A Barlow lens lets you increase the focal length of your telescope and hence the magnification. The Barlow lens is placed between the telescope and the eyepiece.
The high-quality 2.25x Barlow lens made of special glass has the rare extension factor of 2.25. This means that with the eyepieces of the Q-Turret Set, no focal lengths are duplicated, and if you plan to take pictures of planets with a web or video camera later on, you will achieve a higher magnification than with normal 2x Barlow lenses and can make better use of the camera's resolution.
The Barlow can either be screwed into the Q-Turret eyepiece revolver or combined with all 1.25" eyepieces using the special adaptors supplied. It then provides a magnification factor of 2.25x. This results in the following focal lengths on the Q-Turret eyepiece turret: 2.7 mm, 4.5 mm, 8 mm and 14 mm.
Alternatively, it can also be screwed directly into an eyepiece, increasing the telescope focal length by a factor of 1.3. The Barlow element has an M28.5 external thread (corresponding to the filter thread of 1.25" eyepieces), and the housing has an M28.5 internal thread. The factor of 1.3x is achieved when the Q-Barlow is screwed completely into the 24 mm long barrel of an eyepiece whose field stop is 2 mm inside the barrel. This corresponds to the design of the Classic Plössl/Ortho eyepieces; with other eyepiece designs, the magnification factor may vary slightly.
Please note: When fully screwed in, the Barlow element protrudes 13 mm into the 1.25" barrel. On eyepieces where lens elements in the 1.25" barrel prevent the Q-Barlow from being screwed in, it can only be used in the 1.25" adaptor with a factor of 2.25x.
Use with a planetary camera: The 2.25x Q Barlow lens (only the lens mount) can be screwed into any 1.25" filter thread. To do this, remove the lens part with the screw mount from the rest of the housing. Without the entire eyepiece mount, this Barlow (due to the reduction in path length between the lenses and the image plane) no longer has a factor of 2.25x but (when fully screwed in) "only" 1.3x.
However, it may not be possible to screw this lens mount in all the way – in which case the extension factor is approx. 1.5x.
However, if the 1.25" plug-in sleeve of the camera has an M28.5 filter thread cut into it, then you can usually screw the Barlow in completely and the factor is then approx. 1.3x due to the short image distance. This capability is not accidental, but was the basic idea behind the design. At 2.25x, an image circle of 25 mm is vignette-free, and at a factor of 1.3x, this fully illuminated image circle is 13 mm.
