# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Camera FAR 2nd Ed.
From: Greg Rudzinski
Date: 2012 May 30, 09:09 -0700

Marcel,

The camera lens I,m using is as you know already perfectly calibrated so when pixels are measured to the reflected image a correct angle is known and added to the fixed angle of the reflector that is also known so that an Hs is obtained and then refraction is applied as per Nautical Almanac for the sum. I think that you could be assuming that I don't know the fixed angle of the reflector. 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.

Greg Rudzinski

[NavList] Re: Camera FAR 2nd Ed.
From: Marcel Tschudin
Date: 30 May 2012 18:18
Greg,

I understand that the reflected angle is constant. The problem is your calibration (pixels per degree) which relate to the angle in the image without Bris. When you measure in the photo the pixels you get your altitude (with refraction related to the image angle). When you add now to this angle the the constant angle of the Bris the refraction difference between smaller and larger altitude remains unconsidered. Try to see in your various Bris observations whether you obtain by systematically adding or subtracting this refraction difference an improvement. This should really lead to an improvement.

Here how you should proceed:
1) Photo and calibration: Measure the pixels and convert to altitude = Alt1
2) Add to Alt1 the Bris constant angle = Alt2
3) Calculate the refraction for the two altitudes: Ref(Alt1) and Ref(Alt2)
4) Calculate the difference of refraction: DiffRef = Ref(Alt1) - Ref(Alt2)
5) Correct Alt2: Alt2corr = Alt2 +(or -) DiffRef (check whether you need the + or - sign)

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