Tilted air-spaced Fabry-Perot etalon in front position or in a telecentric beam

 

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)

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

Daystar filter modelling and additional results

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

Tilted air-spaced etalons in telecentric beam or in a f/115 convergent beam 

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

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

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

Fabry-Perot math and bibliography

 

The two cases under study

In this page, we study two cases :

1- tilted air-spaced etalon placed in a telecentric beam (such as in the Quark + Lunt 40 mod),

2- a very specific case where a tilted air-spaced etalon is exposed directly to the Sun and placed in front of the slit of a spectrograph. 



1 -Tilted air-spaced etalon in a f/50 or f/40 telecentric beam

This is typically the case of  the "Quark + Lunt40 mod", when the Lunt 40 need to be tilted to be tuned on Ha.

When the etalon is tilted in a telecentric beam, the transmission profile becomes asymmetrical:. The transmission on Ha decreases, the CWL shifts while the effective FWHM increases:

   


   
 


The broadening of the FWHM becomes significant when the required tilt is larger than about 0.2 to 0.3°.

2) Tilted air-spaced etalon in a f/115 converging beam



This case is very specific to testing etalon with a SHG, more precisally in the following configuration : Sun => etalon => spectrograph slit => spectrograph
In this configuration the slit "sees" an f/115 light beam coming from the solar disk (0.5° cone angle) passing through the etalon. Accordingly, the etalon "sees" a f/115 convergent beam.
 
 

The degradation of the FWHM is quite significant beyond about 0.25° tilt.
It means it is very important to have the etalon in a normal position with respect to the Sun when measuring its FWHM using this optical configuration.

 

 

 

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