Glossary
Barlow lens
An accessory placed before an eyepiece that multiplies magnification, commonly by two or three times, without buying more eyepieces.
A Barlow lens is a negative lens element that sits in the focuser tube between the telescope and the eyepiece, increasing the effective focal length of the telescope before the light reaches the eyepiece. The most common versions double or triple magnification, so a single eyepiece paired with a 2x Barlow effectively behaves like two eyepieces, which is a budget-friendly way to expand a small eyepiece collection.
Because a Barlow works on the telescope's light path rather than the eyepiece itself, it multiplies whatever magnification that eyepiece already gives, and its effect is consistent across every eyepiece a person owns. Good quality is worth prioritizing here, since a poor Barlow can introduce noticeable softness or color fringing that undoes the sharpness of an otherwise good eyepiece.
A frequent mistake is using a Barlow to chase magnification numbers well beyond what the aperture and the night's seeing can support, which just enlarges a blurry image rather than revealing more detail. Barlows are most useful on nights with steady seeing and on telescopes whose native eyepiece selection stops short of the magnification needed for small, bright targets like planets or tight double stars.