Whether you're a first time buyer or a seasoned professional, choosing among the various telescope types can be a tedious process. So, we've created this Buyer's Guide to provide direction and help you make an informed choice.
Before we dive into the details, let's start high-level and cover an essential point about what a telescope actually is.
A telescope is an optical instrument that uses lenses or mirrors or a combination of both to gather and focus light for the purposes of observing distant objects.
The key concept here is that the telescope's main function is to collect/gather light. Many beginners believe the telescope's purpose is to magnify objects, however, that is not the case.
If you're reading this, then you are most likely in the market for purchasing or upgrading your telescope to explore the stars, planets, or other celestial objects in the night sky.
In this Buyer's Guide, we will cover:
The 3 primary telescope types
The first patent for a telescope was submitted in 1608 by a spectacle maker, Hans Lipperhey, to the Netherlands government. However, in 1609, Galileo Galelei was the first to actually build a functional telescope for observing celestial objects. Thus, the refractor telescope was born.
Subsequently, in 1668, Isaac Newton developed the first reflector telescope and later came the catadioptric telescope in the mid-20th century.
Now that we have a timeline of when each type came about, let's discuss the differences.
- Refractors (dioptrics) - use lenses to form an image
- Reflectors (catoptrics) - use mirrors to form an image
- Catadioptrics - use a combination of lenses and mirrors to form an image
Refractor telescopes, or refractors, are a type of optical telescope that use a glass lens as its primary objective to form an image. Refractor telescopes bend or refract parallel light rays to converge at a focal point.
Refractors are the simplest and most lightweight design. Since light must flow in a direct path to the eyepiece, refractors usually have long optical tubes relative to their size.
Chromatic aberration, or color distortion, can sometimes arise when using refractor telescopes. For that reason, doublet (achromatic) and triplet (apochromatic) refractors were developed.
- Doublets (Achromatic lenses) - bring 2 wavelengths into focus (red and blue) to reduce chromatic aberration.
- Triplets (Apochromatic lenses) - bring 3 wavelengths into focus (red, green and blue) to virtually eliminate chromatic aberration.
3 popular refractor designs are:
In comparison to the other telescope types, refractors tend to be the most portable.
Reflector telescopes, or reflectors, use a single or an arrangement of curved mirrors that reflect light to form an image. Light enters the telescope through the front and bounces off a curved mirror at the rear. It is then reflected to a secondary mirror (or more) before arriving at the eyepiece or camera.
Since the manufacturing of large mirrors is cheaper than large lenses, reflectors are often times less expensive to purchase than refractors. They are also shorter in length.
3 popular reflector designs:
Many of the most famous telescopes used for astronomical research are reflectors, such as the Hubble Space Telescope.
Catadioptric telescopes combine specifically curved mirrors and lenses to form an image. By using both reflection and refraction, catadioptric telescopes are more error-corrective than a reflector or refractor alone.
3 popular catadioptric designs:
- Schmidt Cassegrain (SCT's)
- Maksutov Cassegrain
- Schmidt Astrograph
It is important to note that catadioptric telescopes tend to be on the more expensive side due to the complexity of design. Additionally, they can be heavier to carry and larger in size.
Smart telescopes, or digital telescopes, are another telescope type that has revolutionized astronomy. With artificial intelligence (AI) and advanced camera sensors, smart telescopes are able to hone in on various objects in the night sky, from the touch of a button.
As light pollution gets increasingly worse, especially in urban settings, smart telescopes have drastically increased the ability for both newcomers and veterans to continue enjoying unparalleled crisp and detailed views of the universe.
Why do telescope mounts matter?
The right telescope mount can optimize your observing experience as you look across the night sky by:
- Keeping your telescope securely steady
- Allowing for proper maneuvering of the telescope's position
Alternatively, a cheap wobbly mount will reduce your ability for an optimal viewing experience.
The two main categories of astronomical telescope mounts are Equatorial and Alt-Azimuth mounts.
Equatorial mounts take into consideration the Earth's rotation axis. They do so by keeping the polar axis in alignment with the Earth's axis. This eliminates field rotation and additional adjustment.
Equatorial mounts navigate celestial targets using right ascension (R.A.) and declination (Dec.). Right ascension is similar to longitude moving in an east or west direction and measured in hours from 0h to 23h. Declination is similar to latitude moving in a north or south direction and measured in degrees from 0° to +90° or -90°. For easy tracking, equatorial mounts can also be motorized.
Alt-Azimuth mounts are more simple. They are moved in an up and down (altitude) or side to side (azimuth) motion. An easy way to remember which directional motion is azimuth, is to mentally link the "z" to the word horizon (azimuth moves horizontally).
Alt-Azimuth mounts are quite easy to use and great for short term exposure. For longer term exposure above 5 minutes, the mount will need adjustment on both axes to continue tracking the image.
How to select eyepieces
Eyepieces, sometimes referred to as oculars, have a tremendous effect on telescope performance. Without the right eyepiece, regardless of how high quality the objective lens or mirror is, the observer will lack the fine details of the celestial object being viewed.
Many first time telescope buyers end up sticking with low-quality eyepieces that come with entry-level telescopes without realizing the types of views that they are missing out on. Fortunately, eyepieces are interchangeable.
Understanding magnification is an important part of deciding which eyepiece is best for you. Magnification, put simply, is the ability to enlarge the objects you are looking at. It is measured by how many times larger an image appears compared to the naked eye.
The magnification formula:
Magnification = telescope focal length / eyepiece focal length
For example, if your telescope has a focal length of 1200mm and your eyepiece has a focal length of 30mm, than 1200 / 30 = 40. Therefore your image is magnified 40x. If you are looking to adjust the power of your telescope, you will need to exchange the eyepiece for a different one. Using the same example, if you swapped out the 30mm eyepiece with a 10mm eyepiece, the magnification jumps to 120x (1200 / 10 = 120).
It is important to note that higher power is not always better. A good rule of thumb to figure out your telescope's maximum useful magnification is to take your telescope's aperture in millimeters (mm) and multiply it by 2.
When assessing eyepiece focal lengths, you should be familiar with three categories:
- Long Focal Length (55mm to 25mm)
- Medium Focal Length (24mm to 11mm)
- Short Focal Length (10mm to 3mm)
Longer focal lengths provide low power and are great for viewing wide galaxies and star fields. Medium focal lengths produce medium power and are best for deep sky objects. Shorter focal lengths provide high power and are ideal for viewing the planets.
Apparent Field of View (AFOV) is another important consideration for eyepiece selection. AFOV is the diameter of the circle of sky seen through the eyepiece, measured in degrees. In general, there are three classes for field of view:
- Standard-Field (AFOV = 40° to 60°)
- Wide-Angle (AFOV = 65° to 75°)
- Extreme Wide-Angle (AFOV = 80° to 100°)
Eyepieces are mounted in a barrel that fits into a focuser or a diagonal. Choosing the right barrel size is an important step in the eyepiece selection process.
3 popular eyepiece barrel sizes:
- 1.25" - industry standard size
- 2" - provides a wide field of view and brighter images
- 3" - provides a very large field of view and even brighter images
Let's get started
Now that you have a better understanding of the different telescope types, a clearer knowledge of why telescope mounts matter, and more awareness on eyepiece selection, it's time to go ahead and choose your next (or first) telescope.
The best education always comes from direct experience, so why wait?
And remember, the secret to choosing a telescope is not to simply buy the most elaborate, but rather to select one that you will actually use and enjoy.
If you have any questions or product inquiries, please do not hesitate to connect with a member of our team.
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