From: Alan Friedman <alan@greatarrow.com>

Date: October 25, 2005 11:53:09 AM MDT

To: Astro_IIDC@yahoogroups.com

Subject: Re: [Astro_IIDC] Re: MER B on Mars

Tim -

Thanks for the interesting reply. 

I think of turbulence as movement of the image rather than movement of the air. I've never understood fully all the weather factors that work to influence seeing, but Peter's principle certainly helps to explain it - any time I have the opportunity and clear skies to observe, the seeing is lousy. 

I did have one amazing night in Cedar Key Florida in Feb 2004 when the image of saturn was as stationery as I have ever seen it (or will probably ever see it again). We observed with my 10" mak using a 3-6 Nagler zoom - 600-1200X and you couldn't decide which view was better. At 1200X the image filled the entire FOV. I think I held my breath for a half hour.

I took images that night with my ToUcam. Unfortunately I way over processed the images.  I saved these files with the intention to someday go back and do justice to the once in a lifetime opportunity.

Alan

On Oct 25, 2005, at 1:34 PM, Tim wrote:

Alan:

Good question.  I've not thought about that recently.  Of course, the exposure times from 

orbit looking down are pretty short, and those looking up from the ground are pretty low 

resolution.

One thing that amazed me when I went to the Keys with Dobbins, Parker, Beish and others 

in 2001 is that the seeing is good there even when it's windy.  It was gusty on the ground, 

and there were two cloud decks at different heights moving in different directions and 

rather fast.  Mars moved around a bit (I borrowed Don's 6" f/8 so I wouldn't have to bring 

a scope with me from CA), but it maintained its shape and sharpness all night.  But when I 

got back to CA, my bad seeing was back to greet me.  I think it's worse where I live than it 

was at my previous homes, also in the LA basin.  We live in the "wake" of Mt Washington,

about 2 miles toward the coast from us, and when the onshore flow is dominating, we've 

got the cool air off the coast roiling around Mt W, and mixing with warmer air over our 

house.  I find that the best seeing is when we have a weak offshore flow, or either the 

onshore or offshore flow is about to "take over" and push the other out.  For about an hour 

before the seeing goes to hell, there appears to be a dead zone where the seeing can be 

exquisite.  I was imaging Jupiter once a few years ago when this moved out and the marine 

layer began moving in.  The change from an 8/10 to a 3/10 was sudden, and smaller than 

Jupiter's disk.  I actually SAW it move across the disk!  About 15 minutes later, the clouds 

rolled in and I put the scope away.

Between these experiences and my own travails with my 12.5" Cassegrain and tube 

currents, I'm convinced that it isn't moving air - turblence - that's the problem, it's 

turbulence between air masses of different temperatures and thus refractive indices.  

I used to believe that you needed laminar flow to have good seeing (and that Florida is a 

laminar state!  ;oD), but fans in telescopes don't creat laminar flow, they create turbulence.  

And it isn't cooling the optics down that is the objective, it's mixing all the air so it's the

same temperature (and/or the turbulent "cells" are extremely small).

Boy, I got off topic there!  Back to Mars, another factor that would need to be considered is 

the density of the atmosphere.  The seeing might be "good" because it's only 6-9 

millibars.  Kind of like a night I spent on  Mauna Kea about 10 years ago, guiding the IRTF 

on Mars with a video camera, where the winds outside were around 70mph, but the seeing 

was surprisingly good (not much atmosphere above us at 14K feet!, and Mars would be 

more comparable to 150K feet).

-Tim.