[AZ-Observing] Re: North country windstorm
- From: "Tom Polakis" <polakis@xxxxxxx>
- To: "Reply-To:az-observing"@freelists.org
- Date: Tue, 24 Jun 2003 08:56:55 -0600
My experience regarding seeing and scintillation parallels Brian's. I
haven't found much of a correlation. Here are some examples.
At the Grand Canyon Star Party, I have seen wild scintillation, where the
stars are changing all sorts of pretty colors and varying in brightness.
Then I have looked into the eyepiece to see very steady views of planets.
This dates my observation, but I remember this phenomenon clearly on two
very steady nights in a row back when Saturn's rings were edge-on. I
learned something that night. Another example is when I observed from the
best site I will ever observe from: Las Campanas Observatory in Chile.
There was typically either little or no image motion detectable at 535x in
my 13-inch, yet the stars consistently twinkled about as much as they do
on a typical Arizona night.
On the other hand, it's quite common to find nights when the stars are
rock solid naked-eye, and the seeing is horrible through the eyepiece.
These are often the same nights when we're planted right under the
jetstream. All that turbulence is obviously the result of something
happening many thousands of feet above. The "period" of the twinkling in
this case is extremely short, unlike the scintillation you would detect
with unaided vision.
These days, it would be a simple matter to use a Webcam make movies that
compare scintillation at 3 arcminutes per pixel (through the standard
lens) to seeing at 0.2" per pixel (through the scope).
As an alternative to the Clear Sky Clock, I like the animated map, which
uses the same models:
http://www.cmc.ec.gc.ca/cmc/htmls/seeing_e.html
So far, the maps have correlated well with my recent Mars observations.
They have done a better job telling me when the seeing is going to be bad
than when it's going to be good. In a few cases, the model has indicated
a 5 out of 5, but some local effects have degraded the seeing somewhat.
Every time it has predicted poor seeing, that's what we got.
A discussion of seeing versus wind would require another few paragraphs,
but mercifully, I will stop now.
Tom
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