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Old 03-10-2018, 09:30 PM
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mental4astro (Alexander)
kids+wife+scopes=happyman

mental4astro is offline
 
Join Date: Jun 2008
Location: sydney, australia
Posts: 4,979
The Atmosphere – the Great Equalizer & task-master

While you can get higher magnification with a larger telescope, how high you can actually take it ALL DEPENDS on the atmosphere on any given day! This is called “SEEING”.

Seeing means how thermally stable the atmosphere is. If the atmosphere is thermally active, the image through the telescope will shimmer and look like it’s boiling. It will look very much like it does looking across a hot road and you see the image in the distance just above the road surface shimmer and roll about – essentially it is a mirage. So in “poor seeing”, the image will begin to shimmer and boil as low as 100X (sometimes even lower). In “average seeing”, the max useable magnification will be around 250X. In “very good seeing” it is possible to go to 400X. BUT this is also limited to your aperture!

Here’s the kicker…
Sometimes, a smaller aperture is better to use than a large aperture!

The larger the aperture, while being able to provide higher magnification and having greater resolution, it is also much more sensitive to the prevailing seeing conditions of the night. So while I may prefer to use an 8” scope with the Moon and planets, sometimes a modest 80mm telescope is the better option. While I will only be able to max out the 80mm scope with 160X, my 8” scope at 160X in poor seeing might produce a totally boiled out image, but that 80mm scope at 160X will be just fine!

CHEAT!
Not everyone is in a position to have two or more telescopes. If you have just the one telescope, and seeing conditions are not great on a given night, there is one thing you can do to help sway things a bit more in your favour - crop the aperture of your scope.

Say your scope is 8" in aperture. As a smaller scope is less susceptible to the ravages of poor seeing, you can make a mask that has an opening of say 6" and put this over the front of your scope.

TRADE-OFF
Of course, with any cheat there is cost involved. By making the aperture smaller, you make the image a bit dimmer, and you reduce the resolving power of the neat aperture of the scope. But sometimes this cheat can mean the difference between a productive night and a night of frustration.

Alex.
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