Re: Astronomical Stereos

[bercov@xxxxxxxxxx (John Bercovitz)]

Peter Abrahams writes:

> --On libration: a stereo photographer could get a true stereo portrait 
> by asking the subject to turn slightly left or right.  However, the 
> background would not be shifted.  Would the photo look unusual or less 
> stereoscopic? Addenda?--Assessments???--Replies???

I have a couple or three comments on stereos of the moon if it's not too 
late.  (I was at a convention and am catching up now.)  The first (and 
obvious) comment is that stereos of the entire moon don't show craters 
because of the scale.  Next comment is that if you take a pair of the 
moon using a long lens and then view the results with short lenses, you'll 
get squash of the third dimension.  For a graphical explanation of this, 
see orthomag.gif in the technical directory of bobcat's ftp/web site.  
Lastly, if you depend on libration for the two views, you are actually 
taking convergent views and so will get convergence distortion which will 
look a little like the moon has been squashed in the left-right direction.  
For an explanation of this phenomenon, see the discussion on this list in 
December of '95.  That said, I've seen a pair of the moon taken by libration 
(I think in Waack's pamphlet) and the result was surprisingly good.

John B


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