365Astronomy 1.25" 3x Achromatic Barlow
This is a short, 2-element, 3x achromatic Barlow lens from 365Astronomy
Full metal construction.
The aim of Barlow lenses is to prolong the telescope's focal length, so e.g. with a 3x Barlow, when we divide the tripled focal length of a telescope with the focal length of the eyepiece we got three times the magnification obtained. This is a popular amateur complementary tool, because with the help of a Barlow lens we can achieve more diverse magnification range with the same telescope without having to obtain additional eyepieces. On the other hand, since the aperture is usually smaller and the eye relief is shorter for eyepieces with shorter focal length (higher magnification eyepieces), thus these are less comfortable, therefore with a Barlow we can achieve the same magnification but by using a more comfortable (with a longer focal length) eyepiece.
Barlows are so good that they are being used even in more expensive wide-angle eyepieces where the Barlow is simply integrated into the construction of the eyepiece.
The Barlow lens is always being used between the telescope lens and the eyepiece.
However, there is a trick will teach you now. If you use a refractor telescope, Maksutov or SC with a star diagonal, you would usually us the Barlow between the star diagonal (diagonal mirror) and eyepiece, thus achieving its nominal magnification, e.g. 2x.
Now, if you use the Barlow between the telescope and the diagonal, then you'll gain a different from nominal magnification, i.e. 2.75x for a 2x Barlow, thus further increasing the number of achievable combination of magnifications with just one Barlow and a set of eyepieces...
Barlow lenses will fit a 31.7 mm or 50.8 mm eyepiece holder barrel.
This particular one will fit a 1.25" holder barrel.
Cemented Doublet lens design
Size: 31.7mm, 1.25"
Optical length: 49mm