There are quite a few important factors that determine the capabilities of a telescope:
We shall now see how each parameter affects the capabilities of a telescope.
There are many types of telescope which are classified as per the optics uesd. Telescope can be broadly classified as:
Refractors use lenses to create an image of the faint / distant object. Reflectors use mirrors to focus an image of the object. Catadioptric telescopes use a combination of mirrors and lenses to create an image.
What are the pros and cons of each type? We will discuss refracting and reflecting telescopes as they are popular.
Refractor Telescopes use lenses to have image of the Object. Combination of lenses gives you required mangnification (hyp). Refractors are usually expensive because a well-engineered combination of several lenses is required to create a good image. Creation of Lenses are expensive because it consumes time and also requires a lot of engineering.
Refractors suffer from a defect called 'Chromatic Abbaration'. This defect can be minimised by using special techniques to manufacture lenses and this results in 'Achromatic Lenses' which again makes it expensive. But to remove chromatic abbaration so that the image is more crisp , Other techniques are used to create more error free lenses. Thus, there are two types of popular refractors:
An achromatic refractor will gives you a quality image. Apochromatic lenses produce the most crisp images, and are ideal for photography of celestial objects. Apochromats are more expensive than Achromats.
Refractors are not preferred for visual observations because as the aperture increases the cost increases exponentially, so only small aperture telescopes are used for visual observation to view planetary objects and close by objects.
Reflecting telescopes produce an image by using combination of mirrors.
Newtonian reflectors are the most popular for visual observing of distant celestial objects.
This is the most popular design for visual observing. Newtonian reflectors have two mirrors - a 'primary' concave (actually parabolic) mirror and a 'secondary' flat mirror. The primary mirror gathers the light from the celestial object and focusses it. The secondary mirror only changes the direction of the light, so that it is suitable for viewing. Big Aperture Mirrors are economical as compared to big aperture Lenses and the cost reduces heavily. Newtonian reflectors are the most suitable for visual observing and can also be used for Photography.
This design is usually preferred for astrophotography of small celestial objects. This design accomodates long focal lengths in a shorter tube length, making it less bulky. A favourite amongst astrophotographers.
To summarize:Schmidt Cassegrains are suitable when you want large magnifications, especially for photography.
The clear aperture of a telescope is the diameter of the objective lens or primary mirror specified in either inches or millimeters (mm). The larger the aperture, the more light it collects and the brighter (and better) the image will be. Greater detail and image clarity will be apparent as aperture increases. In astronomical telescopes, the light-gathering power is more important than magnification.
The light-gathering ability of a telescope varies as the square of the diameter of its aperture. Which means that a 8' telescope is 4 times more powerful than a 4' telescope, which in turn is 4 times more powerful than a 2' telescope.
To summarize: Larger aperture helps you view objects more clearer by gathering more light from teh object.
Focal length, usually measured in mm, refers to the focal length of the primary optical element (objective lens in refractors, primary mirror in reflectors). Larger focal length gives more magnification with the same eyepiece. Smaller focal length gives more field-of-view with the same eyepiece, but less magnification.
To observe planets in great detail, an objective with a long focal length and high optical quality is preferred. To observe deep-sky objects, an objective with a short focal length is preferred.
Most telescope users would like to see both planets and deep-sky objects, so intermediate focal lengths are preferred. A focal length of anywhere between 40' (about 1000mm) and 64' (about 1600mm) is preferrable. Magnification (HYP) can always be increased or decreased by changing the eyepiece.
Astrophotographers who want to photograph small objects will prefer long focal lengths and go in for Schmidt Cassegrain telescopes to reduce the bulkiness of the telescopes.
Type of Mount
There are two basic classes of telescope mounts:
Equatorial mounts are required for photographing celestial objects, and for tracking objects as they drift across the sky.
Altazimuth mounts are ideal for observing as they are simple to operate. A type of Altazimuth mount called the 'Dobsonian Mount' is very popular for visual observing. Most large Newtonian Reflectors use Dobsonian mounts.
Pros of Equatorial Mount
Cons of Equatorial Mount
Pros of Altazimuth Mount
Cons of Altazimuth Mount
Motorization / Computer Control
Motorization is usually required for astrophotography setups, except when doing wide-field astrophotography. Computer control is hardly ever required, unless you want to slew to objects automatically, rather than finding them manually.
Motorization is required for astrophotography, except for wide-field astrophotography.
Compatibility with Standard Eyepieces
Eyepieces come with two standard outer diameters: 1.25' and 2'. Telescopes with focussers that do not comply with these sizes should be avoided, because you might not be able to buy separate eyepieces that are compatible with your telescope.
Buying a Telescope - Recommendations
For Visual Observing
It is ideal that a beginner should not focus on planetary observing unless he is planning to do research on them. The planets, and bright 'stellar' planetary nebulae are limited in number and hence only few objects can be studied with a planetary (long-focal-length) telescope.
Newtonian Reflector telescopes on Dobsonian or other Altazimuth mounts are ideal for visual observing.
For Planetary Observing
A telescope with very high optical quality and long focal length with moderate apertures is ideal to study planets and planetary nebulae in great detail.
For General Purpose Observing
Choose an aperture as large as your budget can accomodate. Choose reasonable optical quality upto 6' of aperture, good optical quality for upto 12' and very high optical quality for larger telescopes. Buy an eyepiece of very short focal length (4.8mm), of a very good design (Nagler / Panoptic / Ethos) if your budget permits. For lower budgets, cheaper Barlow lenses and low-grade eyepieces can help instead. Focal length could be in the range of 1000mm to 2000mm.
Choose an aperture of 4' - 8', unless you plan to observe faint objects, or your budget can fit in more. Choose a moderate focal length. A focal length that is 8 to 10 times the aperture diameter should do. Ensure that the focusser is either 1.25' or 2' in barrel diameter.
A normal Refractor, Achromat or apochromatic refractor and/or a Schmidt Cassegrain telescope of high optical quality, with Motorized/GoTo equatorial mount are ideal for astrophotography. For Schmidt Cassegrains, choose an optimum aperture and high optical quality.