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 conventional spectrographs. Under normal working conditions with a 1-2 arcsec slit, the resolution is 15-20Å in first order (8-10Å in second order). Thus 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. One 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.