Re: Comet Hyakutake

[John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Mon, 25 Mar 1996 09:16:50 -0600
>From: Joel.Alpers@xxxxxxxxxxx
>Subject: Re: Comet Hyakutake

>An update: I overlayed the 6x7 negatives taken 1 hour, 10 min
>apart, and there was a deviation of the comet's position of
>almost 1.0 mm.  Recall that for a MF stereo slide, Ferwerda
>gives a max divergence of 2.5mm, so this is only a mild
>stereo effect. BUT this is in only 1 hour, so photos taken
>two nights apart would have WAY too much divergence. Also, these
>were taken Thursday night. The speed of the comet's track across
>the sky is increasing. 

It's interesting that the apparent motion of the comet across the star
field is roughly perpendicular to the direction of the tail - good for "3D"
(since the camera should be positioned so that the apparent motion of the
comet is side to side).

Joel - did you use any kind of tracking to compensate for the "rotation"
of the star field near the pole? Also, in your previous post on film
speed, exposure time, etc., what was the f-ratio?

>DO get out to a dark site if at all possible! This is something
>to see (at least if you can get over your jaded Hollywood idea
>of what a comet looks like ;)

A lot of the reason for confusion is a very sketchy explanation in the
scientific press, plus those spectacular time exposures. I've been
reading amateur astronomy magazines for years, and never got a clear
impression from them of what a comet would actually look like. A brief 
comment in a newspaper - that the entire comet, including the head and the
huge tail sweeping 20-30 degrees across the sky, would give off about as much
light (as seen from Earth) as a bright star - gave me a much better idea of
what to expect.

There's another celestial phenomenon (and one for which *real* stereo
photography would be possible) for which I have a similar problem - the 
aurora. At its best, an aurora looks like a huge rippling curtain in the
sky, and since it's an atmospheric effect, hyperstereo photos could be
taken. I've never seen an aurora in person, but I've seen them many times
on television - the rippling folds shift very rapidly, completely changing
their configuration every few seconds. In these innumerable playbacks I've
seen, it is *never* revealed whether the playback is normal speed or 
time-lapse. I strongly suspect it's time-lapse, but don't know for sure.
I've even questioned a number of people who have personally witnessed
auroras - they remember the rippled ("curtain") pattern, and they remember 
that the pattern shifts over time, but *none* of them can remember whether 
the shift is over a period of seconds or a period of minutes, which would
be important to know in conducting a feasibility study for a 3D photograph.
(Maybe solar ions affect the brain's perception or recall of time 
intervals. :-)

While I would very much like to know the answer to the question in the above
paragraph (anyone know?), I think that the fact that such a question exists
is indicative of the fact Joel referred to - that there are plenty of people
willing to present a fine show to the public, but there are few who care 
enough to explain it sufficiently for clear understanding. This is a 
continuing problem for 3D enthusiasts trying to get the word out to the 
general public - people have heard about 3D, but with important parts of 
the information omitted. I've spoken with people who were amazed to learn 
that 3D photographs could be in color! The lack of information eventually
gets back to people who try to use 3D, with results like the "3D" television
shows in which those making the show didn't know how to use the Pulfrich
effect. (And of course that gives more people a negative impression of 3D.)
(And to conclude this long philosophical treatise, that's why resources such
as this list and the 3D FAQ are so valuable.)

John R


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