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    Re: Modifications to 2102-D
    From: Gary LaPook
    Date: 2009 Aug 13, 23:24 +0200

    Lucky you, you have a real Bygrave : )
    It was a real thrill for me to hold the real thing in my hands in London 
    last month.
    Regarding your comment that you lack certainty as to which star you are 
    looking at, there are only 57 navigational stars (58 if you include 
    Polaris.) Twenty one of those you will never see from your latitude so 
    you only have to learn 36 stars, it isn't that hard......
    Presetting the sextant is real important when using the periscopic 
    sextant due to the very limited field of view available through the 
    sextant and no broader view available to the navigator. Since in flight 
    navigators pre-plan their shots and pre-compute Hc, (actually called Hp, 
    Height-precomputed, which is Hc with sextant corrections applied using 
    reversed signs and also advanced mathematically or graphically to the 
    scheduled fix time) so he always has an altitude and azimuth to use in 
    aligning his sextant. The sextant mount has an azimuth scale so you 
    preset the altitude, turn it to the correct azimuth and, if the aircraft 
    heading is correct, you should see the right star in the sextant. (This 
    also allows you to check the aircraft's compass.) The Air Almanac also 
    has a series of diagrams showing each of the navigational stars and the 
    surrounding star pattern as seen through the periscopic sextant. I have 
    include thesed diagrams, they might help in identifying the navigational 
    Brad Morris wrote:
    > Hi Gary
    > Yes, that is an approximation that can and does work. Off list, Chief Franklin recommended just the
    > same thing. I felt that there was enough error to not make me happy.
    > What I wanted was to have the sextant preset to nearly the correct value.  Doing
    > so eliminates (at least for me) the lack of certainty as to exactly which star is the right one and
    > makes it easier to bring the star to the horizon.  I assume the midpoint of twilight for time and go
    > from there.  The altitudes and azimuths are so close for the 15 minutes (or so at my latitude) as to
    > not make a difference.
    > To get the star in the view, yes, I use my Bygrave.  Why?  Because I can and it is fun!  Nothing like
    > the feel of that instrument in practice.
    > Best Regards
    > Brad
    > -----Original Message-----
    > From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of glapook@pacbell.net
    > Sent: Thursday, August 13, 2009 1:12 PM
    > To: NavList
    > Subject: [NavList 9494] Re: Modifications to 2102-D
    > Brad wrote:
    > "I just use my to determine which stars would be practical.  Since my
    > latitude is 40 north, it places me right between the N35 and the N45
    > blue templates.  Therefore, the altitudes and azimuths given really
    > don't do the trick. "
    > And:
    > "Once I have the list, I swap over to the Bygrave to calculate the
    > altitude and azimuth of each.  That way, when I am ready to swing the
    > arc, I can preset the sextant to the expected altitude and point via
    > the compass. Generally, the star is right in the telescope! "
    > A trick you can use to get better accuracy for latitudes between the
    > template latitudes is to set LHA Aries (either using the almanac
    > method or my modification or Chief Franklin's modification) and then
    > take the template off the center pin and slide it north of south until
    > the zenith is aligned with your DR latitude. This isn't perfect but
    > will be accurate enough for you to be able to preset your sextant.
    > gl
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