NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Dual Polarized Hybrid Digital SLR Sextant
From: Greg Rudzinski
Date: 2009 Jan 20, 18:01 -0800
From: Greg Rudzinski
Date: 2009 Jan 20, 18:01 -0800
The attached photo shows a 135mm lens and a pair of circular polarizers
with a homemade bris mini sextant fixed to the forward polarizer surface
(double stick tape was used). Celestial navigators with an interest in
photography will enjoy building and using this system.
Preliminary calibration was performed from a known position and height of
eye with a hand held 10 mega pixel SLR Canon Rebel Xti using an adapted fixed
135mm Super Takumar Pentax lens. A precision of two pixels (or .29 minutes of
arc) was achieved with Acorn photo software. The attached calibration image
(shutter 1/4000, ISO 200, f 8, Monochromatic) for 10° 08.6' hs does show
multiple overlapping images of the sun which could be caused by extra
reflections of an optically imperfect handmade mini sextant. Consistency can
be maintained by always using the lowest lower limb of the multiple images.
If aperture priority is used try ISO 200 then work up to ISO 800 for a
less bright sun so that shutter speed is kept high. This will help reduce the
effects of hand shake.
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