From: "Tim" <tjp314@pacbell.net>

Date: October 25, 2005 11:34:36 AM MDT

To: Astro_IIDC@yahoogroups.com

Subject: Re: MER B on Mars

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.

--- In Astro_IIDC@yahoogroups.com, Alan Friedman <alan@g...> wrote:

In addition to transparency I was wondering about turbulence - is there 

"seeing" in the martian atmosphere to contend with?

thanks -

Alan

 On Oct 25, 2005, at 12:55 PM, Tim wrote:

Alan:

Very much so.  Dust clouds in particular can be pretty opaque.  Water 

or CO2 clouds aren't

always (though the polar hoods are).

Right now at Meridiani Planum, the atmosphere is pretty dusty due to 

the regional storm

to the west.  This affects the solar power available to the rovers, 

but not as much as dust

settling on the panels themselves.  Fortunately, both rovers have 

been "cleaned" from time

to time by local winds.  I'm wondering whether we'll start having 

problems again (like

around the end of the nominal mission) with settling outpacing these 

cleaning events as

Mars' atmosphere "settles down" again after the current dust storm 

season is over.  Only

time will tell.

-Tim.

--- In Astro_IIDC@yahoogroups.com, Alan Friedman <alan@g...> wrote:

Hi Tim -

Fascinating. Does the martian atmosphere affect imaging from the

orbiter looking in as ours does looking out?

Alan

On Oct 24, 2005, at 6:18 PM, Tim wrote:

Hi guys:

  Check this out:

http://www.msss.com/mars_images/moc/2005/10/24/index.html

  Before and after images, taken by the Mars Orbiter Camera (about 

a

14" Cass) on Mars Global

  Surveyor, of the rover near "Erebus Crater" on October 5th.  I 

keep

our location map up to

  date with daily  panoramas from the rover mosaicked with the MOC

images taken before and

  after landing, so it was cool to have independent verification 

from

orbit of my prediction

  (within a meter or so) of where the rover was with respect to 

ground

features.

  It's also weird going from centimeter/pixel (or higher 

resolution)

images of Mars at my "day

  job" to something more like hundreds of kilometers/pixel at night

from home! 

  Of course, I haven't even seen the sky since Friday, since we're

socked in with the marine

  layer!

  -Tim.

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