It is specially useful for accurate measurements, comparison of wavelengths and examination of hyperfine structure of spectral lines. It is also used for measuring wavelength changes by application of magnetic field.
It consist of a partially coated mirror mounted in an alumminium housing. This module is placed in front of the moving mirror to set up the Fabry Perot Interferometer. Two adjustable screws are provided for making the two etalons parallel.
In built Telescope has a magnification of 3x and is fitted with a Ramsden eyepiece & a cross line graticule.