Aperture – Amount of light
The most important specification of any telescope is the aperture, the diameter of the main lens (or mirror) of the telescope. The bigger the aperture, the brighter the image, and when looking for celestial objects in the night sky it is really important.
When a telescope is defined as 400x80mm, it means that it has 80mm of aperture.
Focal Length - Magnification
Another important number is the focal length. Once light goes through a lens, it’s directed by the curvature of the optic to come to a focus at a plane some distance away. The length over which this happens is called the focal length of the objective.
The focal length of the lens of a telescope will directly impact the telescope’s ability to magnify objects, along with the eyepiece focal length. The formula is:
Magnification = Focal Length / Eyepiece focal length x Magnification of extender
Note that when the magnification of a telescope goes over twice the aperture in millimeters (mm), then the image starts progressively to get more and more dim and fuzzy. With high-quality optics and steady seeing, you might get up 4x per mm of aperture, but this is rare.
When a telescope is defined as 400x80mm, it means that the focal length is 400mm for this telescope.
Focal Ratio – Brighter smaller
The third key specification of a telescope is the focal ratio, which is the focal length divided by the aperture. A long focal ratio implies higher magnification and narrower field of view with a given eyepiece, which is great for observing the moon and planets. For this purpose, a focal ratio of f/10 or more is ideal. But if you want to see wide views of star clusters, galaxies, and the Milky Way, a lower focal ratio is better. You get less magnification, but you see more of the sky. Wide field telescopes have a focal ratio of f/7 or less.
Focal ratio also influences the brightness of extended objects like a nebula or galaxy. For example, a telescope with focal ratio of f/5 will show an image of four times the brightness as a telescope with focal ratio of f/10, all other things being equal, but the image at f/5 will be only half as large. However, the brightness of stars, which are point sources of light, is influenced only by the telescope aperture.
Resolving Power – Image resolution
Finally, the last important number of any telescope is the resolution. The resolution of a telescope is a measure of its ability to distinguish small details of an object or to distinguish two objects very close from each other. Resolution is important when you’re trying to separate two closely-spaced stars or fine detail on a planet. The resolving power of a telescope with an objective of aperture D (in millimeters) is
Resolving Power = 116/D (in arcseconds, which is 1/3600 of a degree)
Resolution is directly proportional to the aperture of a telescope. A 200 mm scope can resolve details as close as 0.58 arcseconds, twice as well as a 100 mm scope, all other things being equal. But the motion and instabilities in the Earth’s atmosphere often limit the practical resolution of any telescope to 1″ or more, which is equivalent to 116mm of aperture.
What telescope to see what
With telescopes of 60-70mm aperture, you'll be able to see the moon and its craters, as well as some of the bigger planets. You will be able to see the rings of Saturn, though not in great details, as well as most nebulae. However, seeing nebulae is very dependent on the light pollution in your area.
Celestial sky objects you'll be able to see with an aperture size between 90mm to 130mm:
- sunspots (with appropriate solar filter)
- phases of Mercury
- depressions and craters smaller than 5km in the Moon's surface
- Martian polar caps and major dark surface features on Mars:
- additional cloud belts on Jupiter, and in better detail than the 60-70mm aperture range
- shadows of Jupiter's moons as they revolve around it
- Saturn's rings and four or five of its moons
- Uranus and Neptune are also visible as very small discs
- double stars separated by 1.5 arcseconds or more in good seeing conditions
- dozens of globular clusters, emission nebulas, planetary nebulas, and galaxies