| Test method | f-ratio | Surface sampled | FSR | FWHM | Delta CWL | Note | |
| Heliostar 100 mm (#5) | Spectro SHG700 (slit à f/16 focus) |
full aperture | 11.3 A 11.2 A |
Etalon tuned on Ha: FWHM = 0.31 A Etalon not compressed: FWHM = 0.29 A. |
+ 0.43 A |
Measurement with the spectrosocope using the Sun as a light source
Optical setup : Sun => Heliostar 100 complete with in internal etalon and associated collimating lenses => 2× Astrophysics Barlow lens => SHG 700 => IMX 585
Configuration of the SHG 700 : 10 microns slit, f_c= 125 mm, 2400 lpm, f_im = 200 mm,. Dispersion = 0.0471 A/pixel
Thanks to the use of the 2× Barlow lens, the f-ratio falling on the slit is f/16, which means that there is no vigneting of the light beam by the grating. Accordingly, the full-aperture of the etalon is measured.
The following three images show:
- the solar spectrum,
- the solar spectrum transmitted by the etalon when not compressed,
- the solar spectrum transmitted by etalon when tuned on Ha.
Etalon transmission (wavelength) = Transmitted solar spectrum (wavelength) / Solar spectrum (wavelength).
The etalon transmission profile is curved fitted with Voigt functions using Fityk.
Without compression:
FWHM = 0.29 A, FSR = 11.2 A, delta CWL = + 0.43 A
With the etalon tuned on Ha:
FWHM = 0.31 A, FSR = 11.3A.