"Sweet spot", "banding" and air-spaced Ha tilted etalon

(Copyright  Christian Viladrich)



This page presents:
1) How the sweet spot evolves into a sweet ring when an air-spaced etalon is to be tilted to tune its bandpass on Ha.
2) Hhe difference in size of the sweet spots between an air-spaced etalon placed in front position and in internal position (and collimated beam) halfway between the obective of the refractor and its focus.



1) Sweet spot and sweet ring

Case 1 : sweet spot for an air-spaced etalon having its CWL on Halpha at normal incidence

Application of (1) gives :

        -0.25 = -i_max 2, thus i_max = 0.5°

The diameter of the sweet spot is equal to twice de diameter of the solar disk (central cercle on the following figure) :











2) Sweet spot for an etalon placed in internal position
When an air-spaced etalon is place in a internal position in collimated beam, we have:
   
        delta CWL = -i^2  × f^2/F^2

with:
f = focal length of the collimator,
F = focal lenght of the telescope.

                   
                          For an air-spaced etalon placed in front position                                                                               For an air-spaced etalon placed halfway in the refractor OTA               
                       

Click on the images to scroll and display the numerical value of delta CWL for any points in the field.

Color scale:
- red : delta CWL = 0 A,
- bleu : delta CWL = 0.5 A or larger.


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