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I thought I'd deliberately try some experiments in bright moonlight to see if any of the filters help with a star cluster, M41. I wanted to see which filter blocked the most moonlight and showed the most stars. Note that it would be a different result for nebula. I also used an achro refractor for the experiments to see which filter had any effect on chromatic aberration. All pics with each filter are 1 minute each at ISO 400. I didn't test blue as that would be no good in moonlight, but then again it is a test so I should have. Oh well next time... I'll do some nebula test next time as well.
The pics I've uploaded are clear (no filter), 81B light yellow, Baader Semi-Apo filter, K2 yellow and YG yellow green. I used in camera colour balance and a little post processing to make the colour balance neutral.
Moonlight has a blue colour cast and most of these filters reduced that. I was surprised that out of all the filters only the yellow showed fainter stars and the effect was minimal. But this was with the same exposure. Given that filters reduce the amount of total light, I could have exposed longer with the filters before the Moon fogged the frame. In effect this means that most of the filters I tested would go deeper in moonlight for the same amount of sky fog making them worthwhile using for this application.
The next obvious advantage is the amount of CA some of them reduce. The 81B and Semi-Apo had the least effect on CA, while the yellow and yellow / green has the greatest effect. In fact the yellow had the best result of reducing CA while cutting through the moonlight and showing faintest stars, nearly making this achro refractor semi apo like, far better than the much more expensive Baader semi-apo filter!
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