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    Re: Bubble Sextant
    From: Gary LaPook
    Date: 2008 Jul 13, 00:12 -0700
    I've taken hundreds of sextant sights through regular aircraft windows and have found no problem with refraction. What causes refraction in a lack of parallel surfaces on the inside and outside of the window. As long as the two surfaces that the ray of light passes through are parallel to each other there will be no refraction of the ray of light, refer to any text on optics and you will see that this is true. So when taking sights make sure you shoot through a non curved section of the windscreen. Domes on WW2 aircraft were curved and caused refraction that had to be corrected with information from the dome refraction table for the particular dome or from a standard table found in the Air Almanac. (see attached) For example, don't take sights from the portion of a Cessna 172 windshield up near the wing root where it is highly curved, you can see the distortion by moving your head around while looking through this part of the windshield. Pretty much anyplace else is o.k. You can test your windshield by looking through it and moving your head around and if the image wiggles avoid that part.

    Get a copy of the Air Almanac, it is easier to use in flight than the Nautical Almanac, you get it on disk from the Naval Observatory, cost thirty one bucks, here is the link:

    http://aa.usno.navy.mil/publications/docs/ord_info.php

    Just print out the pages you need for your flight, about 7 pages of interpolation and correction tables (which don't change and only need to be printed out one time) and two daily pages for each day of your trip.

    There is a MK IXA for sale on ebay right now:

    http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=150269277288&ssPageName=STRK:MEWA:IT&ih=005


    I was going to bid on it but I will leave it for you. Let me know if you aren't going to bid on it.

    Here is a link to an article with a quick explanation of  celnav in flight:

    http://www.oceannavigator.com/ME2/dirmod.asp?sid=&nm=&type=Publishing&mod=Publications%3A%3AArticle&mid=8F3A7027421841978F18BE895F87F791&tier=4&id=6C9EECDA2E6B47FF9CED1004D3437685





    gl





    Greg R. wrote:

    Otherwise you're probably going to have to determine the
    refraction correction through the windscreen yourself (seems like I
    read somewhere that WWII bomber navigators had to do that themselves
    too (or at least confirm prior calculations), so maybe that's a
    starting point for your research?).

    Hi and welcome to the list.
    
    I'm curious if the aircraft you fly have a dome fitted for celestial
    navigation (which would usually include a hook to hang the bubble
    sextant from). Otherwise you're probably going to have to determine the
    refraction correction through the windscreen yourself (seems like I
    read somewhere that WWII bomber navigators had to do that themselves
    too (or at least confirm prior calculations), so maybe that's a
    starting point for your research?).
    
    Good luck and keep us posted on how your project progresses.
    
    --
    GregR
    
    
    --- bubi352 <benjaminriecken@hotmail.com> wrote:
    
      
    I am new to this group and new to celestial navigation. I currently
    work as an airline pilot and fly extensively over water at night. I
    have developed a keen interest in determining my position the old
    fashion way.
    
    Could someone tell me which bubble sextant I should buy?
    What should I be looking on a bubble sextant?
    Where can I buy one?
    
    Thank you in advance.
    
    Bubi
    
        
    
    
    
    
      


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