Re: Hale-Bopp

[P3D John Ohrt <johrt@xxxxxxx>]

P3D Larry Berlin wrote:
> 
> >Date: Thu, 15 May 1997
> > John Ohrt writes:
> >...........
> >As I understand it, you would select the time interval such that the
> >background stars would register as "infinity" and the movement of the
> >comet relative to the stars would provide the stero effect.
> 
> ***** Are you saying that the comet didn't move relative to the background
> as much as it moves relative to us? I thought it made pretty steady headway
> across the sky.

I think we are talking about different reference frames.  The movement
that counts for a stereo effect in the case of a sun orbitting body or
Received: by bobcat.etsu.edu; id AA24360; Sun, 18 May 1997 15:34:56 -0500
Date: Sun, 18 May 1997 15:34:56 -0500
Message-Id: <19970518202249.AAB17564@xxxxxxxxx>
Errors-To: 3d-moderators@xxxxxxxxxxxxxxx
Reply-To: photo-3d@xxxxxxxxxxxxxxx
Originator: photo-3d@xxxxxxxxxxxxxxx
Sender: photo-3d@xxxxxxxxxxxxxxx
Precedence: bulk
From: photo-3d@xxxxxxxxxxxxxxx
To: Multiple recipients of list <photo-3d@xxxxxxxxxxxxxxx>
Subject: PHOTO-3D digest 2061
X-Listprocessor-Version: 6.0c -- ListProcessor by Anastasios Kotsikonas
X-Comment:   The Stereoscopic Image (Photo-3D) Mailing List  
Status:   

interstellar body is its movement against the stars.  The apparent
movement through the sky relative to a terrestrial reference is caused
by Earth rotation and will not produce any significant stereo effect
with a body as far away as HB. 

> 
> Of course you are trying for the stars at infinity. That doesn't happen
> naturally due to the movement of the comet over the time it takes to reach a
> significant change in baseline. I saw a stereo image composed of images from
> a base of several hours, proclaimed as the ideal baseline. It had slight if
> any stereo of the comet and the stars were twisted in their locations due to
> earth's rotation relative to a fixed camera.

It never occurred to me that someone would use a fixed camera, but if
you did, rotate one image relative to the other until the stars match
and then crop appropriately.

> 
> If one waited longer to take another picture, the comet would have moved too
> far to get the background stars to be coincident in the same frame. For
> example, images taken a day or so apart and with the sky of equal
> brightness. I don't know just how far it moved each day but it's more than
> over a couple hours. It's distance of travel at some point affords a
> reasonable change in perspective for a possibly satisfactory stereo image.

You are right, you can't do it with a fixed camera. You must rotate the
camera and still crop the final images.  You can't fool mother nature.

> 
> There are two problems with this process. A comet is not a stationery object
> nor static of form.

Relative to a 50 mm lens, it is of static form as long as you arrange
things so that differential atmospheric refraction doesn't distort one
image relative to another.  Even that is fixable by the determined.


> In a computer I can separate the comet from the
> surroundings and place it relative to it's paired image over an identical
> copy of the background stars. If the comet remains similar enough in
> appearance over 1 to 3 days, perhaps due to the distance from which we are
> viewing it, the change in perspective could provide a more satisfactory
> stereo experience than what I've seen so far.

That's what I understand.


> The results of the process
> would be composited, but as close to reality as is possible by using the
> existing data. I would like to composit a series of pairs from increasing
> numbers of days separation from some common starting point. They would have
> to be taken essentially with the same telescope/camera/optical
> configuration. In the reslult you would be able to freeview several pairs,
> with an increasing baseline. They could be strung together in an AVI file as
> a repeating loop.

You might be able to simulate this with RedShift and other astronomy
software.

 
> One set of stars + several perspective instances of the comet = 2d
> background and 3d comet. It would be interesting to see how far into such a
> sequence the image develops optimum stereo and at what point in number of
> days it stops working at all.

I have been told that you can simulate stereo by rotating an object
slightly between views, assuming the background is non-descript.  Is
this right and does anyone know the math?  See you in Tech-3d!

> ******  That's understandable. In N. California there were several clear
> nights which would have afforded a series of at least several days. I
> suspect some places in big sky country had more of those clear days.

We are in big sky country, just north of Montana.  But in the winter,
ice crystals form at different times and densities during clear winter
conditions.  I do understand that California was better skywise and
probably good enough aspect wise.  HB has a highly inclined orbit, hence
those in the extreme south (initialy) and those in the extreme north
(later) had the best views of HB and it was high enough that
differential refraction may not have been a problem, IMO.

 
> If I don't find real stereo images I may synthesize one from a good
> original... Anyone know of a good physical 3D description for the tail
> structures?

I'll reply privately if I hear of anything.

Regards,
--
John Ohrt,  Regina, SK, Canada
johrt@xxxxxxx



------------------------------