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    Re: Whole Horizon Mirrors
    From: Lu Abel
    Date: 2011 Apr 25, 12:41 -0700
    http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=4,421,407.PN.&OS=PN/4,421,407&RS=PN/4,421,407

    or

    http://tinyurl.com/43wg3pn

    Patent was issued in 1983

    In fact, this looks like it might be a patent on the concept of a whole-horizon mirror.   Is anyone aware of whole-horizon mirrors being used before 1983?

    The patent does specifically say that the preferred embodiment is one that uses a "spectrally sensitive dielectric" for the mirror, "mainly transmitting light in one spectral region and substantially reflecting it in another"

    Reading the Claims (which are the meat of the patent) they do seem to focus more on the spectrally sensitive dielectric.  They say that the mirror should be transmissive in the red end of the spectrum and reflective in the blue end.   Makes sense -- presumably our horizon view has a lot of reds in it, while the bodies being sighted are bluer.

    I'll have to grab a full-horizon Davis (or Astra) and see if I can see a difference in how horizon appears to the naked eye versus through the mirror.

    Lu


    From: Ken Gebhart <gebhart{at}celestaire.com>
    To: NavList@fer3.com
    Sent: Mon, April 25, 2011 12:40:01 PM
    Subject: [NavList] Re: Whole Horizon Mirrors

    Lu,

    I don't know the exact number for transmissivity and reflectivity.  I remember years ago reading the patent details ( I believe as filed by Davis Instruments) that as I recall was about 80% for each. Of course this could vary by manufacturer.

    Ken
    On Apr 25, 2011, at 1:37 PM, Lu Abel wrote:

    Kind of what I figured might be the case (asymmetric transmissivity).  Is your 80% transmission figure for the film?

    Lu


    From: Ken Gebhart <gebhart{at}celestaire.com>
    To: NavList@fer3.com
    Sent: Mon, April 25, 2011 11:17:39 AM
    Subject: [NavList] Re: Whole Horizon Mirrors

    Lu,

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

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

    Ken:

    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?  

    Lu


    From: Ken Gebhart <gebhart{at}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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