The separation of successive spectral orders on the detector is a function of both wavelength and echelle grating in use. The function has a minimum near 7500 Å, where adjacent orders are separated by 1.24 and 0.49 mm with the 79 and 31.6 echelle gratings respectively. The corresponding separations on the sky, which in turn determine the maximum slit lengths usable before orders overlap, are 18.6 and 7.5 arcsec respectively; in practice somewhat smaller values ( and arcsec) would be used to ease data extraction from the 2-D frames. Maximum order separation is in the ultraviolet; for example at 3500 Å successive orders are separated by 35 and 14 arcsec with the 79 and 31.6 echelle gratings respectively. For this reason, the 31.6 echelle is normally used for observations of objects which are much brighter than the sky, while the 79 grating is more appropriate for fainter objects, requiring accurate sky subtraction with minimum deterioration of the S/N.
By using an order sorting filter to separate a single echelle order (at
H, for example), it is possible to use UES to obtain spatially resolved
observations of extended objects up to the full 5 arcmin length of the slit
(with some minor vignetting at the edges). Any of the TAURUS narrow band
filters listed in Appendix can be used for this purpose.