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    Re: shooting indoors
    From: George Huxtable
    Date: 2010 Mar 24, 14:00 -0000

    I'll argue, a bit, with Brad here.
    If the glass surfaces are non-parallel then that will lead to prismatic 
    deviation in angle; that's true But that doesn't depend very greatly on the 
    angle at which the object is being viewed through the glass. It can apply, 
    in a very similar way, whether or not the glass is at right angles to the 
    line of view. Same is true of the shades.
    It depends, mostly, on the age of the glass window. Modern float glass is 
    remarkably plane-parallel and shows little prismatic error. Older glass is 
    not, and shows a bit of ripple as you view a scene through it.
    If any prismatic effect of the glass happens to be the same, or nearly so, 
    for the horizon view and the index view. then the overall error in a 
    sextant reading will be minimised. What the sextant will be specially 
    susceptible to, is a varying ripple over different parts of the window 
    surface. But as long as the glass is plane-parallel, there's nothing to 
    worry about.
    Another effect that might need considering, when the glass isn't at 
    right-angles, is that of the wedge-shaped bit of air, between observer and 
    window, which won't quite match the refraction from the similar 
    wedge-shaped bit of air outside the window if the air temperatures differ. 
    Although this might deviate the light path a bit, it will deviate both 
    light paths the same amount, so will have no effect on as sextant reading.
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ----- Original Message ----- 
    From: "Brad Morris" 
    Sent: Wednesday, March 24, 2010 12:45 PM
    Subject: [NavList] Re: shooting indoors
    | Hi Scott
    | There may be some degree of prismatic error, depending upon the angles 
    that you are viewing objects thru your glass.  As light passes thru regions 
    of differing indices of refraction, the light will be refracted towards or 
    away from a normal to the interface as a function of Snell’s Law.  The same 
    prismatic error can occur in your sextant’s shades, should they not be 
    perpendicular to the optical path.  Think of your glass doors as yet one 
    more “shade”, which must be considered in the optical path.
    | Another issue you may have is the Dip Short.  If the distant lakeshore is 
    visible, then you do NOT have a horizon, you have a dip short horizon. 
    There are ways to deal with this, you are not alone with this issue. 
    Bowditch, and others like Norie, will provide the appropriate correction.
    | Best Regards
    | Brad
    | From: navlist-bounce@fer3.com [mailto:navlist-bounce@fer3.com] 
    On Behalf Of Scott O'Connor
    | Sent: Tuesday, March 23, 2010 8:04 PM
    | To: NavList@fer3.com
    | Subject: [NavList] shooting indoors
    | Maybe a dumb question. I live where it is very cold much of the year, but 
    can get sextant shots through a sliding glass door to a distant lakeshore 
    as horizon. Does shooting through the glass affect accuracy of the sights, 
    from a scientific basis? Thanks for the input.
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