A Barlow lens (invented by Peter Barlow in 1833) is a negative (concave) lens that inserts into the telescope ahead of the eyepiece and enlarges the image (Figure 6.7). In effect, the Barlow lens increases the focal length and f -ratio of the telescope. In photography, a Barlow lens is known as a teleconverter and increases the focal length of a telephoto lens.
The Barlow lens does not change the range of powers that work well with a telescope, but it makes higher powers easier to use. Consider for example an 8-inch (20-cm) f /10 telescope. The highest usable power for such a telescope is about 400x and requires a 5-mm eyepiece (Table 6.1, p. 81). With a 2x Barlow, you can get 400x with a 10-mm eyepiece, which is much more likely to provide comfortable eye relief.
The gain in magnification depends on the distance from the Barlow lens to the eyepiece. Most 2x Barlows become 3x when inserted ahead of the diagonal instead of after it.
Single-element Barlow lenses are sometimes provided with small refractors, but good achromatic Barlow lenses have two or more elements. I use a Celestron Ultima 2x Barlow, which is a three-element apochromatic design. Tele Vue makes a four-element 5x Barlow that preserves parfocality with many Tele Vue eyepieces - unlike most Barlows, it does not require you to refocus after inserting it.
Higher-quality Barlow lenses are more critical for telescopes that have low f -ratios, since (just like eyepieces) they face a greater challenge intercepting a wider cone of light. The eyepiece after the Barlow, however, has a much easier
job. Inexpensive eyepieces that do not work well in an f /4 telescope may perform quite well when the telescope is converted to f /8 by a 2x Barlow - but the Barlow itself must be of top quality.
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