Astronomy telescopes come in a wide variety, ranging from the plastic tube sold in toy stores to the famous Hubble telescope flying up in the sky.
Here, we only focus on the most basic & essential information necessary for a backyard astronomer.
At the end of this guide, you’ll be educated enough to choose telescope on your own.
The important components of the telescope
- Aperture (or Objective Lenses)
- Focal Length
Aperture or objective lens
An astronomical telescope is used to collect light from a celestial object and produce a magnified image. There are two basic types of optical telescopes: refractor telescopes and reflector telescopes. The first one uses lenses and the other one uses mirrors. The primary lens or mirror that focuses light is called the aperture of the telescope. Both of this types use an eyepiece to magnify the image formed by the aperture.
In space observations, refractor telescopes are commonly used for lunar and planetary studies, while reflector telescopes are preferred for extragalactic studies.
In addition to these two common types of telescopes, there are a number of other designs which use a combination of lenses and mirrors to take advantages of both refractors and reflectors, such as Schmidt-Cassegrain telescopes and Maksutov-Cassegrain telescopes.
The refracting telescope (shown above) was invented in 1609 by Galileo Galilei who used it to discover Jovian satellites, lunar mountains etc. Galileo’s refracting telescope suffered from a number of defects in image formation, particularly aberrations, due to the use of simple lenses.
Modern optical telescopes use compound objective lenses to minimize this aberration. The compound lens consists of one convex lens of crown glass and one concave lens of flint glass. In addition, all lenses are coated with anti-reflective coatings to maximize transmission. In common designs for amateur astronomy, light from the objective lens is often turned 90 degrees by a flat mirror before entering the eyepiece.
Refractors provide the best image quality among amateur telescopes, thanks in particular to the compound objective lens. Images of planets and the moon are extremely crispy and sharp. They are also used for viewing objects on land when an erecting eyepiece is used.
In addition to their high performance, refractor telescopes are also maintenance-free and very portable due to their light weight and compactness. So they have always been one of the most favorable among amateur astronomers. The disadvantage of a refractor telescope, however, is that it is the most expensive in terms of dollar/aperture ratio. Celestron 50mm is one of the most budget-friendly models.
The reflecting telescope (shown above) was first designed by Isaac Newton in 1668. Here, a primary parabolic mirror reflects light rays to an inclined flat mirror, which, in turn, reflects the rays to an observer located at the side of the tube. This design has essentially remained the same today in most amateur reflectors.
The most attractive feature of a reflector telescopes is that it provides the lowest dollar/aperture ratio of any telescope type. Therefore, for the same money, a reflector gathers most light, which is important in deep space viewing.
Small amateur reflectors, such as a 3″ (76 mm) or a 4.5″ (114 mm) are also light enough for easy transportation. But there’s a problem. Because of the open tube design, a reflector requires alignment and cleaning once a while. The image from a reflector is usually upside-down and thus not all reflector are good for terrestrial viewing.
[RELATED: Celestron 76mm is the cheapest reflector in the market right now.]
The dream and the reality…
The larger the objective diameter, the more light it collects. Frankly, I would choose the largest objective possible: the Hubble in the sky or the 200-in reflector at Palomar Observatory comes into mind.
This is of course not possible for an amateur, particularly with a limited budget. Thus, we choose a telescope based on use, value, and ease of operation.
The refractor vs reflector telescope
Most backyard astronomers use telescopes to observe the Planets, the Moon, as well as some of the largest star clusters. I would like something which is easy to carry around and set up in the backyard or up on a hill. A small astronomical refractor makes a good candidate. It offers the best image quality among all amateur telescopes. It is also suitable for terrestrial viewing.
Frankly, I am always excited when I see the details of Jupiter’s moons and Saturn’s rings. For amateur astronomy, the 60 mm is a starter but its small aperture size is a limiting factor. The 80 mm refractor is most popular. Unfortunately, the price goes exponentially with increasing objective size in refractors. So, larger refractors are usually too expensive for the majority of users.
The Newtonian reflector is another popular and economical telescope for amateur astronomy. It offers the best objective size per dollar value among all designs and is suited for deep space viewing.
Reflectors are a bit heavier and thus more difficult to transport than refractors. You may want to use one for a more permanent location. The open design of the reflector also makes periodic cleaning and alignment necessary, but this is something you can do by yourself.
The telescope power/magnification
One thing you should always remember, power of magnification is not everything. In fact, power is not a factor you should even consider when buying a telescope.
The higher the magnifying power, the less light a telescope gathers and the more sensitive it is to any small movement (wind or even your own heartbeat).
Generally speaking, good image quality is obtained for magnification less than x100. A rule of thumb for the upper limit is ~24x magnification per cm of objective lens diameter. For example, a practical limit for an 80 mm telescope is ~200 x. Anything higher is useless and can only degrade its performance.
The telescope mount is a very important component. Some cheap telescopes do not come with a real mount and adjustment is only made with a screw. Avoid those!
Locating a star or planet in the vast space requires a reliable mount which enables you to make adjustment precisely. I would highly recommend the German-design equatorial mount with slow motion controls.
More expensive versions can be equipped with motorized drives and computer interfaces. The Dobsonian mount is also popular for reflectors, due to its low cost and stability. But it is very heavy and not suitable when portability is of concern. [RELETED: Travelscope is the most portable telescope.]
User level suitability
And if money is not an issue…
If money is not an issue, I would recommend compound telescopes that combine the advantages of reflectors and refractors. These high-performance instruments usually come in very compact sizes, thus making them ideally suited when portability is a major concern.
When it comes to a telescope, do you think of names like Meade, Orion, Celestron? Here is a dose of reality: Find our popular list of 8 best telescopes in India here and save the time & frustration of doing research.