The instrumental resolution (both spectral and spatial) is a convolution of the camera resolution, dispersion mode, spectrograph entrance aperture, telescope focus, and spacecraft pointing stability. While the dominant effect is due to the camera, telescope focus and spacecraft pointing stability also play a major role in defining the resolution. In addition, it is well known that the camera resolution is highly wavelength dependent. According to the IUE Camera Users Guide (Coleman et al. 1977), the camera point spread function (PSF) consists of a narrow gaussian-like core having a full width at half maximum (FWHM) of 2 to 5 pixels and a weak long-range tail. The actual resolution in either the spatial or spectral direction can be defined as a function of the FWHM. The Rayleigh criterion of instrumental resolution specifies that two spectra (spatial direction) or two spectral features (spectral direction) can be resolved provided their separation is as follows (Weinstein and Pérez 1992):
