For Barlow Lens we have a term and definition in Telescope.

Barlow Lens (Telescope)

A negative (diverging) lens that fits into your telescope's focuser, a little way inside the focal plane. It acts as a focal-length multiplier for the telescope. Thus, suppose you have a telescope with a focal length of 1000 mm. If you use a 10 mm focal-length eyepiece, you will obtain a magnification of 1000/10 = 100x. With a 2x Barlow lens used in front of the eyepiece, the telescope's focal length will be increased to 2000 mm, and the same eyepiece will give a magnification of 2000/10 = 200x.
The multiplication ratios of commercially available Barlow lenses vary from 1.75 to 5. Barlow lenses are sometimes also called telextenders.
Not all Barlow lenses work with all eyepieces and all telescopes. Try before you buy.

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