Re: [APML]: Atmospheric Turbulence

["Kal Krishnan" <krishnan@novatech.com>]

This would make sense if there is a known atmospheric phenomenon that
extends over a large enough scale, so that the target and the guide star
move around in a synchronized manner. I.e, lets say I am imaging a galaxy,
and I pick a close by guide star; when the guide star moves up in the Alt
direction (lets say), if the galaxy moved up by the same increment in the
same direction, you could indeed guide that out.

However, if it is a fairly localized phenomenon, then the galaxy could be
moving down while the star is moving up, and correcting using the star
would just add more randomness to the actual target.

Does your friend have a theory one way or the other? (I am no meterologist,
so I have absolutely no idea!)

-Kal.

----------
> From: Steve Bell <sb635@delphi.com>
> To: astro-photo@nightsky.com
> Subject: [APML]: Atmospheric Turbulence
> Date: Thursday, October 03, 1996 2:33 PM
> 
> 
> 
> This is what my friend John (the meterologist - he's also an amateur
astronomer - in 
> fact, he's just ordered a 12.5" JMI NGT) and myself believe. A light ray
from a star, 
> while outside of our atmosphere, travels in a straight line and doesn't
scintillate. 
> It hits the atmosphere, and due to the small scale temperature
differences from one 
> air packet to the next, with their differing refractive indices, the ray
is bent first 
> this way and then that way, causing the scintillation. The ray is
certainly not 
> travelling on a straight line any longer, but we can consider fitting the
best 
> straight line pathway through all the smaller, jinking about pathways.
This is the 
> average pathway of the ray, and it is where this average pathway points
at on the 
> celestial sphere that we see the star at any instant in time. Because all
of this is 
> dynamic, ie, continually changing over time, the direction of the average
pathway is 
> not constant, and the whole scintillating image of the star appears to
move around in 
> a random fashion on the celestial sphere (but confined to a small area -
how small is 
> a function of the actual magnitude of the atmosphere's turbulence).  The 
> directionality of the average pathway changes at a much slower rate than
the rapid 
> rate of the scintillations. John and myself feel it is this slower
"waving" of the 
> average pathway, with its associated shift in the star's apparent RA and
DEC, that we 
> are guiding out with our 1-2 sec guidance updates.
> 
> Please bear in mind that I am only paraphrasing what John and I
discussed. Maybe we're 
> all wet, but it seems to make sense to us. I'm going to see him again
this weekend, 
> and we'll talk about it some more. Being a trained meterologist, he has
more insight 
> into the dynamic nature of the atmosphere than I have.
> 
> Also note that I've changed the subject of this thread, since we're not
really talking 
> about the ST-4 anymore.
> 
> 
> -- 
> Clear skies,
> Steve Bell
> sb635@delphi.com
> http://people.delphi.com/sb635 - Astrophoto page