Astro stereo again

[Paul S. Boyer <boyer@xxxxxxxxxxxxx>]

More about Astro Stereo
Recently I posted some thoughts on the attempt to take stereograms of
stars.  I suddenly realize that I was assuming the use of normal
focal-length lenses.  The intent was to use a 4-inch (100-mm)
reflecting lens.  I am not sure what the effective focal length would
be for this lens, but it is a long one.  This makes a difference in
my calculations, and it is a difference worth thinking about.
It also is the one case I have found of an error by McKay in his
classic book on stereo.  [McKay, H.C. 1953.  Three-dimensional
photography, principles of stereoscopy.  American Photography Book
Department, New York.  xii, 333 pp.]  McKay writes that when using
longer-than-normal taking lenses, one should increase the base by a
factor equal to the ratio of the focal length of the new lens divided
by that of the normal taking lens.  This might be a crude rule of
thumb if one assumes that a longer lens implies a proportionately
more distant object, but optically it is fallacious.  Actually, a
longer lens gives one a smaller angle of view, without changing the
lateral parallax effect.  The displacement of objects of different
distance is still there, and is unchanged, but the picture shows a
smaller angular view, so that when the picture is made, the
displacement will actually appear as a larger proportion of the view.
 Stereo will be *enhanced* by the longer lens.  (Hope that you can
follow my someowhat clumsy explanation!)  Therefore, a long lens
would extend proportionately the effective distance available for
stereo.  It might enable stereograms of some of the planets, for
example.
Kittrosser's formula substantiates this.  [Kittrosser, S.  1953.
Polaroid interocular calculator.  Photographic Society of America
Journal, Section B: Photographic Science and Technique, 19B, no. 2,
pp. 74-76.]
I doubt that stars would work in stereo, though, without *really*
long lenses.  Stars are just too darn far away.  That's why we will
never be able to visit them, in spite of all the science fiction
stories.  (Sorry about that, you trekkies out there!)  It also points
out the foolishness of all those movies ostensibly simulating space
travel, where stars whiz by on all sides: actually, in interstellar
travel, most of the time your view would look like the view of a
night sky from earth.  Pretty boring, particularly if the trip were
to last n human generations.
--Paul S. Boyer  <boyer@xxxxxxxxxxxxx>


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