NavList:

While George is concerned about all of these possible issues for a relatively new methodology to measure celestial bodies, and perhaps rightly so; to me the real issue is whether Greg's techniques can produce reliably usable LOP's.  In my opinion. this is the only true test of this, or any, method.

 

What intrigues me, being a photo enthusiast as well, is the way that the polarizer filters held at some distance work in the method instead of being in front of the first element as is normal for photography.

 

Jeremy

 

Thanks to Greg Rudzinsky for letting us in on his celestial camera
techniques. They leave me keen to learn more detail.

The following may just show my ignorance about modern digital cameras,
fitted with interchangeable lenses. Greg refers to a 50mm lens, and a 24
degree field. Is 24 degrees the angle subtended right across the frame's
diagonal, corner-to-corner? It must, presumably, depend on the dimensions
of the sensor array, which I would expect to be somewhat smaller than the
36mm x 24mm of standard 35mm film. With a 50mm lens, I would expect such a
film frame to cover nearly 47 degrees across the diagonals.

Greg's technique should work well. If he clamps the camera firmly to look
at the sky, in such a direction that the Sun will pass close to the centre
of the frame as it transits roughly across the diagonal, at carefully timed
regular intervals, that should tell him everything he needs to know about
the central calibration and the radial distortion. What's more, it should
also tell him if his deduced Sun diameters really do correspond well with
the Sun's true diameter. He will see a string of Sun images that, at
2-minute spacings, will just slightly overlap at the limbs, which should
allow for precise measurement in terms of pixels. It's the along-track
motion that is most relevant, and it should be kept, as closely as
possible. along a radial line passing through the centre of the frame.

However, that was based on the assumption that with a camera, the
distortion function must be symmetrical about the centre point of the
frame. All very well for a film camera, if it's been assembled
well-aligned, on centre. But there are other possibilities with a digital
camera that may well need checking. After all, the array itself is not the
same at all angles, but has defined x and y directions. Does the
pixel-array have the same pitch spacing across its two dimensions, or
alternatively a precisely-known ratio between them? Are the rows and
columns precisely at right-angles? Is there scatter in the pixel
positioning? I would expect that the answer to these questions is that
there's nothing to worry about, and the pixel placement in the array is
highly precise, but still, it might well be worthwhile asking such
questions. They would be difficult to answer by observation.

Greg refers to "lens distortion and tangent geometry", which is exactly the
concept we have been struggling with in recent postings. I would be
interested to learn what the results of his observation and analysis have
been, in his widest-angle configuration, in providing a correcting function
to allow for these effects.

Greg refers to a simple formula, "(.371 x pixels + 12.35 = MOA for a fixed
Pentax 50mm lens). ", and this leaves me wondering exactly what is being
plotted against what.

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" <gregrudzinski@yahoo.com>
To: <NavList@fer3.com>
Sent: Tuesday, June 15, 2010 12:18 AM
Subject: [NavList] Re: NG's "Midnight Fun"


I would like to explain how I have been effectively performing CN with a
SLR 10mp digital camera using 50mm, 100mm, and 200mm lenses. Lens
distortion and tangent geometry are corrected for together for each
individual fixed lens by observing the Sun or Moon every two minutes then
graphing the minutes of arc per pixel derived from reduced observation at a
known GPS position. This allows the entire field of view along a central
axis to be usable (24� for a 50mm lens). Marcel helped convert my graphed
data to a simple formula (.371 x pixels + 12.35 = MOA for a fixed Pentax
50mm lens). This formula works great at +/- one MOA through the entire 24�
field. The trick for consistency is in how the camera settings and
polarizers are used. All my images are shot at f22 and infinite focus (any
deviation causes problems). The polarizers are held out in front of the
lens at arms length. This mysteriously improves results. I strongly
encourage others to try this neat way of generating an LOP from the Sun or
Moon during the day.

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
----------------------------------------------------------------