Re: Stars in stereo

[bercov@xxxxxxxxxxx (John Bercovitz)]

Kirk Bender writes:

> Err... I don't know what you mean exactly by stereoacuity,

Stereoacuity is like vernier acuity, the ability to align lines.
It is commonly given as 10" of arc but the best ever measured is 
3".  It is thought that the reason vernier and stereo acuity are 
so very good is that the retina/brain system has an algorithm 
which finds the centerline of a line or edge by looking at how the 
line excites a number of cones - sort of finds the average path 
through the excited cones.

Ordinary acuity is more like resolution.  The best ever measured 
there is around 1/2 MOA under the very best of conditions which 
figure matches the densest spacing of the cones in the eye.  
However, I've heard typical for a well-corrected eye with no 
problems is around 1 moa under the very best of conditions.  I 
have no doubt your 3 moa figure is much closer to the truth under 
normal conditions.  

One should also consider the way these acuity tests are done.  The
subject is asked to guess and when the guesses statistically beat
chance probability, the subject is said to have that acuity.  Not a 
very fair figure for average use.

> The Astronomy book I have (Astronomy by Jay Pasachoff, 1993) says 
> that [...] Proxima centauri has a parallax of .772  seconds of arc.
> A parsec is the distance at which 1 AU subtends only 1 arc second.

Then we're in complete agreement.

Just to review how I calculated it (apologies to the average member
of this group).  Your 4.23 light years is 1.335E8 light seconds.
Earth orbit diameter (2 AU) is 1000 light seconds across.  The ratio 
is 1 to 1.335E5 which is 1.545 seconds of arc, twice the value  
Pasachoff gives but Pasachoff's value is based on 1 AU rather than 
two so it had better be half my value or I've done something 
dreadfully wrong.  Not that you could really get a full earth orbit 
diameter of separation - you'd be shooting through dawn or gloaming 
which would give you poor contrast.  Rigil Kent is bright enough but
we also need the background stars.  Hmmm...  I wonder how much havoc 
the curvature of the earth's atmosphere would play with this.  I guess 
this is a pretty marginal idea.  Oh well.

Along these same lines, I looked up Rigil Kent over the weekend, which
I probably should have done before starting this thread in the first
place, and found that it is a double star with an 80-year period.  Do 
you know if this binary is resolvable at 10X?  I didn't see any data on 
its separation.  The reason I ask is that if it's a resolvable binary, 
this whole idea of a stereo pair of Rigil Kent won't work because of 
subject motion.  If it's not a resolvable binary, some of our southerners
in NSW or NZ could take a pair with a 500 mm lens and the result should
be viewable stereo in a red button.  Anyone in Tierra del Fuego?  8-)
Dec is about -60 putting it 20 degrees or so south of the zenith.  RA 
is between 14 and 15 hours so it ought to be on the zenith at midnight 
in November meaning a shot a few months before and a few months after 
November should give you your pair.

Thanks,
John B


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