"Astronomical Seeing" is a broad term that describes the condition of the night
sky and how suitable it is for astronomical observing. It is comprised of
two parts - the steadiness of the image through the telescope and the opacity of
the atmosphere (how much stuff is floating around in the air).
Astronomical Seeing is rated on a scale from 1 (bad) to 10 (the best).
The practical lowest power magnification for any telescope is approximately 7
times each inch of aperature. For example 28X for a 4 inch diameter
telescope. The practical highest power magnification for any telescope is
approximately 50 times each inch of aperature. For example 200X for a 4
inch telescope.
1 Severly Disturbed Skies. Even low power views are uselessly
shaky. Opacity is bad, with a lot of dust or smog present. Bright
sky objects like Venus or the Moon may even look yellowish.
2
3 Poor Seeing. Low power images are pretty steady, but medium powers
are not. Opacity is still low.
4
5 Good Seeing. You can use about half the usual magnification of your
scope. High powers produce fidgety planets. A moderate level of
opacity is present. You have to look low along the horizon line to see any
dust or haze in the air.
6
7 Excellent Seeing. Medium powers are crisp and stable. High
powers are good, but a little soft. Opacity is very good. The air is
largely free of dust or smog haze.
8
9 Superb Seeing. Even high powers produce a good crisp image.
Opacity is not an issue at all.
10 Night To Remember. If you're lucky, you'll get a two or
three nights in this category over many years of observing. You will never
forget these moments of perfection, as they don't last. Tack sharp images
at any power you can throw at them. You're limited only by your
optics. You smile all night.
