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    Re: Camera FAR 2nd Ed.
    From: Marcel Tschudin
    Date: 2012 May 31, 00:55 +0300
    Sorry, I only realised afterwards that Greg continued this part of our off-list discussion on-list. Additionally I missed his first of the two mails, only noticing before the one with the schematic drawing. I reply you here now to the mail which you sent before sending the drawing. However, I do not know whether this discussion is really of interest to other NavList members. 

    Greg, you wrote:
    There are three ways to determine the fixed angle:

    1. Observe and time the reflected Sun is on the horizon directly without using a camera.

    2. Observe the same with the camera for better results because the setting image can be zoomed. No pixels get counted in this procedure.

    3. Observe with the Sun off the horizon then subtract the angular value determined from the pixel count to get the reflector fixed angle.

    Note that there is no calculating of refraction for the lower reflected images because the light is not traveling through the atmosphere at that lower altitude but at the true higher altitude so there is no Ref(Alt1) only Ref(Alt1 + Alt2). Don't forget that the light is traveling back and forth between two pieces of glass on the reflector just as mirrors on a sextant

    The angles measured with 1. and 2. do contain refraction. This fixed Bris angle represents then the apparent altitude and contains therefore the refraction corresponding to this apparent altitude (=fixed Bris angle). In 3. you really deal with different refractions and the difference in refraction between sun's altitude and image altitude needs to be considered.

    Marcel
       
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