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George,

     Very good observations of my post. I did a high altitude check
using the Cassens and plath with a 4 power scope and had everything
within a mile and a half with an extreme difference of half a minute.
So yes there was refraction but that should be expected at very low
altitude. I did not adjust for flatness so there is another small
factor. I do have some pictures and I will let you be the judge. I
believe the pixels are to the nearest pixel which implies a half pixel
potential error. Keep in mind that all of this is done with a $125
camera.

On Aug 8, 4:09�pm, "George Huxtable" 
wrote:
> Thanks to Greg Rudzinski for posting his interleaved sextant and camera
>  observations of a setting sun; just the thing we need to assess the accuracy
> of one against the other.
>
> His stated conclusion was this-
>
> "CONCLUSION
>
> The results demonstrate the ability of a 7 mega pixel camera at optical zoom
> 4 power �to accurately function �as a sextant "
>
> However, I think we need to examine the details first, before agreeing with
> that conclusion.
>
> Because this also is stated-
>
> "Sextant observations �observed with CASSENS & PLATH using sight tube and
> whole horizon mirror."
>
> And when we compare the results of the camera and a sextant USED WITH A
> SIGHT TUBE (i.e., without a telescope) we see that the camera gives an
> average difference between calculated and observed altitudes of 3.5',
> against 2.1' for that sextant, �the scatter being about the same, in the two
> cases,. of 2.2' or 2.1' between extreme values. Not a wonderful performance,
> for an observation made from firm ground in what are described as ideal
> conditions. Neither were the sextant observations anything to be proud of;
> but that may be due to the difficuly of working without a telescope,
> combined with the problems inherent in correcting for refraction at such low
> altitudes, and perhaps an unusual value (unknown) for local dip.
>
> In which case I would rephrase that conclusion to state only that the camera
> gave a mediocre performance which was significantly worse than that of a
> sextant without a telescope.
>
> I wonder if Greg Rudzinski has retained any of the images from that set; in
> particular, the image(s) which were used to obtain the Sun diameter, and
> from that the overall calibration factor. I am interested to see how sharp
> and well-defined the Sun limbs were, and whether there was any leeway in
> assessing the Sun's diameter, at 108 pixels, by choosing a different
> brightness contour for the fit to the disc. After all, changing that contour
> by just one pixel, from 108 to 107 pixels, would cause a difference of
> nearly 1% to the calibration factor, and shift the first of the camera
> observations in the set by more than 3 arc-minutes
>
> Finally, I wonder if Greg has made any assessment for non-linearity of the
> angular scale that was used, and made any correction based on that
> assessment.
>
> George.
>
> contact George Huxtable at geo...@huxtable.u-net.com
> or at +44 1865 820222 (from UK, 01865 820222)
> or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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