Types of Telescopes
Refractor
A refractor is what most people recognize. It has a lens that bends (refracts) light into the eyepiece.
Uses lenses to bend light and send it through the telescope to the eyepiece at the back end of the scope.
- Rugged, needs little maintenance
- Light passes un-obstructed to the eye
- Requires little maintenance (sealed tube)
- Most expensive (inch per inch)
- Typically small aperture 3” to 5”
- Suffers from “chromatic aberration”
- View is a mirror image
Reflector
A reflector is sometimes called a Newtonian. It has a primary and secondary mirror that bends (reflects) light to the eyepiece.
Light is reflected from the curved primary mirror in the back of the telescope to the secondary mirror in the front of the scope. The secondary mirror reflects light into the eyepiece in the front side of the telescope.
Smaller reflectors have closed tubes. Larger reflectors have truss tubes.
- Affordable: inch for inch the least expensive
- Eye piece at top of tube for easy viewing
- Correct image (not mirror), although upside down
- Reflectors with Dobsonian mounts (“Dobs”) are very simple and sturdy
- Bulky (over 6” to 8”)
- Open tube can collect dust
- Requires occasional alignment (collimation)
- Viewing height will vary considerably
Compound/Catadioptric
A compound telescope is more complex and has a combination of lenses and mirrors. The most common types are the Schmidt-Cassegrain and the Maksutov-Cassegrain.
Light passes through the corrector plate in the front of the telescope to the primary mirror in the back of the scope. It is then reflected by the primary mirror to the secondary mirror in the front of the scope. The secondary mirror reflects light to the eyepiece in the back of the scope.
- Compact and portable system (1/3 length of equivalent reflector)
- Sealed tube
- Eye piece remains close to the same height
- Typically has computerized controls
- More expensive than equivalent reflectors
- Requires occasional alignment (collimation)
- Heavy (8” and over)
- View is a mirror image
- Complicated to use
Telescope Mounts
Altitude-Azimuth
Many telescopes have Altitude-Azimuth mounts, which move horizontally and vertically.
These are the traditional tripods most are familiar with.
Equatorial
Some have Equatorial mounts that can be aligned to track the movement of the night sky.
Equatorial mounts are typically harder for a beginner to use.
Dobsonian
Some reflector telescopes (“Dobs”) have Dobsonian mounts, which sit on the ground and allow the telescope to move horizontally and vertically.
Dobs are often considered good telescopes for beginners.
Fork
The more complicated compound telescopes typically have Fork mounts, which are part of the telescope assembly and attach to a tripod.
Some compound telescopes utilize equatorial mounts.
Smart Telescopes
The world of telescopes is changing. Recently, several companies have introduced “smart” telescopes for astrophotography.
These allow even novice astronomers to jump into the world of imaging. This might be an option for you.
More On This Page
(Telescope Characteristics and Accessories)
Characteristics and Accessories
Aperture
A telescope’s aperture is the diameter of the telescope’s primary lens or mirror. With a larger aperture, the telescope gathers more light, and it is easier to see dim objects. When determining the light that is captured, it is the area that is important (Area = pi * radius2). As an example, a 12” aperture will capture 9 times more light than a 6” aperture.
However, the larger the aperture, the heavier and more expensive the telescope. Therefore, there is a tradeoff between APERTURE vs. COST/SIZE.
Some of the larger Dobsonian reflectors come as truss tubes, where the primary mirror and secondary mirror/focuser are two separate pieces. They are lighter than equivalent single tube scopes, but it takes longer to set them up.
Focal Length
A telescope’s focal length is the distance light travels from the primary mirror or lens to the eyepiece. The longer the focal length, the higher the magnification for a given eyepiece.
For most telescopes, the focal length is equivalent to the length of the tube. For compound telescopes, its is longer because light makes multiple passes through the tube.
A telescope’s focal ratio is the aperture divided by the focal length. “Fast” telescopes have a f/ ratio of f/5 or less. “Slow” telescopes have a f/ ratio of f/8 or more.
Focuser
The Focuser holds the telescope’s eyepiece (see below) and allows you to focus the telescope to obtain a sharper image. Focusers and eyepieces come in two sizes: 1 ¼” and 2”. Almost every scope supports 1 ¼”, while larger scopes support 2”.
The advantage of a 2” focuser, and the corresponding eyepieces, is that they provide a wider field of view. However, they are more expensive.
Focusers also have single or dual adjustment knobs, the latter for fine adjustment.
The highest magnification isn’t always the best.
Look for a “sweet spot” with a lower magnification that gives a clear image and possibly a wider field of view.
Eyepieces
A telescope Eyepiece is required to focus and magnify the light captured by the telescope so that you can view a large and sharp image. Eyepieces come in 1 ¼” and 2”.
One of the most common eyepieces is the Plössl with focal lengths ranging from 40mm to 6mm and apparent field of view (AFOV) around 50o. However, the eyepiece will typically magnify the view creating a true field of view (TFOV) that is much less (maybe around 1/2o)
A telescope’s magnification is:
TELESCOPE FOCAL LENGTH /
EYEPIECE FOCAL LENGTH
500mm telescope / 25mm eyepiece will magnify 20 times
1200mm telescope / 25mm eyepiece will magnify 48 times
Barlow Lens
A Barlow Lens is an eyepiece that doubles (2X) or triples (3X) your magnification.
Simply insert it into the focuser, then place your eyepiece into the Barlow rather than directly into the focuser.
Eyepiece Filters
Eyepiece filters block portions of the electromagnetic spectrum (specific wavelengths of light) and allow the glare from bright objects such as the Moon to be reduced and the contrast of other objects such as planets and nebulae to be enhanced. Most filters are sized to fit either a 1 ¼” or 2” eyepiece. DO NOT use these to observe the Sun!
- A Moon filter (typically a 13% transmitting filter for larger telescopes and 25% transmitting filter for smaller telescopes) is recommended.
- A nebula or light-pollution filter can be useful in bringing out the shape of a nebula. These are most useful with larger telescopes.
- Colored filters are helpful when viewing the planets but are most useful with a larger telescope and under dark skies.
Finders
A Finder sits on the telescope tube and allows you to point the scope toward an object in the night sky. Finders come in three varieties:
Small auxiliary scopes – These are “miniature” telescopes that provide a wide field of view and a small magnification allowing you to locate an object. Some have a right-angle shape, which makes it easier to look through.
Reflex (“Red Dot”) finders – These LED finders display a red dot or circle on a small transparent screen. To aim the telescope, simply line up the LED target with the object in the sky. A reflex finder (such as a Telrad) is often a better choice than the more traditional auxiliary scope.
Laser mounts – These allow one to mount a green laser pointer on the telescope and to aim the scope using the laser pointer.
More Information
- Elena Stone. “Telescope Specs Explained By An Expert.” Little Astronomy. September 9, 2023.
- Elena Stone. “Telescope Accessories.” Little Astronomy.
We have some information about Suppliers of Telescopes and Accessories.
Find out about taking care of and cleaning your telescope and optics here:
Here is a handout summarizing some of the above information.
You must be logged in to post a comment.