It all depends on the telescope aperture and the observing conditions – darkness of the sky and atmospheric turbulence. Here are what you can hope to view when conditions are good.
With a 60 mm (about 2 in) refractor: the surface of the Moon, the phases of Venus, sunspots (with a ﬁlter in front of the instrument – whether using binoculars or a telescope, never look at the Sun without a ﬁlter!), double stars separated by more than 2 arcseconds, stars up to magnitude 11.5, large globular clusters, the brightest nebulae, and most Messier objects (but with few details).
With an 80 mm (about 3 in) refractor or a 10 cm (4 in) reﬂector: the phases of Mercury, smaller craters on the Moon, the polar caps on Mars, the bands on Jupiter’s disk and four or ﬁve of its satellites, the divisions in Saturn’s rings, double stars separated by more than 1.5 arcseconds, stars up to magnitude 12, dozens of globular clusters, several planetary nebulae, galaxies, and all Messier objects.
With a 15 cm (6 in) reﬂector: all the previous objects in greater detail, the centers of globular clusters, and (under a very dark sky) many details in nebulae and galaxies.
With a 20 cm (8 in) reﬂector: details on the Moon of the order of 1 km, several asteroids, stars up to magnitude 14, the core of globular clusters, and (with a very dark sky) exquisite details in nebulae and galaxies.
Visual observation has its limits. Although the eye is very sensitive (equivalent to an ISO 800 ﬁlm) once dark-adapted, it can integrate (accumulate and process)
A 15–20 cm (6–8 in) telescope can produce very ﬁne astronomical images, such as these of the Andromeda Galaxy and of a nebula, with an exposure time of 1–2h, captured by an experienced amateur astronomer. Credit: J. Lanoue, BedfordNights.com.
low-intensity light for a few seconds at most before sending an image to the brain . After that it resets itself. If, instead, a photographic camera and a motorized mount are used, exposures only a few minutes long will reveal details invisible to the naked eye. A simple webcam modiﬁed for long exposures can also give good results.
The electronic age has revolutionized amateur astrophotography. Not only are digital detectors much more sensitive than photographic emulsions, they allow multiple expo- sures. With the proper software, poor-quality individual images can be removed and the best ones added together. The recent appearance on the market of low-cost electronic detectors (like charge couple devices – CCDs) specialized for astronomy has provided amateurs with powerful tools to produce spectacular color images, rivaling in beauty those made by professionals.