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

(Copyright  Christian Viladrich)

This page presents how the "sweet spot" evolves into a "sweet ring" when an air-spaced etalon is tilted to tune its bandpass on Ha.

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) :

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