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Performance

The main factors governing the suitability of FOS for astronomical observations are the spectral resolution and the efficiency. As far as resolution goes, FOS works at a lower resolution than most other conventional spectrographs. The dispersion is 450Å /mm; camera resolution with a narrow slit (0.25 arcsec) is 1.2pixels (12.8Å) fwhm, independent of wavelength along the full width of the 4 arcmin slit. Under normal working conditions with a 1-2 arcsec slit, the resolution is 15-20Å in first order (8-10Å in second order). Hence it would appear that FOS is suitable for studying emission line objects (AGN, HII regions, planetary nebulae), or continuum studies. It is questionable whether FOS is suited to studies of narrow absorption line objects (solar type stars etc.) where absorption features would fill only 1-2 pixels. The principle advantage of FOS is that it is considerably faster than a conventional spectrograph, and so is likely to be useful for those objects which are too faint for their spectra to be obtained at a high enough signal to noise in reasonable integration times with other instruments. The total throughput of the entire system (atmosphere,telescope and FOS with the extended blue CCD) in first order is 12.5% at peak. Only a small proportion of this loss occurs in the FOS optics - the throughput of the FOS minus detector is approximately 70% at blaze. The efficiency of FOS in first and second order is shown in Figure 1.5.

In Figure 1.6 we show the sky subtracted signal to noise per first order 10.7Å pixel, assuming the object occupies 3 columns of the chip, and adopting mode D operation (25 arcsec slot for sky + object). Adequate classifications of broad features (QSOs, emission line galaxies, M stars) can generally be made with a s/n of around 5; absorption line objects usually demand a s/n of better than 15. The format of the spectrum is shown in Figure 1.7. If an object is blue, the curved third order is seen to the left of the first two. This is faint; there is no software developed to make use of it.