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.