RESOLUTION OF A TELESCOPE
Whatever the quality of your own eyes, every telescope has a limit to
the size of the details it can see. This is a physical limit imposed by
the wave nature of light passing through our telescopes. That being said,
it is rare that we get to experience the sharpest and smallest details
our telescopes can give. The reason is that thick soup we call an atmosphere
through which we must view. Our atmosphere is never stationary and the
turbulence in the air actually modifies the light rays that pass through
it, similar to a lens¡ªonly in this case the stack of lenses is many miles
thick¡ª10 miles or so when we look straight up and nearly 100 miles thick
when we look toward the horizon.
Our atmosphere usually limits what we can see, in the way of small details,
and limits how much magnification we can use¡ªno matter what the size of
the telescope! Yes, the big telescopes on remote mountaintops are limited
by the atmosphere, too¡ªoften to the same limit as a 4¡± to 5¡± scope. [Of
course, the images in big scopes will be brighter, even if the small details
aren¡¯t any more visible].
Due to the wavelike nature of light and the infinitesimally small apparent
size of stars (due to their distances), a good test of resolution for
a telescope is the appearance of star images. With smaller telescopes,
a star appears as a round disc with a visible size. This disc is not reality;
it is an illusion. The actual stars are too distant and small to resolve
into discs. Larger scopes will show these discs as smaller and brighter.
The apparent star image¡¯s disc is called the Airy Disc. Smaller disc sizes
improve resolution in both close star images and in extended objects like
Astronomers usually talk about how close together two stars of a double
star can be in seconds of arc and still be seen as two separate images.
The ultimate limit, called Dawes Limit, can be expressed as 4.5/aperture
of telescope in inches. This is where two star images will overlap to
form a long oval with a wasp waist in the middle. In practice, the atmosphere
must be exceptionally steady to see this, and a more practical limit,
called the Rayleigh Limit is used: 5.45/aperture of telescope in inches.
This second limit is easier to see, and the star images look like a figure
8 on its side.
In practice, doubling this separation limit will provide a better and
more sensible viewing experience. If you want to be guaranteed you will
see the double star as two separate stars, figure out what magnification
will result in an 8¡¯ separation: divide 480 by the separation of the double
star in seconds of arc. The star separation figure is usually listed in
books, and in the hand controller of computerized telescopes. Even 20/40
vision will resolve this small a separation. There will be a little dark
space between the two double stars.
The larger a scope, as you see from the formulas, the smaller the separation
can be between two points and still see the two points as separate. This
applies to extended objects like the Moon and planets as well. So, resolution-wise,
though it may be tough on the pocketbook, there is still justification
for the larger scope, even if all you do is observe the Moon and planets
from your backyard.