If you are purchasing a telescope, you may ask yourself “what is the best type of telescope?”. There is no single answer to this question. Let’s start out by giving the characteristics of each type.
There are three main types of telescopes
The three primary types of telescopes are refractor telescopes (also called refracting telescopes), reflector telescopes (also called reflecting telescopes) , and catadioptric telescopes. Each type of telescope has its own advantages and disadvantages.
The choice of which to purchase often comes down to cost , whether they are to be used for visual observation or for astrophotography, and what type of object will be observed or photographed.
Refracting telescopes use lenses to gather and focus light. The lenses are usually made of glass or plastic and are arranged in a specific pattern to bend the light and bring it to a focus. The light travels directly through the telescope from one end to the other, being bent or refracted by the lens(es) to reach focus at the opposite end. An example of a refractor, and the one I personally use, is the William Optics FLT120.
- Provides high contrast and sharp images
- Refracting telescopes have a simple design, making them easy to use and maintain
- Low maintenance
- Can be used for both astronomy and terrestrial viewing
- They are ideal for viewing bright objects, such as the Moon, planets, and stars.
- Can be expensive for large apertures due to the cost of large lenses
- They are limited in size, because larger lenses become heavy, requiring robust support structures
- Susceptible to dew and atmospheric distortion.
- Chromatic aberration is a common issue, which causes color distortion. This simply means that all visible wavelengths do not reach focus at the same point. This results in colorful halos around the edges of stars and other objects.
Apochromatic and Achromatic versions of refractors use additional lenses to make different wavelengths come to focus at the same point. This adds to the cost
Reflecting telescopes use mirrors to gather and focus light. The mirrors are arranged in a specific pattern to reflect the light and bring it to a focus. Light enters the telescope at one end and travels all the way to the opposite end where the primary mirror lies. The mirror has a curve designed to bring the light into focus after it gets reflected. A smaller secondary mirror lies somewhere in the middle of the telescope and it redirects the light to the eyepiece opening, where it reaches focus for observation with an eyepiece or with an attached camera. An example of a reflector telescope is the Celestron – NexStar 130SLT, a good quality telescope for beginners. It comes with a computerized mount, which makes it ideal for pointing to the desired object and keeping it in view.
- Generally less expensive than refracting telescopes for the same aperture size
- Reflecting telescopes can have a larger aperture than refracting telescopes, allowing them to gather more light and provide brighter images.
- Do not suffer from chromatic aberration
- No lens dewing issues as with refracting telescopes
- They are ideal for viewing faint objects, such as galaxies and nebulae simply because of the larger aperture and excellent contrast.
- The folded light path results in longer focal lengths and higher magnifications
- Can suffer from coma, which causes distortion at the edges of the field of view
- Can be heavier and bulkier and more difficult to move due to the size of the mirrors
- May require more maintenance than refracting telescopes as the mirrors may need to be cleaned
- Reflecting telescopes require frequent alignment (also called collimation) and maintenance of their mirrors
- Very large reflecting telescopes can be so tall, they require a ladder to observe through the eyepiece opening
Catadioptric telescopes use a combination of lenses and mirrors to gather and focus light. The design typically uses a combination of a corrector lens and a primary mirror or secondary mirror to reflect and refract light. Two common versions of catadioptric telescopes are the Schmidt-Cassegrain and the Maksutov-Cassegrain. An example of a Schmidt-Cassegrain catadioptric telescope is Celestron – NexStar 8SE. This is a computerized telescope suitable for beginners and more advanced users.
The arrangement of the mirrors and lenses varies a bit, but the concept is the same. The light path is lengthened by reflection by the mirror and brought into focus by the corrector lens.
- Catadioptric telescopes combine the advantages of both refracting and reflecting telescopes, allowing for a larger aperture
- Compact and portable design
- Because the light is folded by the mirror to achieve focus, this type of telescope usually has higher focal length, which generally results in higher magnification, or larger apparent images of small objects
- Generally less prone to coma than reflecting telescopes
- They are versatile and can be used for a wide range of astronomical observations
- Can suffer from a small amount of chromatic aberration
- Typically more expensive than reflecting telescopes
- More complex design than either refracting or reflecting telescopes
- Because the secondary mirror is in the light path, contrast can be reduced
What are the important considerations?
There are many considerations when choosing a telescope. Good quality telescopes can be found in various configurations. And the method used to bring light to focus should be considered depending on what your plans are. Are you going to use it at home? Then you can consider a telescope that is not compact. Are going to travel to a dark sky sight? Then portability is important. Are you interested in observing through an eyepiece? Are you interested in photography (if you are, you need to consider a tracking mount in your budget)? Are you interested in planets or galaxies or nebulae?
Before you buy
Before jumping in, decide what your priorities are. You don’t need to spend a fortune either. Obviously the highest quality telescopes are going to cost at least $1000. But there are plenty of decent quality telescopes for a few hundred dollars. Once you have some experience with a good beginner telescope, you can decide to spend more on a more advanced. or higher quality telescope. If you already have some experience and a larger budget, don’t be cheap! You won’t be sorry.