UES offers two Echelle gratings: E31, with 31 rules/mm, and E79,
with 79 rules/mm. It is important to realise that both gratings give
the same resolution, and the same wavelength range.
The orders of E79 are 2.55 times as wide as
those of E31 (in the spectral direction), and consequently the
extracted continuum is less curved for E79. For E79 the interorder
distance is 2.55 times as large as that of E31, allowing for longer
slits, better sky subtraction, better flatfielding, better scattered
light subtraction. Owing to these characteristics, the E79 data
are easier to reduce with high precision than the E31 data.
As the orders of E79 are wide, the far ends of the orders are affected
by camera vignetting. In the red no order overlap is present because
of this. The E31 Echelle offers a more complete wavelength range,
with better order overlap. Use the ECHWIND program to investigate the
effects of order overlap and the position on the echellogram of the
lines you are interested in.
