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The ability to see details on a celestial object ultimately depends on how far away it is, and how big it is. Stars are so very far away that they will never show a real disk or ball shape in a telescope. Planets, the Moon, and the Sun are much closer and will show discernible disks and details even at low or medium powers in most telescopes. Nebulae and galaxies are also very far away but are so enormous that they will also show details in many telescopes. What you will see looking at stars at high magnifications (assuming a steady atmosphere) is an optical pattern known as the diffraction pattern. It appears as a bull’s-eye with a bright, central area or disk surrounded by one or more concentric rings but is not the actual disk of the star you are seeing. The diffraction pattern is due to how the telescope’s circular lens or mirror acts on light from a pinpoint source like a star. source: Celestron.com

Every scope Celestron manufactures includes the following information in its specifications chart: the minimum useful magnification and the maximum useful magnification. This information provides the theoretical limits for each telescope model. These limits assume a night of good stable atmosphere, good transparency, a scope that has been allowed to cool to ambient air temperature, and well collimated optics (for Newtonians and Cassegrains). If you choose an eyepiece that causes the telescope to exceed its highest useful magnification, the image will be magnified but will not carry or enhance any more details. This is generally defined as 60x per inch (25.4mm) of aperture. If you choose an eyepiece that provides magnification lower than the lowest useful magnification, the exit pupil becomes larger than what the human eye can support. Your view will become vignetted (encircled by a black ring) and you’ll never see the entire field of view. The lowest useful magnification is 3.6x per inch (25.4mm) of aperture. For this article, we’ll use the AstroMaster 130EQ as an example. This telescope’s focal length is 650mm. (To find the focal length of your telescope, look at the telescope’s nameplate or retaining ring.) The aperture is 130mm. The scope’s highest useful magnification is 307x and the lowest is roughly 19x. To determine the focal lengths of eyepieces that fall within the magnification limits, you must take the focal length and divide by the magnification. For the AstroMaster 130EQ, the highest useful magnification is achieved with a 2.1mm focal length eyepiece. The lowest useful magnification is achieved with a 34mm focal length eyepiece. Celestron offers several options for eyepieces that fall within these limits. A 2.3mm X-Cel LX eyepiece will give you 282x, which is close to the highest useful magnification of the optics. An Omni 4mm eyepiece with a 2x Barlow lens will exceed your highest useful magnification at 325x. It’s unlikely that any eyepiece will be a perfect match, but with the right combinations of eyepieces and Barlows, you can achieve numbers that are very close. Remember, the performance of your telescope will always be limited by your seeing conditions. Choosing an eyepiece with too much magnification on a night with poor seeing conditions will only magnify the turbulence in the atmosphere, providing poor quality views. To determine the magnification of a given eyepiece when paired with your telescope, use this equation: source: Celestron.com

While small telescopes can show a lot of objects, they may only be seen as colorless dots or smudges. Size does matter in astronomy–bigger is better in many cases. The larger the lens or mirror diameter or aperture, the more light your scope gathers and the higher resolution (ability to see fine detail) it has. Larger scopes also have longer focal lengths, meaning greater magnifications and image sizes are possible with both the eye and camera. Faint objects like nebulae and galaxies demand large aperture scopes. Planets need longer focal lengths for higher magnification views and large apertures for high resolution. Nonetheless, here are some recommendations for specific scopes versus specific types of astronomical objects and observing: The Sun: A small scope properly equipped with a full-aperture solar filter will provide very good views of the Sun under typical daytime seeing (atmospheric steadiness) conditions. A moderate focal length will reveal granulation and sunspot details with your eyepieces. Apertures of 6 inches (150 mm) or less and focal lengths of 1000 to 1500 mm are good for looking at our closest star. Planets: Here you’ll need a long focal length scope to take advantage of high magnifications (when the atmosphere is steady enough to use them). to be able to see details in Jupiter’s clouds, Mars’ features, etc. Large aperture is also great for high resolution when using with high magnifications. Therefore, the bigger the scope, the better to take advantage of good seeing for planetary viewing. Big scopes will also let you see faint Pluto, planetary satellites and minor planets. The best scopes are at least 5 inches (125 mm) aperture and 1250 mm focal length or more. Open Clusters: Except for the very biggest and brightest star clusters (Pleiades, Hyades, Beehive, Coma Cluster, etc.). You will need as large an aperture as possible and moderate to long focal length as well for the most interesting and star-filled views. The best scopes are at least 5 inches (125 mm) aperture and 1250 mm focal length or more. Globular clusters: While pleasing views can be seen with smaller apertures, in order to resolve the cores of many of these clusters. You will need scopes on the order of 11 inches (280 mm) or larger. Long focal lengths scopes of 1500 mm or longer are required to provide higher magnification. Nebulae: Clouds of gas and dust vary enormously in size and brightness. The bright, larger ones like the Great Nebula in Orion is best seen in relatively small scopes. Larger scopes targeting this nebula and other objects of its class will reveal details that cannot be seen in smaller scopes. The only sacrifice going to larger scopes is the loss of a wide field of view encompassing the entire nebula. (This is because they will usually have longer focal lengths, resulting in higher magnifications and smaller fields of view with typical eyepieces.) For bright nebulae, choose medium aperture scopes with relatively short focal lengths to capture the entire gas-dust cloud in the field of view. Use apertures of 5-8 inches (125 to 200 mm) with focal lengths of 1000 mm or less. On the other hand, faint nebulae demand larger aperture just to capture enough light to see them. Some are also very large–many degrees across. Use scopes of at least 8 inches (200 mm) and focal lengths of less than a 1000 mm for faint, larger nebulae. For fainter, smaller nebulae, scopes of at least 8 inches and focal lengths of 1000 mm or more are recommended. Planetary nebulae: large scopes with long focal lengths are superior. Faint central stars are visually challenging in even the very largest amateur scopes. Use at least 11 inches (280 mm) aperture and 2000 mm focal length or greater for best results. Galaxies simply demand large apertures and longer focal lengths. With a very few exceptions, they are so far away and so faint that small telescopes will only show them as smudges. If you are a galaxy hunter, do not start out with anything less than 8-11 inches (200 to 280 mm) to be able to see details in these other "island universes." source: Celestron.com