# 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: Marcel Tschudin
Date: 2012 May 30, 14:08 +0300
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

On Wed, May 30, 2012 at 4:12 AM, Greg Rudzinski wrote:

The second edition of the camera FAR has a 1 in. x 1 in. cut microscope slide glass attached and angled between a pair of 52 mm linear polarizers. This can be called a camera FAR system which is then stepped down to 49 mm so that it fits a Pentax 50 mm prime lens. There are two reflections at 14* 23' and 28* 46' for a theoretical maximum observable altitude of 52*. An initial trial at the beach was performed with intercepts in the 2' range. Sea trials will naturally have a larger intercept range. The attached image is for an altitude of about 39* with the camera pixel angle measuring about 10* between the horizon and the second reflection of the Sun.

Greg Rudzinski
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