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Measurement in diffuse light using a hydrogen discharge lamp

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

Optical setup:hydrogren lamp => diffuser => W25 filter => Heliostar etalon (with its convergent beam facing the diffuser, but without its divergent lens)  => 85 mm f/1.8 S Nikkor lens (focused to the infinite) with Nikon Z7 II.



Etalon not compressed:

Interference fringes obtained when the etalon is not compressed.
Nikkor 85 mm  f/1.8S - Nikon Z7 II - RAW mode - 14-bit acquisition - 2 s exposure time - 100 ISO


Raw FWHM = 0.36 A, deconvoluated FWHM < 0.30 A, delta CWL = + 0.76 A, FSR = 12.2 A, gap = 0.177 mm.


Etalon tuned on Ha:


Raw FWHM = 0.42 A, deconvoluated FWHM = 0.32 A, FSR = 12.2 A, gap = 0.177 mm.