The Short Answer
Telescopes differ mainly in how they collect and focus light:
- Refracting telescopes use lenses.
- Reflecting telescopes use mirrors.
- Catadioptric telescopes use a combination of mirrors and lenses.
Within each design, telescopes also differ in aperture (the diameter of the main lens or mirror), focal length, and the kind of mount they sit on. Aperture matters most, because it decides how much light the telescope gathers and how much detail it can resolve.
This site is mainly about microscopes, but the two instruments share a lot of optics. If you came here wondering how telescopes and microscopes differ from each other, see our article on how a simple microscope works and how it differs from a telescope.
Refracting Telescopes
A refractor has a large objective lens at the front of a closed tube, which bends (refracts) light to a focus near the eyepiece at the back. This is the classic “spyglass” design, and the oldest type. Galileo’s instruments of 1609 were refractors.
- Strengths: Sealed tubes that stay clean, little maintenance, and sharp, high-contrast views. Often good for the Moon, planets and double stars.
- Weaknesses: Large lenses are expensive to make, so apertures tend to be small. Simple lenses suffer from chromatic aberration, which shows up as color fringes around bright objects.
To tackle color fringing, refractors are often sold as achromatic (two lens elements, reasonably corrected) or apochromatic (more highly corrected, usually with special low-dispersion glass, and more expensive). Microscope objectives use the same terms for the same reason, as explained in how lenses magnify.
Reflecting Telescopes
A reflector gathers light with a curved primary mirror at the bottom of the tube. The most common design, the Newtonian (first built by Isaac Newton in the late 1660s), uses a small flat secondary mirror to send the light out to an eyepiece on the side of the tube.
- Strengths: Mirrors are cheaper to make in large sizes than lenses, so reflectors offer the most aperture for the money. Mirrors also don’t produce chromatic aberration.
- Weaknesses: The open tube can collect dust. The mirrors sometimes need collimation (realignment). The large tube can be bulky.
A Dobsonian is often listed as a separate type, but it’s really a Newtonian reflector on a simple, sturdy swivel base. It’s popular with beginners because it’s easy to set up and gives a large aperture at a modest cost.
Catadioptric Telescopes
Catadioptric (or “compound”) telescopes combine a mirror with a corrector lens. The two most common designs are:
- Schmidt-Cassegrain (SCT): A thin corrector plate across the front, and a mirror system that folds the light path back on itself.
- Maksutov-Cassegrain: A thick, strongly curved meniscus corrector lens at the front, often used in smaller, very compact telescopes.
Folding the light path packs a long focal length into a short tube, which makes these telescopes portable for their aperture. On the downside, the large secondary mirror in the light path can slightly reduce contrast. Larger models can also take time to cool down to outdoor temperature.
Comparison Table
| Feature | Refractor | Reflector (e.g. Newtonian) | Catadioptric (SCT, Maksutov) |
|---|---|---|---|
| Collects light with | Lens | Mirror | Mirror plus corrector lens |
| Color fringing | Possible in simpler designs | None from the mirror itself | Minimal |
| Cost per unit of aperture | Highest | Lowest | In between |
| Tube | Long, sealed | Open, can be bulky | Short, sealed |
| Maintenance | Very low | Occasional collimation and cleaning | Low to moderate |
| Typical strengths | Sharp, high-contrast views | Large aperture for faint objects | Compact, long focal length |
Galilean vs Keplerian Refractors
Early refractors came in two designs, and the difference is a nice link to microscope optics:
- Galilean telescopes use a converging objective and a diverging (concave) eyepiece. The image is upright, but the field of view is narrow. Opera glasses still use this layout.
- Keplerian telescopes use converging lenses for both the objective and the eyepiece. The image is inverted, but the field of view is wider, and higher magnifications are practical. Almost all astronomical refractors follow this pattern.
The Keplerian telescope inverts the image because the objective forms a real image that the eyepiece then magnifies. A compound microscope does exactly the same. Our article on real vs virtual images in a microscope explains why. Inverted images don’t matter in astronomy. Spotting scopes and binoculars add prisms to turn the image the right way up.
Specs That Make Telescopes Different
- Aperture: The diameter of the main lens or mirror, usually given in millimeters or inches. A larger aperture gathers more light (to show fainter objects) and resolves finer detail.
- Focal length: The distance from the main lens or mirror to where it brings light to a focus. Longer focal lengths give higher magnification with a given eyepiece.
- Focal ratio (f-number): Focal length divided by aperture. Lower f-numbers (such as f/4 or f/5) give wider fields of view. Higher ones (such as f/10 or above) are often favored for high-magnification views of planets.
What Magnification Do Telescopes Have?
A telescope doesn’t have a single magnification. It depends on the eyepiece you use:
Magnification = telescope focal length ÷ eyepiece focal length
For example, a telescope with a 1200 mm focal length and a 25 mm eyepiece gives 48x. Switching to a 10 mm eyepiece gives 120x.
There is a practical upper limit. A widely used rule of thumb puts the maximum useful magnification at about 2x per millimeter of aperture (around 50x per inch), and turbulence in the atmosphere often limits it further. A box promising extreme magnification from a small telescope is a warning sign. Microscopes have the same problem, which is called empty magnification.
Mounts: Alt-Azimuth, Equatorial, GoTo
The mount matters almost as much as the optics, because a wobbly mount makes any telescope frustrating to use.
- Alt-azimuth mounts move up-down and left-right. They’re simple and intuitive. Dobsonian bases are a type of alt-azimuth mount.
- Equatorial mounts are tilted to line up with Earth’s axis, so one slow movement follows a star across the sky. They’re useful for long observations and photography, but take more setting up.
- GoTo and smartphone-assisted mounts use motors or apps to find and track objects automatically.
Beyond Visible Light
Not all telescopes are optical. Radio telescopes use large dishes to collect radio waves. Space telescopes sit above the atmosphere to avoid its blurring effect and to observe wavelengths, such as much of the infrared and ultraviolet, that the atmosphere blocks. They follow the same basic principle: gather as much of the incoming signal as possible and bring it to a focus.
How Telescopes Compare With Microscopes
Telescopes and microscopes are optical cousins. Both grew out of the spectacle-making trade around 1600, as our history of the microscope describes. Both often combine an objective with an eyepiece. The difference is the job:
- A telescope uses a long focal length and large aperture to collect faint light from distant objects.
- A microscope uses a very short focal length to spread out the detail of something tiny and close.
The full side-by-side comparison is in our guide to the simple microscope and how it differs from a telescope.