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Monday, 3 January 2011

What are advantages and disadvantages of using a Barlow lens?

Some major advantages:

  • Achieve higher magnifications.

  • Provide flexibility of magnification levels with your existing eyepieces. For example, if you have 2 Plössl eyepieces with 16mm and 26mm focal length and use them on a telescope with 800mm focal length, you have a 800/16=50x and 800/26=31x magnification levels respectively. With a 2x Barlow lens, you will get 100x and 62x magnifications as well.

  • Increase eye relief (distance of exit pupil from eye lens). Many eyepieces have an eye relief which is directly related to its focal length. For example, the eye relief of a Plössl is 0.73 x its focal length. Therefore, for these eyepieces, there will be a greater eye relief with a Barlow than without one.

  • Disadvantage: a major disadvantage of adding a Barlow is a slightly decreased brightness in the produced image.

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