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Assignment 24-5


Do you need a telescope to enjoy the sky?

This is a very important question to answer. The truth is, you do not need a telescope to enjoy looking at the sky. You will never see anything as pretty through a telescope as you will looking at the sky with your naked eyes. Even using binoculars is more fun. That is because the more powerful the object you use, the smaller the field of view gets. With very powerful telescopes, you only get to look at a very small part of the sky.

I have decided I want to buy a telescope:

If you have set your mind on buying a telescope, let me give you some advice. First of all, before you even open the box to your new telescope, make sure you have already learned the constellations. Otherwise you will not be able to find anything in the sky. You will get frustrated and probably put your new telescope back in its box.


There are a few numbers you need to find on the telescope which will help you decide whether or not you want to purchase it. The first number is the Focal Length. That is the distance from the mirror, or objective lens, to the lens in your eyepiece.

If the telescope has a high focal length, it is going to be more powerful, but it will give you dimmer views of whatever you are looking at. If the telescope has a low focal length, your telescope will be less powerful, but have brighter views. I can’t tell you what a good focal length is because that depends on the size of the mirror, or object lens, and what you want to use the telescope for.

The next number, though, will help you determine whether or not the focal length is what you are looking for. This number is called the Focal Ratio. A high focal ratio means your focal length is also high. It will look like this on the telescope: F/6, or F/4, or F/(any number). F/8 is high, or what we call slow. If you are only after magnification power, this is the best telescope for you. F/4 is low, or what we call fast. If you want bright images such as for taking pictures, then you want a fast focal ratio.

The last number you need to be aware of, and by far the most important, is the diameter of the mirror, or objective lens. The bigger the diameter the more light your telescope will be able to gather. This will allow you to see faint objects because the larger mirror will gather more light.

A four inch mirror, which is what many people start out with, is a good beginner size, mainly because it is the cheapest. With a four inch mirror you will be able to see many neat objects, including the rings of Saturn and the clouds of Jupiter. My telescope has a ten inch mirror.


Notice that I did not include power as a number you need to watch for when buying a telescope. That is because power is not important. The truth is, any telescope can have any power.

When you buy your telescope it will probably include two or three eyepieces. Each eyepiece will have a number written somewhere on it. This is the focal length of the eyepiece. To find the power that your telescope will have with an eyepiece, divide the focal length of the telescope by the focal length of your eyepiece.

If my telescope’s focal length is 910 mm and my eyepiece has a focal length of 25 mm, then the power of my telescope with that eyepiece is 910 divided by 25, which is 36. So the power of my telescope is 36, or in other words, objects will be magnified 36 times.

You can also buy a special lens called a Barlow lens. This lens will double or even triple the power of your telescope.

If my telescope has a focal length of 910 mm and the eyepiece has a focal length of 4 mm, then my telescope’s power will be 910 divided by 4. This gives a power of 227. Objects will be magnified 227 times. If I have a Barlow lens, I can increase that to 445 or even 682. However, if your mirror is not large enough, objects at that distance will be too dim for you to see them. All you will see is darkness.


Assignment # 5

Is the power of a telescope important? Why or why not?

If my telescope has a focal length of 1000 and my eyepiece has a focal length of 10, how many times will my telescope magnify objects? What if I use a 2x Barlow lens which doubles the power? What if I use a 3x Barlow lens which triples the power of an eyepiece?


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The power of a telescope can be increased simply by changing the eyepiece.

100 times, 200 times, 300 times.