The maximum slit length available with the 31.6 lines mm
grating is
often insufficient to provide a clear sky spectrum on the same frame as the
object. A periscope assembly is available which redirects light from two
apertures 
long and separated on the sky by 40". Each echelle
order is therefore split into two sections, one containing light from the
object and the other from the sky. Because of the narrow aperture size needed
to prevent order overlap with the 31.6 lines mm
grating, there will be
a loss of light in poor seeing conditions. There are two periscopes available
which are mounted at one end of the dekker slide and are optimized for the red
and blue ends of the spectrum. The control program will automatically select
the correct one to use.
The periscopes automatically use the focal modifiers in order to gain adequate
order separation.
Main commands: PERI IN/OUT