What Is A Telescope Aperture? Explanation and Size Comparison

diameter of telescope objective

Telescope aperture is the size of the primary optical element. Knowing the aperture of a telescope allows observers to understand the light-gathering ability and resolution. For amateur telescopes, apertures range from 50mm to 130mm in diameter. Knowing the aperture diameter size also allows telescope users to understand the focal ratio and focal length. These values help determine the field of view, magnification, and suitability for observations. Combining focal length with the eyepiece diameter allows astronomers to calculate the magnification power. 

What is Telescope Aperture?

The telescope aperture is the diameter of the primary optic. The primary optic is either a lens or mirror, depending on the type of telescope. Different types of telescopes cater to unique observational goals, and therefore will also vary in lens size. The aperture size plays a crucial role in the telescope’s light-gathering ability and resolution, directly influencing the quality of the image. The light-gathering ability is determined by the surface area of the primary optical, or aperture size.

To identify a telescope’s aperture, look for the telescope’s specifications near its focuser, at the front of the tube, or on the box. The aperture size is typically expressed in millimeters (mm). Knowing this size allows observers to understand key elements of their viewing experience. 

The optimal size of an aperture will depend on personal observational goals. For basic observations, such as the moon and stars, apertures of 70mm (2.8 inches) to 130mm (5.1 inches) are considered good. However, larger apertures, above 130mm (5.1 inches), allow observers to get a brighter, sharper and more detailed view. The view is generally clearer because a larger diameter allows the telescope to gather more light. Larger apertures are capable of revealing faint and distant objects that would be challenging to see through smaller apertures. Therefore, as the aperture size increases, the telescope’s ability to showcase celestial bodies improves significantly.

What are Common Telescope Aperture Sizes?

The most common telescope primary optical diameters, or aperture sizes, are 70mm (2.8 inches), 80mm (3.1 inches), 90mm (3.5 inches), 100mm (3.9 inches), 130mm (5.1 inches), and 200mm (7.9 inches). Aperture sizes of 90mm and lower are considered small, but remain prevalent due to their affordability and user-friendly nature. 

Smaller apertures are suitable for casual stargazing and beginners, offering a cost-effective entry point into astronomy. Apertures ranging from 100mm to 130mm are considered medium in size. Medium-sized apertures strike a balance between portability and light-gathering capability, making them adequate for common observations of planets, the Moon, and brighter deep-sky objects. Aperture sizes of 200mm and above are considered large, catering to serious astronomers and astrophotographers. These telescopes excel in capturing faint and distant celestial objects, providing detailed views of smaller or more remote astronomical bodies. Overall, different telescope sizes are ideal for different observational goals.

Does a Larger Telescope Aperture Mean It’s Better?

Yes, higher telescope apertures are better at capturing light, allowing for a brighter, sharper and more detailed image. As the diameter of the primary optic increases, the telescope is able to gather more light. The increase in light-capturing ability allows observers to see faint or distant objects that wouldn’t be visible through a smaller aperture. This is because lower apertures allow for less light, resulting in lower brightness, resolution, and overall image quality. Because of this, most scientific telescopes have extremely large apertures, allowing them to study details of distant celestial bodies.

Is it Possible to Increase the Size of Telescope Aperture?

No, it is not possible to increase the size of a telescope’s aperture due to the delicacy of its optics. When light enters the telescope, it interacts with the primary lens or mirror, which acts as the first optical element. The incoming light rays converge at a specific point, known as the focal point, where they create an image that is then magnified and viewed through an eyepiece. Because of this, increasing the aperture size requires a larger primary lens or mirror. While it’s a simple concept, implementing such a change poses significant technical challenges. Crafting and aligning larger optics with the required precision becomes increasingly more difficult as the size increases. Even minor imperfections in the surface shape or alignment will severely degrade image quality, making it nearly impossible to increase a telescope’s aperture without damaging the telescope or viewing experience. Because of this, the only way to increase a telescope’s aperture is to use a different telescope.

What is the Largest Aperture in Telescopes?

The largest aperture size to date was designed for the James Webb Space Telescope, with a primary mirror of 6.5 meters (256 feet) in diameter. The largest amateur telescope, at the ​​Stansbury Park Observatory Complex (SPOC), has an aperture size of 1.8 meters (70 inches). These large-aperture telescopes allow for impressive light-gathering capability, as well as extreme focal lengths, focal ratios and magnifications. 

How Does Telescope Aperture Affect Focal Length?

Telescope aperture does not affect the focal length of a telescope because focal length is determined by the telescope design. Focal length is determined by the specific optical design and represents the distance from the lens or mirror to where light converges to form an image. Because of this, focal length is not directly related to aperture size. However, larger aperture telescopes often have longer focal lengths, providing higher magnification and narrower fields of view. Smaller aperture telescopes tend to have shorter focal lengths, offering wider fields of view, suitable for observing larger celestial objects or engaging in astrophotography. 

The interaction between aperture and focal length is crucial in determining the telescope’s focal ratio (f/number), which affects image brightness and the ability to capture faint objects. Determining the ideal aperture and focal length combination is essential to achieving the desired magnification and field of view.

How Does Telescope Aperture Affect Focal Ratio?

A larger aperture affects the focal ratio by decreasing it, but increasing the light-gathering capability. The increased light-gathering ability results in brighter, sharper and more detailed views, making it ideal for observing faint objects and astrophotography. However, a higher, or faster focal ratio results in narrower views, making it difficult to observe larger celestial objects. Conversely, a smaller aperture increases the f/number, making the telescope slower, with wider views, but requires longer exposure times for astrophotography. What is the difference between fast and slow focal ratios? A faster focal ratio benefits astrophotography with shorter exposure times, reducing the risk of star trailing. Meanwhile, a slower focal ratio offers wider views and is useful for observing larger objects. Balancing aperture, focal ratio, and magnification requirements ensures an optimal stargazing and imaging experience tailored to individual astronomical interests.

How Do Telescope Aperture Sizes Affect Magnification?

Telescope aperture sizes allow observers to determine a telescope’s maximum useful magnification. Telescope magnification is primarily determined by the telescope’s focal length and the diameter of the eyepiece, but magnification is limited by the telescope’s aperture. A larger aperture telescope, with its greater light-gathering capacity, allows for higher magnification possibilities when combined with appropriate eyepieces. 

The increased light-gathering ability makes larger aperture telescopes well-suited for celestial objects that require higher magnification to observe. On the other hand, smaller aperture telescopes have limitations in their light-gathering power, which affects their ability to achieve high magnification. While small apertures still provide a view of the Moon and brighter objects, they will not offer the same level of detail when observing more distant or fainter celestial targets.

The maximum useful magnification allows the observer to identify how far a telescope is able to see clearly. To calculate a telescope’s maximum useful magnification, multiply the aperture, in millimeters, by 2. For example, an aperture of 70mm will have a maximum useful magnification of 140x. The table below compares the maximum useful magnification for each of the most common aperture sizes.

Aperture SizeMaximum Useful Magnification
70mm (2.8 inches)140x
80mm (3.1 inches)160x
90mm (3.5 inches)180x
100mm (3.9 inches)200x
130mm (5.1 inches)260x
200mm (7.9 inches)400x