Coronado SM 40 test

(Copyright François Rouvière  - Christian Viladrich)






Coronado SM 40 mm: qualitative evaluation

Optical setup (from left to right) : Ha lamp, diffuser, Coronado SM 40 mm etalon, ASI 290 camera with 4 mm FL f/1.2 lens (12-bit acquisition, 90 s cumulated exposure).


The diameter of the area sampled by each position of the camera is given by the aperture of the lens. Here : 4 mm /1.2 = 3.3 mm.
Three positions are sampled along a radius : one close to the edge of central spacer, one half way between the edge of the central spacer and the edge of the etalon aperture, the last close to the edge of the etalon aperture.
For each position, the profile of the central fringe is drawn in order to have a better assessement of the CWL offset compared to the Ha line.

Scanning the etalon along a radius from left edge of the aperture to the edge of the central spacer:


The peak of the central interference fringe is a bit flatten on the left edge of the etalon and at half distance from center spacer. A small dip appears close to the central spacer betraying a slight offset of the CWL.


Scanning the etalon
along a radius from the central spacer to the right edge:

Again, there is a very slight CWL shift close to central spacer. However, the CWL is right on Ha line from the right edge of the aperture to half distance from the central spacer. 

Scanning the etalon along a radius from central spacer to right edge:

There again, there is a slight CWL shift close to the central spacer.The CWL is right on the Ha line from the bottom edge of the aperture to half distance to central spacer. 

Conclusion : This etalon has quite an uniform CWL accross its aperture. There is a minimal and negligeable CWL offset in a very small ring around the central spacer.



Coronado SM 40 mm: qualitative evaluation (FSR and FWM)


Interference fringe pattern

Nikon Z6 - 85 mm S  f/1.8 Nikkor lens at f/2 (i.e. 42 mm aperture) - 12-bit RAW file - 1/4s - 200 ISO

Results:

FSR = 12.1 A and FWHM = 0.74 A (average value over the full aperture of the etalon).  



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