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    Re: Whole Horizon Mirrors
    From: Ken Gebhart
    Date: 2011 Apr 25, 13:17 -0500

    Most of them go right on through due to the special (patented) film.

    On Apr 25, 2011, at 12:40 PM, Lu Abel wrote:


    While the transmissivity of the side of the mirror facing the horizon may indeed be high because the mirror is silvered on the side towards the sight-taker, what happens when those photons from the horizon get to the mirror's silvering?  


    From: Ken Gebhart <gebhart@celestaire.com>
    To: NavList@fer3.com
    Sent: Mon, April 25, 2011 9:54:11 AM
    Subject: [NavList] Re: Whole Horizon Mirrors

    It should be noted that the whole horizon mirror has better than 50% transmissivity (say 80% maybe) through the side facing the horizon, and better than 50% reflectivity from the side facing the index mirror.  The sum of the two is not limited to 100% as it would be if only one side of the mirror were considered.

    Ken Gebhart
    On Apr 25, 2011, at 1:06 AM, Geoffrey Kolbe wrote:

    > At 20:44 24/04/2011, Gary wrote:
    >> 50% throughput, on the average for both the horizon and the star for the whole horizon mirror. But for the split mirror almost 100% throughput of the dim horizon and almost 100% reflected of the dim star.
    > Ya-but.... When using a telescope of reasonable power only half the light input to the scope is coming from the star and half from the horizon, so there should be no difference whether whole-horizon or split-mirror horizon mirrors are used.
    > I can see that when using a sighting tube the eye will see an uninterrupted view of the horizon on one side and of the star on the other. For low power sighting systems the images will not be super-imposed with a split mirror system but at least you will see the star.
    > But.... When trying to see faint stars, it would be better to avail yourself of the increased light gathering power of a telescope - would it not? In that case, does it matter which system you use?
    > Geoffrey

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