P3D Re: Aurora

["KENNETH LUKER" <KLUKER@xxxxxxxxxxxxxxxxxxxxxxx>]

>From Eric G.:
> Which leads to the question: for stereo astrophotography purposes,
> what's at infinity? And within this application, should infinity be
> defined as an object from which we are too distant to achieve a
> photograph with perceptable parallax? What would this distance be?

I considered the background stars to be at infinity.  From earth, no
matter what baseline length is used, the angle between baseline and
stars remains virtually constant all along the baseline (any
parallax is not detectable).  If any other object is at a distance
such that the angle between baseline and object is different at the
two ends of the baseline, then enough parallax exists to provide a
stereoscopic effect, I believe.  Such seemed to be the case with the
aurora.  As the airplane moved along, the stars seemed to be
stationary behind the aurora, which the airplane passed as it would a 
very distant cloud.  The discernible structure within the aurora was 
changing over time, also, so it wouldn't have worked well to take a 
time-separated stereo pair based on waiting for the airplane to move 
along the baseline.  Simultaneous, widely separated exposures would 
have been best.  How far separated?  Perhaps one thirtieth to one 
sixtieth of the distance to the aurora.  Meteorites occur about sixty 
miles high.  I have no idea how high the aurora are, nor how far away 
to the north.  I'd begin with a two-mile baseline if I could.

Ken Luker
_______________________________________________________________
Kenneth Luker
Marriott Library Systems and Technical Services
KLUKER@xxxxxxxxxxxxxxxx


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