Analysis of Fabry-Perot mica spaced Ha filters


1) Center Wave Length (CWL) and bandpass (FWHM) as a function of f-ratio of the telescope and tilt of the F-P filter (in collimated beam, telecentric beam or non-optimized telecentric system)

2) CWL shift and FWHM broadening in non telecentric systems (refractors and Cassegrain telescopes) and useful formulae

3) Daystar filter modelling and additional results

4) Air-spaced F-P etalon theoritical performances and comparison with mica-spaced etalons

5) Analysis of the PST modification (air-spaced F-P etalon) and comparison between collimated and telecentric mounts

6) Contrast factor of the F-P etalon and blocking filter assembly

7) Contrast factor of the F-P etalon : test of various stacking schemes

8) Fabry-Perot math and bibliography


Fabry-Perot Math

The math of a F-P etalon can be expressed in a number of ways. Here is the formulation given in ref W3 :

These formulae are valid for a perfect F-P in a collimated beam (parallel beam with no fied angle). Various defects (non parallelism between the plates, roughness of plate, ...) would reduce the finesse (see ref W4).


Jacquinot spot and sweet spot :

The Jacquinot spot is defined to be the region over which the change in wavelenght does not exceed sqrt (2) times the etalon bandpass. Straightforward calculations give :


Some good references on Fabry-Perot etalons and telecentric systems :

Web ressources :

[W1] : Degradation of etalon performance by using a non-optimized telecentric lens system - Gene A. Baraff (for telecentric systems)

[W2] : On telecentric systems

[W2] :

[W3] : Teha - Telescope H Alpha pour la surveillance de l'activité solaire et la météorologie spatiale - Blanco, Malherbe - Observatoire de Paris-Meudon - LESIA - 2006 (Comparison of air space FP and Lyot filters between other things)

[W4] : Study of a Fabry-Perot interformeter prototype for space application - Luca Giovannelli - PhD Thesis

[W5] : A dual Fabry-Perot based imaging spectrometer for the National Large Solar Telescope - Indian Institute - 2010

[W6] :The Fabry-Perot interferometer :

[W7] :Notes on Fabry-Perot interferometers :

[W8] Thin-film optical filters - Third Edition - See chapter 7 for everything on F-P :

[W9] Filtre Daystar Halpha, calibration sur le soleil - Measurements of a Daystar 0.3A Ha filter at the Meudon observatory solar tower - JM. Malherbe with measurements from D. Crusaire and M. Bonafous

[W9b] : Monitoring fast solar chromospheric activity: the MeteoSpace project. Jean‑Marie Malherbe et al. Experimental Astromony. March 2022.

[W10] D-film optical filters - Third Edition - See chapter 7 for everything on F-P interferometer :

[W11] Photometric properties of new solar Ha commercial Fabry-Perot etalons (Coronado) - Cyril Bazin and Serge Koutchmy - Astrophysical Institute of Paris

[W12] Principe du filtre solaire Ha Coronado - Principle of Coronado Ha solar filter - JM. Malherbe, N. Mein - Meudon Observatory

Other interesting references :

Fabrication of high-precision solar filters : variety and limitations - Y Sidorin, A G Lunt ..- Coronado Technology Groupe - Proc. of SPIE Vol 5494 - 519

Patent US 6181 726 - Jan 30,2001 - High performance telescopic etalon filters

Patent US 6215 802- April 10,2001 - Thermally stable air-gap etalon for dense wavelenght-division multiplexing applications

Patent US 7142 573 - Nov 28,2006 - Etalon caviry with filler layer for thermal tuning

Patent US 7149 377 - Dec 12,2006 - Solar tunable filter assembly



Return to home page