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    Re: Aldebaran occultation
    From: Peter Monta
    Date: 2017 Mar 6, 17:51 -0500
    Hi Alex,

    I was a little unclear I think.  For your question about using an occulting mask to observe bright stars near the Moon in antiquity:  I don't know if they used that technique, but it seems to me they should have.  A simple circular aperture centered on the star would do the trick.  The observer would have to gradually move the mask, or his eye, or both, but that's not difficult since the movement is so slow.  I'd like to try it during the next good opportunity.

    Since I was using a telescope, none of this applies directly to my observation session; but I was having a similar problem that a different sort of masking was helping with.  The star was approaching the dark limb of the Moon, which was lit only by earthshine (reflected light from the Earth), making its contrast pretty low with the dark sky.  So the idea was to move the bright part of the Moon outside of the eyepiece field of view, using the baffles of the telescope as a kind of crude mask.  That tones done stray light from the bright portion of the Moon and allows the dark limb to be seen more clearly as the star gradually approaches it.

    I'm sure experienced observers just do this by reflex, or perhaps they use fancy coronagraph-like hardware.  For the couple of Jovian-moon eclipses I've seen, I used a small piece of aluminum foil in the eyepiece focal plane to mask off Jupiter, but for Aldebaran I didn't have time for anything elaborate, and I'm not sure an eyepiece solution would have been more effective for a large low-contrast object like the dark limb.


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