# 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, 07:31 -0700

Marcel,

It is not as complicated as you think. The reflected Sun image is a constant angle that repeats exactly and requires only one single calibration to determine and is transferable to any lens or camera unchanged or simply used without a camera and timed as if a sunset. The loss of precision is in finding the vertical to the horizon since the Sun isn't near the horizon to be rocked back and forth for the low point. There is in addition a loss of image sharpness from added glass reflections that disrupt the symmetry of the Sun's reflected image.

Greg Rudzinski

[NavList] Re: Camera FAR 2nd Ed.
From: Marcel Tschudin
Date: 30 May 2012 14:08
Greg,

Regarding the accuracy you attain with your various FAR systems: Could it be that the loss of accuracy results from an error in not considering or wrongly considering differential refraction? The calibration of your lens considers refraction between actual altitude and apparent horizon. The reflection of the Bris moves the image to an altitude where the original calibration (refraction) is no longer true. May be one can obtain a good approximation with the original curve by adding or subtracting the difference in refraction. Imagine the sun being at 50* altitude being refracted by Ref(50). This image is now shifted down by e.g. 28* 46' producing an image at 21* 14'. Your calibration curve considers refraction between this altitude and the apparent horizon, but not between 50* and the horizon. As a gross approximation you may try to correct this (by adding or subtracting ?) with the difference between Ref(50) and Ref(21* 14'). It is however my understanding that in order to do it correct requires to derive separate calibration curves for each of the two Bris images.

Regards,
Marcel

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