If you’re somebody who’s just gotten into the hobby of stargazing, then you’re probably struggling to grasp all of the jargon associated with telescopes. In this article, we explore one of them and describe what does focal mean.
Some terminology can be incredibly difficult to wrap your head around. This seems especially so when you’re trying to read a technical manual.
Many people don’t actually realize how difficult it can be to get to grips with using a telescope. However, the good news is that once you’ve figured out all of the different names and terms, you’ll be on your way to success.
One of the words that we hear most often within the context of telescopes is ‘focal’. It’s important to learn what this word means in order to understand how to operate your telescope properly. Understanding the term will help you get the most out of your stargazing experience.
Hopefully, you have discovered this article before you have purchased a telescope. This is because a telescope’s focal length is an extremely important telescope characteristic.
In order to learn more about telescope focal length, keep reading, as we take a closer look.
What Does The Word Focal Mean In Telescopes?
So, why not just jump straight into the titular question and investigate what focal means in relation to telescopes? Chances are that you’ve already searched this question before and were met with either very little or very confusing answers.
Operating telescopes is a technical business. Because of this, some articles explaining the matter can be incredibly complicated.
The word ‘focal’, in relation to telescopes can be easily explained. Focal (or more correctly focal length) refers to the distance between the primary element of your telescope and the focal point. By element, we mean either the objective lens in a refractor or the primary mirror in a reflector telescope. The focal point is the point where all the light from the primary element is brought together in a point.
The diagram on the right illustrates focal length of a refractor telescope. The diagram is a simplification at the focal point will not be at the position shown but rather at a point closer to the position you would look into the telescope.
Why Is This Important?
Now that we have a definition of focal length, we can move on to our next question. Why is focal length so important in telescopes?
The answer is incredibly simple. You must pay attention to the focal length of your telescope. The focal length will determine the magnifying power of the telescope.
The focal length is the item that you need to consider in order to determine the level of magnification that results. If you want to see something as closely as possible, you’ll want to make sure that it’s properly magnified.
However, there is a sweet spot. If you magnify your telescope too heavily, it can cause the image to become incredibly blurry and dim.
If you don’t magnify it enough, the image will be incredibly clear, but you won’t be able to see details. You then need to adjust the focal length in order to make sure it matches the object you are viewing.
Now that we’ve taken a closer look at why focal length is so important when you’re using your telescope, we can discuss the best focal length you can purchase.
If you’re a beginner who’s just getting started with the hobby, we’d recommend choosing one with a focal length that’s easier to adjust. You’ll want to look out for ones that have a focal length of at least 600mm.
If you go for anything that’s lower than 600mm your viewing experience will be limited. This is because you will have limited ability to magnify your targets. You can purchase telescopes with a shorter focal length of less than 600mm. However, these are typically designed for children who are just getting started with the hobby.
The focal length you choose for your telescope should also be based on what you want from the experience. For example, if you want to use it mainly to observe the moon and planets, we’d recommend that you choose a longer length.
If, however, you want to observe deep sky objects, we’d recommend choosing a shorter focal length telescope. By deep sky objects we mean objects such as galaxies, nebulae and stars.
Is Focal Length Or Aperture More Important?
If you’ve been reading, you will probably also have encountered a term called aperture. Aperture is an incredibly important aspect of using your telescope.
The aperture determines how much light will come through your telescope when you’re viewing the image.
This leads us to the question: what should you look out for the most when choosing your perfect telescope, aperture or focal length?
We’d recommend always considering aperture and focal length in combination. It is these two parameters that determines the performance of a telescope.
After all, there’s no point in being able to increase or decrease the magnification if the image isn’t even bright enough to see. They’re both very important when choosing a good microscope.
The focal ratio of a telescope is its focal length divided by its aperture. For example, if a telescope has a focal length of 1000mm and an aperture of 200mm, its focal ratio is 5. It is usually represented as f/5.
The focal ratio indicates how bright objects will appear in a telescope. When the ratio is larger (eg f/10) the image will appear dimmer than that of a telescope with a smaller number. When the number is large the telescope is said to be slow and when small it is said to be fast.
A telescope with a high focal ratio will have a narrow field of view. A fast telescope will have a wider field of view.
Which is best for you will depend on what you plan to use it for. If you are interested in viewing the Moon and planets, then a slow telescope is recommended. As they are bright a slower telescope is of little consequence. Look for telescopes for f/10 to f/15.
A telescope with a focal ratio of about f/5 is a good choice for observing dim deep-sky objects and wide-field objects.
Telescopes with focal ratios between the two will be sufficient for both purposes.
Final Thoughts
To sum up, focal length essentially means the distance between the mirror or lens of your telescope and where the light is focused. The reason why it’s so important to get your focal length right, is because it essentially determines the magnification of your microscope (Check out How To Find Total Magnification).
If you’re using your telescope for gazing at galaxies or stars, you will need to choose one with a shorter focal length. If you’re using it to gaze at brighter objects like the moon, you should choose a telescope with a longer focal length.
Hi, my name is Jason Anderson, and I am a Physics Professor. Ever since I was a kid, I’ve been fascinated with space, the universe, the moon, you name it. I spent hours and hours at the planetarium close to my hometown, wondering what else could be out in the universe.
Since then, I’ve been an avid stargazer and astronomer, and love nothing more than spending my time charting stars, observing planets, and finding constellations.
This is why I decided to start Telescope Guru. I only wish to share this fun pastime with the world. With this site, I hope to answer all of your questions relating to astronomy, telescopes, and stargazing.
