Aperture for Refractor Telescopes: How Much You Actually Need

What happens if you go under or over on Aperture?

The risks of under-provisioning aperture

If you choose a refractor telescope with an aperture smaller than 70mm, you will find that the image becomes increasingly dim as you attempt to zoom in. Smaller apertures collect fewer photons, which means that at higher magnifications, the light is spread too thin to reveal detail. You may see the Moon clearly, but smaller apertures will struggle to show the rings of Saturn or the bands on Jupiter with any sharpness. This limitation is a physical property of the glass: a small opening simply cannot gather enough light to produce a bright image at high power.

For those needing a compact setup, view our list of the best portable refractor telescope models.

The practical costs of over-buying aperture

Selecting a refractor telescope with an aperture much larger than your requirements introduces significant physical and financial costs. Larger apertures require larger tubes, which are harder to transport and require more space in your home. Because the glass must be larger, the weight of the telescope increases significantly, often requiring a much heavier and more expensive mount to keep the image steady. Furthermore, larger refractors are more expensive to manufacture because the optical surfaces must be polished to a higher degree of precision to avoid distortion across the wider opening.

What is the most common mistake with Aperture?

The most common mistake is assuming that a larger aperture automatically results in a better image regardless of other specifications. Many buyers focus solely on the mm measurement, but aperture only determines the amount of light collected; it does not determine the clarity of the image. If a large aperture is paired with low-quality glass or a poor focal ratio, the result is often a blurry or distorted view that is no better than a smaller, higher-quality telescope.

What you should optimize instead

You should optimize for the quality of the glass and the focal ratio rather than just the raw aperture size. For refractor telescopes, the quality of the lens determines how well the telescope handles chromatic aberration, which is the purple fringing seen around bright objects. A high-quality 90mm refractor with apochromatic (ED) glass will often provide a sharper, more usable image than a lower-quality 120mm achromatic refractor. Balancing the aperture with a focal ratio that suits your goals—such as a “fast” scope for wide views or a “slow” scope for high-power planets—is more important than simply choosing the largest number available.

How does Aperture interact with other deciding specs?

Aperture interacts with the focal ratio to determine the telescope’s field of view and its suitability for specific targets. The focal ratio is the relationship between the focal length and the aperture; a lower number indicates a “faster” telescope. If you choose a large aperture but a very long focal length, the telescope becomes extremely long and heavy, making it difficult to use for wide-field views of large nebulae.

The impact of mount stability on large apertures

Aperture also dictates the requirements for your mount. As the aperture increases, the weight of the telescope front-end increases, and the vibrations caused by the wind or small movements become more pronounced. A 150mm refractor telescope requires a much more stable and robust mount than a 70mm model. If the mount is not heavy enough to counter the weight of the larger aperture, the image will drift off-target the moment you try to observe at high magnification. Therefore, your choice of aperture must be matched with a mount capable of supporting that specific weight and balance.

The limits of glass quality on large apertures

The larger the aperture, the more difficult it becomes to maintain image quality across the entire field of view. In a large refractor telescope, the edges of the image can become distorted if the lens is not perfectly figured. This is why high-quality glass standards, such as apochromatic (ED) glass, become more critical as the aperture increases. While a 70mm telescope can be forgiving of some optical imperfections, a 120mm telescope requires high-quality glass to ensure that the image remains sharp from the center all the way to the edges of the view.

Reviewed by the RecommendedOptics editorial team. RecommendedOptics writes these guides from the 119 most-reviewed refractor telescope listings in our retail dataset, not from manufacturer copy.