Should I buy a neutral density moon filter in order to view the moon through my telescope?
I have a 5" reflector telescope with a 5" cover that allows a smaller hole to be opened for reduced light transmission. It seemed to make the moon dimmer, but I also lost quite a bit of contrast (or resolution?) for some reason. I wonder if that little opening in the telescope cover has any purpose in protecting the internal optics during solar viewing, but all the solar filters seem to be aimed at covering the aperture, 5" in my case (and they are expensive, too). Anyhow, the moon filter seems like it would do the very same thing as making the aperture smaller, but people sell 1.25" moon filters, and 5" moon filters with neutral density and with other colors bias, like green. Do I need a neutral density filter, or some other kind, or does my aperture limiting with the hole in the cover already allow good moon viewing?
Is there any advantage for a 5" aperture ND moon filter over a 1.25" eyepiece ND moon filter?
Reducing the aperture is the same as using a smaller telescope, thus the reduced resolution. But I cannot imagine a real need for a lunar filter. It is not that bright, so unless you are concerned about reduced dark adaptation after lunar viewing, why bother. As for solar filters, a lot of the stuff that is provided with a telescope, especially the smaller and less expensive ones, is pure junk. Go with a good 5.0 neutral density filter. If you really feel the Moon is too bright, a 1.0 ND should be enough. Remember, at Full Moon you can’t see much in the way of craters or mountains anyway from lack of shadows.
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