Barlow lenses are accessories used to increase the effective focal length of an optical system. Inserting a 2x Barlow results to doubling of the telescope’s focal length. For my telescope which has a focal length of 900 mm, inserting a 2x Barlow in series results to an effective focal length of 1800 mm. Inserting yet another 2x Barlow, results to an effective focal length of about 3600 mm (increasing the separation between the two Barlow lenses by not fully inserting the second Barlow yields a slight increase in the magnification of the image).
The Barlow lenses shown here are the Celestron Omni 2x Barlow lenses which I use extensively in imaging planets. These Barlows feature dual-element multi-coated lenses which produce acceptable results, even when stacked. Note that stacking Barlows is a useful workaround if you already have the Barlows and need more magnification. A better alternative would be to use a single but more poweful 5x Barlow, rather than stacking less powerful 2x or 3x Barlows.
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Image of Jupiter using Stacked Barlow Lenses
Image of Saturn using Stacked Barlow Lenses
Night Sky in Focus | Astronomy and Amateur Radio © Anthony Urbano | Manila, Philippines