# NavList:

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

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Re: Angular Distance Between Stars By Camera and Sextant
Date: 2012 Sep 16, 20:31 -0400

For the Alioth Alkaid pair

I get refraction corrections for each object at
1 minute 35 seconds
1 minute 19 seconds

Corner Cosines
0.61255
-0.52942

Which has the affect of a delta refraction correction of
18.75 seconds
This agrees closely with your +0.3 minute correction.

ADDING the refraction correction to the true distance, I get an observed distance of
10 Degrees 28 minutes 1 second

Please note that this differs from your method whereby you added the delta refraction correction to the observed distance and compared that to the true distance.

If I add the refraction correction to each true altitude, and compute the spherical distance I get the same result (10 28 1).

We obviously both can't be correct with such an apparent divergence in procedure.  I expect to be proven in error shortly!  Hopefully an independent colleague will weigh in.

Best Regards

On Sep 16, 2012 12:59 AM, "Greg Rudzinski" <gregrudzinski@yahoo.com> wrote:
>
> This evenings star pair observations included Vega Deneb (by sextant) and Alioth Alkaid (by sextant and camera). This time the on-line William's GC calculator was used properly to calculate true star pair distance. The Vega Deneb pair was corrected for refraction using the approximation method of 0.1' per 5* of angular distance. The Alioth Alkaid pair was corrected for refraction using the table provided by Mike Burkes (attached).
>
> Vega Deneb Hc 85*65*
> 23* 50.1' observed distance by sextant
> +0.5' refraction correction (approximation 0.1' per 5*)
> 23* 50.6' true distance
> 23* 50.8' true calculated distance by William's on-line calculator
> Low 0.2'
>
> Alioth Alkaid Hc 31*37*
> 10* 27.1' observed distance by sextant
> +0.3' refraction correction by table
> 10* 27.4' true distance
> 10* 27.6' true calculated distance by William's on-line calculator
> Low 0.2'
>
> Alioth Alkaid
> 10* 28.0' observed distance by camera
> +0.3' refraction correction by table
> 10* 28.3' true distance
> 10* 27.6' true calculated distance by William's on-line calculator
> Hi 0.7'
>
> The camera image was disappointing showing 4' wide stars. Settings were ISO 1600, f 2, 1/8 sec, infinite focus using a fixed prime 50 mm lens. Sextant results were good.
>
> Greg Rudzinski
>
>
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