Re: Stereo of the Moon

[P3D <ThomGillam@xxxxxxx>]

(Disclaimer):  This message was composed before Dan Shelley's
excellent research was posted to Photo3-D 677.

Fred Peters asks:

> I want to photograph the full moon in stereo and have been told
>that early stereo photographers would make an exposure and wait
>a month for the next full moon to do the second exposure. Does
>anyone know the exact technique.

Mark Chapman replies:

 >As the moon orbits the earth it rotates slightly back and forth
>over time from the point of view of someone on the earth.  By
>taking a picture of the same phase of the moon at the maximally
>different points of the wobble a stereo image can be produced. 
>The best result I believe was taken at a 4 year interval.  It is
>important that the same camera and lens is used both times.  If
>you do not want to juggle positioning on film chips then you
>will also need an alignment guide to center the image in the
>same spot in each picture.  Side to side positioning of the
>camera is irrelevant here since it is the wobble not the camera
>picture spacing that is producing the stereo separation.

R. J. Thorpe writes:

>I know that the 3D effect of the moon pictures comes from the
>wobble (is that perturbation?).
>The earth is about 8000 miles in diameter and the moon is about
>250,000 miles away. If all the conditions were right; moonrise
>at sunset and moonset at sunrise, etc, and you took a picture at
>both times. Then the ratio would be 8000:250,000 or about 1:30
>which is what I think we shoot for. Any comments?

Dr. T replies:

>But, if the same person is going to take two pictures at
>different times from the same location in a 12 hour period, we
>are talking about a different situation.  The moon has the bad
>habit of showing us the same face at all the times, which makes
>stereo of the moon _from the same location_ impossible.  
>Well, almost.  It is this "almost" (small variations in the
>rotation of the moon) that make the stereo of the moon from the
>same location not impossible, but just difficult.

William Carter adds:

>The stereos I've seen of the moon, taken at it's greatest
>angular disparity along it's elliptical orbit, ran about 3
>degrees.  

NOW, here's a topic I can expound on!  Please refer to Stereo
World Magazine for May/June  96, pg. 19 for my own attempt at
stereolunacy.  The backlabel of my view reads (in part):

"The stereo baseline is attained by taking the Moon's libration
into account, making the left and right views at different times,
namely at the full moon during different months [9/94 & 2/95]..."

Mark Chapman has it figured pretty well.  The conventional wisdom
is that the moon shows only one face towards earth, but in fact a
subtle trick of orbital geometry (called libration) allows us to
see more than 50 percent of the lunar surface (the actual amount
of lunar surface visible to us over time is close to 59 percent). 
The trick is to let *time* be your stereo baseline, and capture
different faces of the moon at the same phase.  The wobble
(libration) will actually allow images to be made as if our
baseline was in fact greater than the diameter of the earth.

Two images taken a month apart would theoretically yield a stereo
pair, however, in practice it doesn't work out that way because
the phase isn't necessarily EXACTLY the same at the same time.
This problem has haunted me since I created my view, since these
two images, made 5 months apart, suffer more from the fact of one
being made 5 HOURS BEFORE and the other THREE HOURS AFTER the
true  full' phase than the 5 months between exposures!

The trouble with the method proposed by R. J. Thorpe is not the
baseline, but the fact that in 12 hours the moon will have gone
noticeably towards the next phase, and you will not have
equivalent images.   If you were to coordinate with someone on
the other side of the earth, and made simultaneous views, you
might be able to produce a stereo pair, but it's a lot easier to
take advantage of the celestial quirk called libration and do it
yourself.

The problem of aligning both views is more complex than you might
imagine (I can expand on this off-line if anyone is interested).

Now, to get back to the original thread, Fred, you will want to
use a camera or telescope with at least a 1000 mm focal length,
in order to obtain a reasonably large image on your film,
however, much more than 1500 mm will make the image too large. 
As has been said, you should use the same setup for both
exposures, and that includes your choice of film (a slow speed,
low grain film is best).  I suggest you use print film (Kodak
Royal Gold 100, for example) and mount your views as  Holmes'
style cards, because you can substantially crop your prints to
align the two chips.  Slides would be more difficult to
manipulate, as there is the danger of losing too much of your
image to completely fill the window.

Alluded to earlier in my post, an awareness of the exact time of
each phase during any given month is crucial.  As it happens, the
full moon for November 1996 will occur at 11:12 pm EST, an
auspicious timing if ever there was one!  So get out on that
Sunday night and get your first half-pair exposed.  Then find a
source of astronomical timings (the mag "Sky and Telescope" is
mine), and look for another timing which coincides with
November's.  December will not be good, but January 1997 might
be.

I can tell you I'll be out there, weather permitting: I got my
first half pair already, and I need this one to make my new,
improved stereoview "The Full Moon II".

Thom


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