 5.3: The Faint Object Red Spectrograph (FORS)

Figure 5.3 shows the optical layout of FORS, while Figure 5.4 gives the FORS sensitivity as a function of wavelength. The curve has been determined theoretically and represents the combined throughput of the atmosphere, the telescope, the spectrograph and the CCD. The maximum throughput is at 7000Å, where the detected quantum efficiency reaches ~ 12%. Note that slit losses have not been allowed for, and the throughput will be lower when using FORS with fibres.

Because FORS is sensitive over such a wide wavelength range, blocking filters may be needed to eliminate the second-order spectrum in some applications.

 
Figure 5.4: Overall throughput of FORS as a function of wavelength

Further details of FORS and its operation are given in AAO UM 23: FORS users' manual.



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