Fwd: ATM Star test

[mdholm@telerama.com]

If you are new to this game, you might not realize that some of that 
atmospheric "turbulence" may be inside your scope.

First off, it really isn't turbulence that causes the optical disturbance, it 
is temperature differences in different parcels of air.  Air movement stirs 
them up to make the dancing effect, but it is really the temperature 
differences that do the dirt.

A major source of temperature differences is right inside your scope: the 
primary mirror!!  Another is right outside: you!  Others are often near by: 
your house, a parking lot warmed by the sun and not yet cooled off, a lake that 
will be warmer at night than the air above it, etc.

What to do about these?

If your mirror is 8 inches or less in diameter, and especially if it is 1 inch 
or less in thickness, you can mostly get rid of it's bad thermal effects by 
letting it stand in the outside air for about 2 hours.  If you are not that 
patient, a fan blowing across the front of the mirror can speed things up 
greatly.  If your mirror is over about 8 inch diameter (and not real thin) it 
may never cool down enough in an acceptable amount of time without a fan.  If 
the air temperature outside is falling quickly during the evening, most any 
size mirror may not cool fast enough to avoid warm air current rising off of 
it.  (12 volt computer cooling fans blowing across the face of the primary 
mirror are one of the latest fads in amateur built telescopes, and with good 
reason!)

Try to keep the warmth from your body out of the light path.  Stand downwind of 
the scope, and not under it.

Try to aim the scope so that the light path passes over a large area of grass 
or woods (or snow at this time of year) rather than over a heated structure or 
paved area or bare ground.

Up high in the atmosphere, the jet stream may be blowing a bunch of cold air 
past warmer air and causing the two to mix in such a way as to make lots of 
packets of moving air of slightly different temperature.  An article in Sky & 
Telescope a few years ago discussed this and described how to tell from weather 
reports, when this sort of trouble is likely.  (In the middle USA, winter is a 
bad time for this.)  It also told how to tell from weather reports when good 
seeing is more likely.  I am sorry I don't have the reference right now.  (S&T 
does have a free online index at their web site.  You can order an article from 
their website, or go to your public library once you have the reference.)

Lower down, breezes may mix layers of warmer and colder air.  A lot of the 
lower activity occurs in the first couple of meters of air.  Even a small 
increase in elevation can provide significant improvement in seeing.  Observe 
from the top of a hill, or on an elevated platform, or at least a good ways up 
the side of a hill if at all possible.  Down in the valleys, there is likely to 
be a layer of cold air.  The slightest breeze will stir the interface between 
the cold air and warmer air on top, causing parcels of warmer and colder air 
that bend light.

Technically, none of the air movements mentioned here constitute turbulence.  
All of them are laminar flow.  It isn't the laminarity or turbulence of the air 
that causes bad seeing.  It is the temperature differences.  Even when there is 
very little air movement, those temperature differences can still wreck 
images.  I know it sounds like a technical fine point, but there are practical 
consequences regarding what you can do about the situation.  Having a correct 
idea of the cause of the trouble goes a long way toward finding mitigations.

Mark Holm
mdholm@telerama.com
mdholm@telerama.com