Measurement in diffuse light using a hydrogen discharge lamp

In this test, the Fabry-Perot etalon is used as an interferometer. The whole the surface of the etalon is sampled. The FWHM, FSR and air-gap are derived from the measurements of the fringe system.

The etalon is removed from the Heliostar 100. The convergent lens is removed. The side of the etalon facing the diffuser is the side with the divergent lens. The divergent lens has no impact on the test. The optical setup is:

Optical setup: hydrogren lamp => diffuser => W25 filter (to remove continuum from the hydrogen lamp => divergent lens (no impact on the fringe system) => etalon (without the converging collimating lens) => 135 mm f/1.8 Sigma lens (focused to the infinite) with Nikon Z7 II.


1 - Etalon tuned close to Ha


Interference fringes obtained with the etalon tuned close to Ha.
Sigma 135 mm f/1.8 - Nikon Z7 II - RAW mode - 14-bit acquisition - 1.3 s exposure time - 100 ISO

Raw FWHM = 0.50 A,
deconvoluated FWHM = 0.42 A.

NB : the intensity profile shows that the etalon is not precisally set on Ha (flat top). A more accurate measurement of the FWHM with the etalon tuned right on Ha would require the use of the spectro.


Etalon not compressed (CWL = Ha + 0.65 A)



Raw FWHM = 0.34 A,
deconvoluated FWHM = 0.21 A.


Summary of measurements made: