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George,

It never was suggested that you throw your sextants in the dustbin and
buy a digital camera instead. Each instrument has it's advantages and
disadvantages. Greg pointed out the pros and cons. The main advantage
is the documentation of an observation. Even years later you can pull
the photo out of the drawer (or your data archive) and analyse it
again to see what you measured at that time.

If you think that the advantages of this "technique" can be useful to
you for some applications or if you are just curious how it works then
TRY IT and tell us your experiences, otherwise just leave it. The
recommendations I provided in the various contributions are meant for
those who would like to try it.

That's really all what I can say on this subject.

Marcel


On Tue, Jun 22, 2010 at 2:13 PM, George Huxtable  wrote:
> Richard Reed points towards a telephoto lens , but this is NOT the way to
> go for measuring altitudes above the horizon. The aim is to measure a large
> angle with great accuracy. Both the sky-body and the horizon have to appear
> in the frame. If the two are so close together as to fit into the view of a
> telephoto lens, then the altitude is in the range where refraction becomes
> unpredictable. But Greg's camera will not work with altitudes much greater
> than 20 degrees, which is a severe limitation for celestial navigation.
>
> On the other hand, there may be something to be said in favour of a special
> device which consists of two such narrow-field cameras coupled together,
> with a precise hinge and notched stops at exact angular intervals, say 2
> degrees apart. One camera could carry a very black filter (when necessary,
> for the Sun) and they could work with appropriately different apertures.
> The two images could be superimposed on a common LCD display, but the final
> angle readout could be done, not via that display, but after correction and
> analysis of the two pictures.
>
> The mechanical demands for precise repeatable angular alignment would be
> considerable, to comparable standards as apply to a sextant.
>
> Is there anything in such a notion? Give me a sextant, in preference...
>
> George
>
> contact George Huxtable, at �george@hux.me.uk
> or at +44 1865 820222 (from UK, 01865 820222)
> or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
> ----- Original Message -----
> From: "Richard Reed" 
> To: 
> Sent: Tuesday, June 22, 2010 11:23 AM
> Subject: [NavList] Re: On The Water Trial of Digital Camera CN
>
>
> I've been lurking on this topic and seen the discussions about image
> distortion and pixel size.
>
> Greg said: �"0.4 moa of a single pixel width for the 50mm / 10 mp". �My
> brother, who's become a DSLR fanatic, leads me to believe that 20 mp would
> be really expensive. �Wouldn't it be possible to calibrate the image
> distortion of a prime telephoto (are these available/expensive?) in order
> to get better pixel precision? �Maybe a bit like using a 7x scope. �I
> suppose sensor speed is an issue for magnification, just like it's hard to
> hold a sextant still with the 7x scope. �Can we even dream about accurate
> star sights this way?
>
> I recently surfed up some articles (now lost) on photometry, and this kind
> of thing is at least well-studied academically.
>
> Richard
>
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