The objective of this post is to give the reader a sense for the different types of telescopes typically in use by amateurs and the areas of astronomy for which they are best suited. Like the other beginner topics in the forums, there may be some simplifications here which overlook some of the nuances that more experienced observers just love to discuss with the goal of keeping things simple for the newcomer while capturing the essence of the topic.
Telescopes are categorized in three general categories based on the composition of the optical components. This section will be be broken up into these categories. Telescopes that use only mirrors to gather and focus light are called
reflectors (or rarely catoptrics). Telescopes that use only only lenses to gather and focus light are called
refractors (or rarely dioptrics). The final category of telescopes are those that use both mirrors and lenses to gather and focus the light, and these telescopes are called
catadioptrics.
When shopping for telescopes in the United States, telescopes are nearly always specified by aperture and focal ratio. In Europe, it seems that this is still the most common practice, but you may also see some telescopes specified by aperture and focal length. Examples of telescopes specified by aperture and focal length would be “90mm f/10” or “8” f/5”. An example of a telescope specified by aperture and focal length would be “100/800” where 100 is the aperture in mm and 800 is the focal length in mm.
The five primary aspects when considering a telescope are
size,
optical quality,
aperture,
focal length, and
focal ratio.
SizeSize matters! For newcomers to the hobby, this attribute is really a combination of the size of the telescope and the mount required to use it because you will probably buy both together as part of a single combined package. Size may be more important than any of the optical attributes listed above because if you buy a telescope that is too large or cumbersome, you won't be using it. At that point it doesn't matter what the optical properties are. Fortunately, this isn't a complicated matter to understand. You just need to consider when looking at a telescope if it will fit in your car if you're planning to take it to dark skies outside the city or if the telescope is too heavy for you to move around easily. This is one of the main drivers behind the advice to seek out your local astronomy club. Many areas of the country no longer have shops where telescopes are setup to look at before you buy. Even if you're lucky enough to have such a shop, the local club is likely to have a wider variety of telescopes than would be on display at the shop.
Optical QualityThis needs to mentioned because a simplifying assumption in these beginner topics is that to describe the other metrics of your equipment the optical quality of the equipment being compared is similar. This is intuitively obvious. Telescopes are just tools, and just like, for example, wrenches, some may be manufactured to higher standards or made from better materials than others. With telescopes it can be that on paper one telescope could look better than another, but poor optical quality can override other factors. Unfortunately, manufacturer specifications can be difficult to find or vague when they are found. One can rely on price to some extent, because if something is super cheap its likely to be on the lower end of the quality scale, but there ways other than skimping on optical quality for manufacturers to things cheaper. For example, the mount quality could be lowered. On line reviews can be helpful, but many of those are perhaps too unforgiving at least with respect to the quality standards that one might be using for a beginner's telescope.
ApertureThe brightness of objects in your telescope is a function of the area of the primary objective. In other words, if you double your aperture, the diameter of your objective, objects will appear four times brighter not two times brighter.
The aperture also affects the maximum resolution of your telescope, but this relationship is linear. Thus, if you double the aperture of your telescope, you will get a theoretical doubling in resolution although in actual practice the atmosphere is going to put an upper limit on your resolving power. So what is resolution? Put at its most simple, resolving power is the ability of an optic to distinguish two points from each other. So imagine two stars that are very close to one another. If when viewed through a telescope they appear as two separate stars, they are said to be resolved. This is sufficient depth on this resolution for now. A more in depth discussion of resolution will be provided in a separate post.
Focal LengthThis is the distance from the objective required for the image to come to focus. Simply put, telescopes with longer focal lengths magnify objects more than telescopes with shorter focal lengths. The corollary to this is that telescopes with longer focal lengths smaller areas of sky than telescopes with short focal lengths. The exact magnification produced depends on the eyepiece being used (which will be discussed separately with eyepieces), but when considering multiple telescopes against each other remember the longer focal lengths produce more magnification.
Focal RatioThe focal ratio of the telescope is derived by dividing the focal length of the telescope by the telescope's aperture. Make sure that the focal length and aperture are in the same units (i.e both in inches or both in mm). A focal ratio of 10 would be expressed as “f/10”. This measure is identical to the focal ratio used in camera lenses if you're familiar with how that works.
The focal ratio is used to calculate the exit pupil of an eyepiece used in a particular scope. Telescopes with larger focal ratios (slow telescopes) are also more forgiving of eyepiece aberrations. Correcting for optical aberrations generally increases the cost of an optic, so in general cheaper eyepieces perform better in slow telescopes (telescopes with large focal ratios) than they do in fast telescopes (telescopes with small focal ratios).
See the following posts for a discussion of each telescope design.
ReflectorsRefractorsCatadioptrics
