NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Greg Rudzinski
Date: 2013 Sep 23, 09:53 -0700
Yesterday's morning twilight offered up clear skies to measure star to star distances with a Tamaya sextant w/7x35mm scope and a DSLR camera w/135mm lens. The stars in Orion and Ursa Major were used.
Alkaid to Mizar 6* 40.5' on-line calc. 6* 40.2' Sextant
Alioth to Mizar 4* 21.7' 4* 21.5'
Alnitak to Mintaka 2* 44.2' 2* 44.1'
Alnilam to Mintaka 1* 23.1' 1* 23.4'
Betelgeuse to Bellatrix 7* 31.8' 7* 31.6'
A small 0.1' correction can be subtracted from the on-line calculation or added to the sextant observation to compensate for the effects of refraction. The sextant and on-line calculator are in good agreement here.
Comparing Orion distances to DSLR w/135mm lens pixels.
Betelgeuse to Bellatrix 3115 Px (.145008 moa/Px)
Alnitak to Mintaka 1130 Px (.14522 moa/Px)
Alnilam to Mintaka 572 Px (.145105 moa/Px)
Roughing out a graph using all the observed distances enabled the creation of the following table:
(.145)(Px) +/- moa'
Px
3800 -0.6'
3500 -0.4'
3000 0.0
2500 +0.2'
2000 +0.4'
1500 +0.5'
1000 +0.4'
500 +0.2'
100 +0.1'
A series of sunset images were taken yesterday to test the table with the following intercepts between 8* and 3* of altitude.
Hs
8* 36.0' 0.8' T
7* 28.7' 0.4' T
6* 47.7' 0.4' T
6* 3.6' 0.7' T
5* 6.1' 0.6' T
4* 22.9' 0.0
3* 37.1' 0.0
2* 58.2' 1.0' A
My conclusion is that a single set of star to star distance images will be good enough to calibrate a DSLR fixed prime lens for the purposes of celestial navigation. Star to star DSLR camera settings were 1/10s, f3.5, ISO 1600, infinite focus, hand held. +2 magnitude stars and brighter were used.
The benefit of star to star calibration over Sun to horizon calibration is that dip, refraction, and horizon uncertainties are eliminated.
Greg Rudzinski
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