How to Increase the Magnification of a Telescope

Updated on February 19, 2018
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Unvrso is a lover of nature and technology. He has been writing for HubPages since 2009 on various topics.

Telescope Range

Telescope Range
Telescope Range | Source

How to Calculate a Telescope's Power

The magnification of a telescope is measured by dividing the diameter of the objective lens over the focal distance of the telescope. In this article, I’m going to show you how to increase the magnification of a telescope, specifically the one in the picture above. The telescope has a focal distance of 70 mm and an objective lens of 60 mm, although the optical devices that I'm going to use here can be adapted to other types of telescopes to increase their magnification.

To increase the magnification on a telescope, astronomers use eyepieces, which are usually sold along with a telescope. The eyepiece is the lens that is placed at the end of a telescope's tube, which is where the entering light converges, also called the focal plane of the telescope. Although the eyepiece is the main amplification device in a telescope, there are other devices, which when placed in combination with the eyepiece, can increase the amplification of a telescope even further.

How to Measure the Magnification in Telescope?

The magnification in a telescope is measured by dividing the main objective lens' focal length by the eyepiece's focal length. In this case, I'm trying to show how to magnify the focal length of a telescope with a focal length of 700mm using eyepieces of 9mm and 25mm.

When using the 25mm eyepiece on a telescope, all you have to do is divide the telescope's focal length. In this case, 700mm over the 25mm eyepiece, thus, you obtain 28x, which is the magnification of this telescope using a 25mm eyepiece. This means you'll be able to see objects 28 times their actual size.

In the other instance, if you use the 9mm eyepiece on the 700mm telescope you obtain a magnification of 77x, which is the magnification you get with the 9mm eyepiece. The shorter eyepiece will give a greater magnification than the longer eyepiece. That is so because the shorter eyepiece has a shorter focal length, thus, giving a higher magnification.

Eye Piece, Barlow Lens

Optical Devices
Optical Devices | Source

How to Increase the Magnification of a Telescope Using a Barlow Lens?

There is another technique with which you can increase the magnification of your telescope even further. You can do that by inserting a Barlow lens in front of the eyepiece. A Barlow lens is a diverging lens, that is to say, this type of lens causes light rays to spread out. When used in a telescope, a Barlow lens increases the telescope's focal length, thus, magnifying the image.

Barlow lenses are usually placed before the eyepiece, and they may double or triple the magnification of a telescope. Barlow lenses are usually classified as 2x and 3x, meaning they increase the original magnification of a telescope by two or three times, although, other magnifications exist.

By placing a Barlow lens before the 25mm eyepiece of the 70mm telescope, will double the magnification of the eyepiece. If the magnification without the Barlow lens was 28x, using the Barlow will be 56x. Whereas when placing the Barlow lens in front of the 9mm eyepiece, the magnification will increase to 155x. Thus in these two examples. The magnification is doubled. There other Barlow lenses with higher magnifications.

The same is true for a Barlow lens grated 3x. In this case, the magnification of a telescope will be tripled, depending on the eyepiece used. With the 25mm eyepiece, the magnification will triple to 84x, whereas with the 9mm eyepiece the magnification will triple to 233x.

How to Increase the Magnification of a Telescope with an Extension Tube?

The magnification of a telescope may be increased even further by adding an extension tube between the Barlow lens and the eyepiece. You can buy a 1 inch PVC tube and sand it a little on one end. It will fit perfectly on the Barlow barrel. On the other end, you can cut and attach a piece of the same tube to serve as a holder for the eyepiece. This is what I did with the extension tube on the picture.

By placing an extension tube between the Barlow lens and the eyepiece, you will increase the magnification of a telescope by two three or more times, depending on the size of the extension tube. The idea is that as you increase the distance between the Barlow lens and the eyepiece, you reduce the eyepiece's focal length, thus increasing the magnification of the telescope.

Image Amplified using the Barlow Lens Plus an Extension Tube

Optical Image
Optical Image | Source

The image seen through the telescope on the left with the Barlow lens and the 25mm eyepiece. On the right, the same image with the same optical devices mentioned before and an extension tube which quadruples the distance given by the Barlow lens.

The image is that of a water tank on top a house about two miles away. It would have been almost imposible to have distinguished the letters on the water reservoir without the aid of a telescope.

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Summary

As you can see, the three optical devices explained in this hub will help you increase the magnification of your telescope. The magnifications will let you see features of the moon, although, with less brightness. This is because when you increase the magnification, you also decrease the sharpness of the object.

With the 700mm telescope, I'm able to watch the planets only when using the 25mm eyepiece without the Barlow lens. If I use the Barlow lens, I will still be able to see the planets, although, with a reduced field of view.

You may locate an object in the sky using a low power eyepiece, such as the 25mm eyepiece, and then change it to a higher power eyepiece, such as the 9mm eyepiece or the Barlow lens. You may even insert an extension tube, but the quality of the image will decrease.

The magnifications explained here may allow you to observe birds or other distant objects during a trip to the countryside or a ship from the seashore. The higher magnifications will not increase the quality of the image being viewed.

Questions & Answers

    © 2012 Jose Juan Gutierrez

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