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PSTs and pressure tuned Lunts don't require factory matching since the etalon is perpendicular to the incoming light so adding a tilt-tune double stack is very unlikely to result in ghost images. It's possible to get a more ideal match with the factory testing different filters but non-factory matching rarely results in problems.
Even two tilt-tune etalons won't give ghost images if the primary filter comes 'on band' with minimal tilt and the second filter (double stack) comes on band with more tilt.
Ghost images happen when the two etalons have the same amount of tilt when tuned.
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