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    Re: Navigation with Moon images
    From: Peter Monta
    Date: 2020 Aug 25, 09:15 -0700
    Hi Robert,

    Have you considered using a graduated neutral density filter (sharp edge ) on your camera?  They are available in a range of attenuations and would allow you to attenuate the moon intensity without impacting the sensitivity needed for the star images in the same frame.

    It's possible there's a better filter arrangement than what I'm using.  Briefly, I'm suspending a 58mm ND64 filter (clear aperture 51 mm) about 40cm in front of the 85mm f/1.4 lens, which is stopped down to f/2.0 and thus has an entrance pupil of 42mm.  Ideally, though, the filtering would occur at the focal plane.  If I could somehow deposit an ND filter directly on the image sensor in a spot about 1.5 degrees in diameter, then that would filter the Moon and leave the stars totally untouched.  A more practical alternative might be the sort of clip-in filters that go behind the lens in a mirrorless camera.  I'm a little wary of those, though, because any time you have a plate with nonzero thickness in a converging beam (and f/2.0 is a pretty big beam), you get some spherical aberration.  Anyway, they would not work with my DSLR I think.


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