Calculate the magnification with a Barlow lens.
You can change the power of your telescope by changing the eyepiece
Two important values are:
1. The focal length of your telescope in mm.
2. The length of your eyepiece in mm.
3. Barlow lens
Just enter the values below, click "Calculate the Magnification" and the calculator gives you your answer.
Reference:

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Focal Length:
Every telescope has a stated focal length, which is effectively the distance from the primary lens or mirror to the point at which it forms an image of a very distant object. 
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Length of the your Eyepiece:
Eyepieces have focal lengths, too — 25 or 10mm, for example. 
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Barlow lense:
A Barlow lens is a concave lens that when placed between a telescopes objective lens or mirror and the eyepiece, will increase the magnification of the telescope. A Barlow lens will connect directly to your eyepiece. The most common Barlow is the 2x Barlow.