
13.6.
Fourier Transform Spectrometer 347
For the Fabry-Perot we assume
01
= 4E5, a resolving power appropriate for
the observation of narrow spectral lines, for example. With this ^ we find
Sp =
15
arc-min, which projects to about 23 arc-sec on the sky. Light loss at the
entrance aperture is not a problem with this telescope- spectrometer combination
and the system is far
fi-om
any reasonable seeing limit. It is important to note that
for 1 = 500 nm and N = 40 the fi-ee spectral range of the Fabry-Perot for this
resolving power is only 0.05 nm, compared to 5 or lOnm for an echelle in this
same wavelength range.
These examples illustrate th