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    Re: Angular Distance Between Stars By Camera and Sextant
    From: Brad Morris
    Date: 2012 Sep 16, 05:16 -0400

    Hi Greg

    For Vega-Deneb, the true distance if refraction did not exist, I get
    23.84634 degrees
    23 degrees 50 minutes 47 seconds to the nearest second
    23 degrees 50.7833 minutes

    For Alioth-Alkaid, the true distance if refraction did not exist, I get
    10.46115 Degrees
    10 degrees 27 minutes 40 seconds to the nearest second
    10 degrees 27.6666 minutes

    While these might not be significantly different from the Williams results presented, there may be enough to cause concern when calibrating your instruments. 

    In order for me to calculate the refraction correction, the corner cosines and finally the distance you should observe, I will need your latitude, longitude, date and time of observation.

    In practice, I calculate all of my data without rounding or truncation.  When the distance I should observe is finally determined, I set my vernier to that value as closely as I can.  I lock the instrument and observe. 

    I never had much luck going the other way, which is to observe and reduce the observation to true distance.  I found this to be very fiddly.

    Regards
    Brad

    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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