According to the Clear Sky Clock in my region of the U.S. there will be virtually no cloud cover, it will be completely dark, and there will be high transparency, yet seeing conditions are rated poor. Are seeing conditions reflective of air currents and turbulence or other factors? I note that seeing is calculated based on mid-size to large instruments; so what can I expect using a smaller (4" APO) scope?
What accounts for poor seeing?
Started by davidcolton, 04/28/2006 03:18PM
Posted 04/28/2006 03:18PM
Opening Post
Posted 04/28/2006 03:31PM
#1
Hi David
The seeing portion is concerned with the upper level turbulence as you suspected. In general terms it means you will not be able to push the power to a limiting factor. I had simular conditions last night with a 4"APO and while I could still get upwards of 216x, the image scale got bigger but I couldn't get any better detail than I could at 108x on Saturn. With better seeing conditions at 216x I should have seen much more detail in the rings and more defined bands on the planet. Don't let it stop you from going out and enjoying, just adjust your targets and expectations.
Hope this helps
Buck
The seeing portion is concerned with the upper level turbulence as you suspected. In general terms it means you will not be able to push the power to a limiting factor. I had simular conditions last night with a 4"APO and while I could still get upwards of 216x, the image scale got bigger but I couldn't get any better detail than I could at 108x on Saturn. With better seeing conditions at 216x I should have seen much more detail in the rings and more defined bands on the planet. Don't let it stop you from going out and enjoying, just adjust your targets and expectations.
Hope this helps
Buck
Posted 04/28/2006 03:33PM
#2
Astronomical "Seeing" is strictly stability of the stellar (or other) image. It is not related to clarity, brightness, contrast, or any of the other things that make stars look good. In other words, astronomical "Seeing" is only one part of "seeing" through a telescope.
Seeing is stable if the air masses in the atmosphere are stable. Different temperatures of air refract light differently. They also tend to move around as they try to equalize. As they move, the light from the distant star is bent one way and another. This causes instability in the seeing. Immediately after a storm and a cold front is moving through, for instance, the air is super clear and transparent, and the stars are very bright, but the currents and such make the stars very turbulent.
Seeing can be upper atmosphere (weather patterns causing large cells of air to move around in the upper atmosphere), local (heat generated by parking lots, cars, rooftops causing unstable air), or even in the tube (air circulating at different temperatures in the tube until it all equalizes). Wherever there is air to move around, there can be bad seeing.
It is an ongoing debate whether "seeing" is affected more in small scopes or large.
A 4 inch APO is largely immune from tube problems, and cooling/heating problems that can affect seeing.
However, it can still be afected by other kinds of bad seeing in the upper atmosphere, and by hot rooftops.
Alex
Seeing is stable if the air masses in the atmosphere are stable. Different temperatures of air refract light differently. They also tend to move around as they try to equalize. As they move, the light from the distant star is bent one way and another. This causes instability in the seeing. Immediately after a storm and a cold front is moving through, for instance, the air is super clear and transparent, and the stars are very bright, but the currents and such make the stars very turbulent.
Seeing can be upper atmosphere (weather patterns causing large cells of air to move around in the upper atmosphere), local (heat generated by parking lots, cars, rooftops causing unstable air), or even in the tube (air circulating at different temperatures in the tube until it all equalizes). Wherever there is air to move around, there can be bad seeing.
It is an ongoing debate whether "seeing" is affected more in small scopes or large.
A 4 inch APO is largely immune from tube problems, and cooling/heating problems that can affect seeing.
However, it can still be afected by other kinds of bad seeing in the upper atmosphere, and by hot rooftops.
Alex
Posted 04/30/2006 02:57PM
#3
Dave,
You are seeing the results of the jet stream. I'm in Maryland, and in spite of the fact we've had a nice patch of clear weather lately, there have only been a couple of nights with good seeing (last night was one of them...it was excellent around midnight). Here's a link to a live jet stream map that I find very useful for predicting whether it's worth breaking out the high-power eyepieces:
http://www.wunderground.com/US/Region/US/2xJetStream.html
If you hit the "animate" button, you'll be able to see the trend and predict seeing for the next 24 hours or so. This area is really the pits....I've found maybe 20 good nights per year, usually on hazy summer nights. Transparency is bad, but planetary and lunar views can be very stable. HTH,
Tim Iafolla
You are seeing the results of the jet stream. I'm in Maryland, and in spite of the fact we've had a nice patch of clear weather lately, there have only been a couple of nights with good seeing (last night was one of them...it was excellent around midnight). Here's a link to a live jet stream map that I find very useful for predicting whether it's worth breaking out the high-power eyepieces:
http://www.wunderground.com/US/Region/US/2xJetStream.html
If you hit the "animate" button, you'll be able to see the trend and predict seeing for the next 24 hours or so. This area is really the pits....I've found maybe 20 good nights per year, usually on hazy summer nights. Transparency is bad, but planetary and lunar views can be very stable. HTH,
Tim Iafolla
