Various designs of telescope have four essential factors

Aperture, focal length, focal ratio, and magnification

Telescopes use refractor, reflector, compound and catadioptric

There are various designs of telescopes, each with unique features and uses, such as the refractor, reflector, compound, catadioptric, Cassegrain, Dobsonian, and astrograph, among others.

To evaluate the performance of any telescope, regardless of its type, examine four essential factors: the size of the Aperture, the focal length, the focal ratio, and the maximum useful magnification.

Aperture

The aperture size is the simplest aspect to understand as it refers to the diameter of the Main  lens or mirror of a telescope. It is typically measured in millimeters and can also be expressed in inches.

This measurement indicates the telescope’s ability to gather light, or in other words, the number of photons it can capture.

A larger aperture provides brighter and more detailed views, enhancing contrast with more details.

This is why aperture is frequently regarded as the most critical characteristic of a telescope since it allows for the collection of more light. With an increased aperture, fainter celestial objects become visible.

The amount of light a telescope can collect is directly related to the size of its aperture.

The increase in light gathering is significant: a telescope with a 6-inch aperture gathers four times more light compared to a telescope with a 3-inch aperture, considering the difference in their surface areas.

Focal Length

Before you can clearly see what you’re looking at, the light that comes in through aperture needs to be brought together at one spot. This spot where the light meets is called the focal point.

The length the light travels from the aperture to the focal point is called the focal length. We measure it in millimeters.

The connection between an instrument’s aperture and its focal length has no  fixed relationship; it really depends on how the lenses and mirrors inside the tube are set up.

Focal length is important for two reasons: it mainly decides how much you can zoom in (we’ll explain this soon) and it helps you guess how wide of an area you can see.

Shorter focal lengths give you a bigger view, making them great for looking at large parts of the night sky and jumping from star to star. On the other hand, longer focal lengths give you a smaller view, which is good for looking at planets up close. They also let you use eyepieces that are more comfortable to look through, especially if you have to keep your eye a bit away from the lens, like when wearing glasses.

Focal Ratio

Our third key number is the focal ratio, or f/number, which tells us how the focal length compares to the  Aperture.

You can figure it out by dividing the focal length by the aperture size, and both numbers should be in millimeters.

If you have an instrument with a 130mm aperture a 900mm focal length, its focal ratio would be ‘f/6.92’.

The figure ‘f/6.92’ is arrived at by dividing the focal length (900mm) by the aperture size (130mm), which gives us approximately 6.92 when rounded.

Like focal length, the focal ratio tells you a lot about a telescope: bigger f/numbers mean you can zoom in more with the same eyepiece and see a smaller area, while smaller f/numbers do the opposite, allowing you to see a wider area with less zoom.

Also, an f/number can be called ‘fast’ or ‘slow’, and this shows how well a telescope works for taking pictures of stars and planets.

The words “fast” and “slow” come from the old times when people used to develop pictures using chemical film.

Telescopes with fast ratios (usually f/5 or lower) can take pictures faster than those with slow ratios (f/9 and higher). However, the downside is in how sharply they focus; telescopes with slow ratios are easier to get a clear focus with.

 Useful Magnification

Every telescope and the eyepiece you use with it have something called a focal length. The magnification, or how much bigger things look through the telescope, depends on comparing these two focal lengths.

To figure out the magnification, just divide the telescope’s focal length by the eyepiece’s focal length.

So, if your telescope has a focal length of 1,200mm and you use a 20mm eyepiece, you’ll get a magnification of 60 times.

If the eyepiece’s focal length is shorter, you’ll get a higher magnification with any telescope. The size of the telescope’s opening (aperture) doesn’t matter for this.

It’s good to know how much you’re zooming in because, unlike the size of the telescope’s opening, bigger zoom isn’t always better.

The maximum useful zoom is double the size of the telescope’s opening in millimeters; so for a 150mm opening, the max zoom is 300 times.

If you zoom in more than the useful limit, you’ll see your target closer, but the image will be blurry and darker.

(The author Girish Linganna is a Defence, Aerospace & Political Analyst based in Bengaluru. He is also Director of ADD Engineering Components, India, Pvt. Ltd, a subsidiary of ADD Engineering GmbH, Germany. You can reach out to him at: [email protected])