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George wrote, and provided an attachment

> It will provide a target to be shot at by any keen proponent of the digital
> camera for navigation at sea.

George

I am not a proponent or detractor of using a digital (or Polaroid) camera
for navigation at sea, especially in a lifeboat situation.  Processing film
onboard a sailboat seems less than practical.

GPS has rendered the futuristic sextant with digital readouts and imbedded
hardware and software obsolete before beta testing (making the leap of faith
that prototypes were created and tested).

You point out many interesting and valid limitations, but after reviewing
your MSW document I find important misconceptions IMHO (if I understand your
arguments correctly) that I would question.

You are indeed correct that *any* lens will introduce distortion in one form
or another. In practical photographic terms, the wider the angle of view
(relative to the film/sensor size) the greater the distortion near the edge
of the image. (Western perspective is a matter of distance, not image size
or lens.) Using a wide-angle lens, the circular top of a flower pot at the
center of the frame rendered as a near-perfect ellipse will record
differently than the same pot near the edge of the frame. I agree it would
be imprudent to use the diameter of the sun or moon at any position in the
image as a yardstick for calibrating the scale of an entire image.

Ansel Adams (a famous US of A photographer) tucked the following tidbit in
along with the main topic. He was speaking of an environmental portrait (of
a male human) taken with a view camera.  It is generally considered bad
composition to center the subject in the frame.  Ansel centered the subject
and then shifted the camera back so the subject would not be centered, but
was in the center of the lens's field of view therefore the head/face would
not be distorted. (Note a view-camera lens has greater coverage than is need
to fill the frame at dead center to allow for swings, tilts and shifts.)

There are exceptions that minimize any distortion.  The famed Hasseblad
Superwide C with a 38mm lens for 6cm x 6 cm film for one.  APO lenses are
designed to photograph flat objects with near zero distortion.

Somehow--as I read it--you confuse cartography with photography. Not like
you. Reducing your argument to the absurd, it is impossible to take
photographs of the heavens with a camera lens, a camera mounted to
telescope, or plates mounted to an observatory telescope and accurately
determine relationships or angles.  You need to share this with the minds
behind the biggest best telescope EVER (now under construction) which will,
BTW, use a digital recording device. 

The above portion of your argument--as I understand it--falls apart IMHO
given the following.  Lens distortion a given, if we are dealing with the
celestial sphere how does a sextant differ from a camera back?  All a
sextant does is measure the angle between two objects, an object and the
horizon, the top and bottom of an object (say lighthouse) etc.  Question.
What is the software imbedded in the sextant that maps the celestial sphere
to a plane? Answer. The NA.

Added to the notion of the geocentric model used in cel nav, it must be
noted that all the bodies (except for major moon corrections) are assumed to
be infinitely far away. Scholarly individuals adjust for the fact that "it
isn't so" (and many other nuances) and make adjustments to the NA and other
data so all *we* have to deal with is the measured angle.

While I am sure theoretical mathematicians can blow me out of the water,
just how does one modify the infinite radius of the celestial sphere to move
bodies towards or away from the observer?  Point being, if all the stars are
at infinity (or adjusted to appear so), it seems like a plane to me.

Which gets me to the point of hanging onto the "celestial sphere" model when
calibrating the camera lens/recording device.  Why do we need to wrap the
measuring tape inside a jig with an arc (in this case using the film plane
or a node in the lens as distance to the tape as the radius)?  I do agree
with the notion that any problems with a lens focused at infinity may not be
mirrored in an image with the focus set at several meters. The calibration
needs to be done at infinity--and yes I did star to star distances with my
sextant sitting right alongside Alex and his SNO-T. ;-)

What can I say?  I have the Russian space pen--a pencil.  My PDA is a small
spiral-bound notepad.  My GPS is very basic and does not have hundreds of
dollars of electronic charts in memory.  I loath spending thousands of
dollars a year on redundant graphic-design computer software upgrades and
new machines to run bloatware that automates tasks I seldom use and learned
how to do manually when needed--just to provide corporate-funded
wet-behind-the-Pentel designers and production people with compatible file
formats.

That being said, I am willing to entertain new concepts, and give them a
fair review.  No sense throwing out the newborn baby with the bath water.

Bill B.




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