Is A Big Telescope Aperture Size Always Better?
As amateur/hobbyist astronomers, we often hear or read about how astronomy newbies get confused whenever they are struggling to purchase their first telescope. More often than not, they tend to examine and focus on the magnification power of a telescope. As seasoned and experienced amateurs, we know that the magnification power isn't the most important characteristic to consider.
What Is A Better Technical Specification To Evaluate?
Let's answer that question as it applies to the aperture of a telescope or binoculars. With any optical viewing device, like a telescope, it is designed to brighten and magnify the object being viewed. There are variations in the way that each telescope type (refractor, reflector, catadioptric) accomplishes it, and each type has it’s own set of advantages and disadvantages.
Independent of the type of telescope, the single most important feature of a telescope is it’s aperture size. In simple terms, the telescope aperture size is the measurement of the diameter of the primary light-gathering lens or mirror. A larger lens or mirror is capable of gathering more light from the viewed object, which means that the telescope is able to provide a brighter and sharper image.
For a refracting type telescope, the telescope aperture is the distance measured across the widest point of the objective lens. This is typically referred to as the diameter of the objective lens and is measured in either inches (in) or millimeters (mm). So in this instance, the lager diameter the objective, the more light the telescope is able to collect and direct towards the eyepiece.
For a reflecting type telescope, the telescope aperture is the distance measured across the widest point of the objective mirror. The mirror is responsible for capturing the incoming light and directing the reflected rays to the focal point. The diameter of the objective mirror is measured in either inches (in) or millimeters (mm). Similar to the refracting type telescope, the larger the size of the objective the more light can be captured.
Why Is The Light Gathering Of A Telescope Aperture Important?
A convenient side-effect of capturing more light from the viewed object, means that you can apply more magnification while maintaining clarity. In simple terms, a larger aperture means that a person can use more magnification without resulting in a blurry image.
A larger aperture allows you to use more magnification. You can configure a telescope with various magnifications at all (just by changing eyepieces), but without a sufficiently large aperture, the high magnification is worthless — it will just result in showing a blurry, dim mess. Not very usable or enjoyable.
So What Can You Do With Aperture? What Are The Limits Of Aperture?
As it turns out, the aperture value of a telescope is a key value in determining the maximum useable magnification power that can be used. This value is known as the MUM Factor (maximum useable magnification), and you can learn more details about it by checking out our article on the MUM Factor.
The MUM Factor of a telescope is used to directly calculate the magnification power that can be used with satisfactory results. In general, a telescope can only be pushed to 50x (50 times magnification) before the view loses clarity, becomes blurry and unusable. Often times 1 50x magnification is good enough for casual lunar and celestial object viewing. But if the expectation is to discern surface features, distinguish both members of a tight double star, view the articulation of rings, then you would really require sharp views at 150x magnification or more.
Here we can see a comparison of pictures of the planet Saturn that were taken with two different telescopes, each with a different size objective. The picture on the left is from a telescope with an objective size of 100mm. The magnification setting was equal to the MUM Factor. As you can see, the planet and rings are just barely distinguishable. Any increase in magnification would have resulted in am image without clarity and the planet and rings would just be a blurry blob.
Next, look at the photo on the right. This picture of Saturn was taken with a telescope that has an objective size of 280mm in diameter. Due to the increased light gathering capability, the telescope was able to use a higher magnification and maintain clarity and sharpness. The result is an image of the planet and rings with with superior quality. The planet and rings are clearly defined. Additionally, with the increased magnification you can easily see at least two separate and distinct bands of Saturns rings. Wow, what a difference !
Depending on optical quality and environmental observation conditions, you can expect to get anywhere from 40x to 50x of useful magnification per inch of aperture. In other words, a 4-inch scope tops out at 200x under ideal conditions, but a 6-inch scope can work well as high as 300x under ideal conditions.
But that's the maximum; most of the time, you'll find that the best views are actually had at the telescope's lower power magnification settings. If the advertising on the box hypes super-high power, then the manufacturer may be trying to fool you into thinking that the aperture size is not important.
The Large And Small Of It
So as it turns out the answer to the question, “Is bigger always better?”, the answer is "Yes". Having a larger aperture means that the telescope is able to gather more light from the viewed object and this results in more clarity when using higher magnification power settings.
Additional background and important information about the telescopes in general can be found in our Telescope 101 article. Or if you'd like to see review and recommendations, be sure to check out our Buyer's Guide.