Principle of the Fabry-Perot interferometer

The Fabry-Perot interferometer is made of two semi-reflecting plates of glass, parallel, producing an interference pattern.

Interference rings produced by a spectral lamp observed through a Fabry-Perot

Schematic drawing of a Fabry-Perot interferometer.

1. Some history

Charles Fabry and Alfred Perot invented this device in their laboratory at Marseille.

One of the first interferometers made by Fabry and Perot

2. Today interferometers

Such devices are today commercially available. Queensgate Fabry-Perot with its CS100 controler :

The spacing between the plates can be adjusted owing to piezoelectric actuators. This allows to scan the interference pattern.

3. Principle of a scanning interferometer

The spacing between the plates is changed by small steps in order to scan the whole free spectral range
(which is the range from one ring to the following one).

An image of the interference pattern produced by a galaxy (or nebula) is made for each scanning step so
that the observation produces a data cube made of a set of several images.

For any pixel in the field one can then plot the change in light intensity observed along the scanning
process, thus drawing the profile of the emission line of interest which has been selected (owing to an
interference filter).
Most often it is the H alpha line of hydrogen which is chosen, because it is the brightest line emitted
by galaxies.

4. Computing the velocity field of a galaxy

The wavelength of an emission line changes with the radial velocity (velocity along the line of sight) of the object observed in the sky, because of the Doppler shift. Objects with approaching velocities appear bluer while those with receding velocities appear redder. Inside the same galaxy, the rotation of the galactic disk will produce changes in radial velocities.

As a consequence, the profile of the line seen in the lower plot of section 3 will be shifted toward the left or the right, depending on the velocity observed for each pixel of the object. A velocity can be computed for each pixel, thus producing a 2D velocity field of a galaxy.