- The interference patern is quite uniform with a nice Lorentzian profile for the rings.
- The mesurements from the different rings are very consistent.


Measurements were made at full-aperture (relevant for a use of the etalon in a collimated beam) and over a 21 mm diameter (relevant for a use of the etalon in a telescentric beam with the IMX432). Both measurements are very similar.

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Measurement of the etalon by spectroscopy in collimated beam using the Sun as a light source


Optical setup: Sun => Lunt 40 => ZWO 80/600 + 2× AP Barlow lens => SGH700

Configuration of the SHG 700 :
- slit placed placed preciselly at the focus of the ZWO80/600 (in order to have a collimated beam on the etalon,
- thanks to the use of the 2× AP Barlow lens, the f-ratio falling on the SGH700 slit is large enough (f/30) to prevent any vigneting on the grating. In other words, all the surface of the etalon is sampled by the grating (see Christian Buil, 2026, p 135 for technical explanations) ,
- 125 mm collimator, 2400 lpmm grating, 200 mm imaging lens, 10 microns × 4.5 mm slit, IMX585

- top image : solar spectrum,
- bottom image : solar spectrum transmitted by the etalon.


The transmission profile of the etalon (= transmitted spectrum:solar spectrum) is curve-fitted using Voigt functions in order to calculate the FWHM, FSR and delta CWL of the etalon.

FWHM = 0.47 A
FSR =  11.3 A
delta CWL = 0.09 A

NB : the FWHM is deconvoluated from the spread function (=spectral resolution) of the SHG700.