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    Camera sextant?
    From: George Huxtable
    Date: 2010 Jul 6, 12:12 +0100

    Greg's interpretation of his scale-factor as varying linearly is an 
    illusion, caused by the restricted range of his observations, which do not 
    include the important central portion of his display.
    
    I have taken the liberty of doing a shrink, cut, and paste job, in the two 
    attachments, of his linked files
    
    
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    in such a way that I can extrapolate them down to small angles, around the central region of the array. And then I have stuck my neck out, in daring to predict the sort of shape that Greg is likely to find if he observed down to those smaller angles. The general shape, similar to a parabola-on-its side, is an inevitable consequence of the axial symmetry. It corresponds closely to one side of the plot of "cal fig" in Marcel's spreadsheet, but placed on its side because of the different way up it's been plotted. It's clear that the observations made with the 50mm Pentax are very compatible with such a shape. Those made with the 100mm Nikon are not; they would be, except for one rogue point, at around 2700 pixels span, which doesn't show up clearly on my attachment, but can be seen better in his linked file. I would be a bit suspicious about that point, and suggest that Greg might give it a closer look. There are no error-bars shown on that plot, and it might be helpful if they were, on account of its highly-expanded scale. It would call for no more that a one-in-2000 change in the scale-factor at that point to bring it exactly on to my predicted curve. Is Greg confident of all his observations, down to that precision? Is the sinuous nature of the curve he has drawn real, or is it no more than the result of experimental scatter? Greg seems confident that his linear variation of scale factor will serve his purposes, and perhaps it will, as long as he measures only within his calibrated spans. But if he starts to measure much smaller angles that the range he has calibrated for, which cover 7.5º to 12º for the 100 mm lens, and 12.5º to 22º for the 50mm lens, he will find large errors in the scale factor, if he tries to extrapolate his derived straight-line fit. We still haven't been told the overall size of the array that Greg is dealing with, which would tell us where its central point would be. Clearly, it must be similar to the array that Marcel refers to, which I take to be 3880 pixels, so that its centre is at 1940. Are they identical? 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: "Greg Rudzinski" To: Sent: Tuesday, July 06, 2010 12:03 AM Subject: [NavList] Re: Camera sextant? was: Re: On The Water Trial of Digital Camera CN George, See the linked graph images of 50mm and 100mm lens moa/px vs. px. I have hand drawn highlighted curves over the linear red lines for direct visual comparison. The maximum discrepancy for the 50mm lens calculates out to 0.35' moa at 3500 pixels. The maximim discrepancy for the 100mm lens calculates out to 0.24' moa also at 3500 pixels. Chasing tenths of a minute of arc by graph or formula won't add much utility for conventional CN applications. If a photographer were trying to shoot a day time lunar then every tenth of a moa would be important. Photographing a sharp crescent Moon 24� from the Sun may not be possible. I'll have to give it a try. Greg Rudzinski ---------------------------------------------------------------- NavList message boards and member settings: www.fer3.com/NavList Members may optionally receive posts by email. To cancel email delivery, send a message to NoMail[at]fer3.com ----------------------------------------------------------------
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