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    Re: Whole Horizon Mirrors
    From: Lu Abel
    Date: 2011 Apr 23, 20:45 -0700
    Greg and James:

    Your pair of emails are an excellent example of the controversy of whole-horizon mirrors.   But I do have to ask Greg whether his ability to capture a daytime sight of Jupiter was due to using a whole-horizon mirror or something else -- like using a scope on a high-quality sextant.

    I know they offer no comparison to a good sextant or even an Astra, but when I took my Power Squadron celestial class for a round sights, some of the students had Davis Mark 25 sextants with whole horizon mirrors.   Others were using Mark 15s or the no-longer-manufactured Mark 20s, both of which have split-horizon mirrors.   Students traded sextants to get experience with other models (both Davis, Astra, and three WW II sextants with split mirrors).   The majority consensus, regardless of what sextant they had started with, was a preference for split horizon.   Comments included things like "it's a heck of a lot easier to measure index error with a split horizon than with a full horizon" and "gee, it's far less difficult to see the body coming down to the horizon with a split field mirror than the whole-horizon proponents would have you believe."

    Bottom line -- it's very much an individual choice; the "advantages" of a whole-horizon mirror are much exaggerated, IMHO.

    Lu Abel


    From: Greg Rudzinski <gregrudzinski---.com>
    To: NavList@fer3.com
    Sent: Sat, April 23, 2011 7:48:44 PM
    Subject: [NavList] Re: Whole Horizon Mirrors

    For many years I had been skeptical of whole horizon mirrors thinking that they were for beginning navigators until I got a Cassens & Plath with a whole horizon on it three years ago and to my surprise the sight picture was fantastic. That didn't end my skepticism though so some varied observations were tried. The most convincing observation for the whole horizon was being able to observe a daytime Jupiter with it using a 4x40mm scope. The lone disappointment would have to be Polaris observations where a poorly defined horizon during late twilight combines with a dim star to really work against the light transmission abilities of the whole horizon. For those who enjoy taking lunars now and then the whole horizon does just fine.

    Greg Rudzinski


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