A Barlow is an auxiliary lens system in a tube that mounts in front of the focal point of a telescope, between the telescope and eyepiece. It increases the power of any eyepiece used with the telescope by increasing the telescope's effective focal length. (Since magnification equals telescope focal length divided by eyepiece focal length, a longer telescope focal length means higher power with any given eyepiece.) A Barlow effectively doubles the number of eyepieces you own - at less cost than buying that many new eyepieces individually.
While a Barlow increases eyepiece power, it does not change the longer eye relief typical of lower power eyepieces (in fact, actually increasing it somewhat in some cases). This allows eyeglass wearers to see a full field of view at high power without having to remove their glasses to get close to the eyepiece, as they would have to with a shorter focal length eyepiece alone.
Barlows have an undeservedly poor reputation among those who start astronomy with inexpensive toy store telescopes that use toy plastic one-lens Barlows. A good Barlow is not a toy, and does not deserve a toy's reputation. It is a precision multi-lens all-glass optical system, just as an eyepiece is, and can actually improve the performance of many eyepieces. While most Barlows are achromatic doublet (two-lens) systems, some are triplet (three-lens) apochromatic systems or even four-lens systems consisting of two complementary doublets.
By increasing the effective focal length of a fast to medium focal ratio telescope, the diverging lens optics of a Barlow give inexpensive or moderately priced eyepieces a slower-converging and easier-to-handle light cone to deal with. This results in lower astigmatism and better color correction at the edge of the eyepiece field.
A Barlow can similarly improve the edge sharpness of good quality wide angle eyepieces used with fast focal ratio telescopes. It should be considered in lieu of higher power eyepieces alone for star cluster and planetary nebula viewing with such a scope.
Barlows that are designed for long focal ratio telescopes may vignette (cut off) the edge of the image when used with low power wide angle eyepieces on a fast focal ratio scope.
An eyepiece and Barlow combination will have a slightly (2%-4%) dimmer image and slightly lower contrast than the short eye relief/short focal length eyepiece alone that is needed to get the same power as the eyepiece/Barlow combination.
The lower-priced Barlows we carry may introduce minor astigmatism and spurious color at the edges of the field with fast focal ratio scopes. However, since a Barlow is normally used to look at objects at high power in the center of the field (where the image is unaffected), this is a scarcely insurmountable drawback. Many observers, particularly eyeglass wearers, find that the potential of long eye relief at high powers with a Barlow, and its ability to multiply their eyepiece collection at modest cost, more than outweigh this minor shortcoming.
Be certain when you buy a Barlow that you do not duplicate eyepiece focal lengths you already have - or plan to buy. If you have a 26mm eyepiece now, a two power (2x) Barlow would effectively give you a new 13mm eyepiece, so it wouldn't make sense to buy a 12mm or 13mm eyepiece later, since you'd merely be duplicating magnifications. A 15mm to 17mm or 8mm to 10mm eyepiece would make more sense, as any of them would give you unduplicated powers when combined with the 2x Barlow and 26mm eyepiece.
Magnifications specified for Barlows only hold true if the eyepiece is mounted directly in the Barlow and the Barlow is used with a reflector, with a refractor used without a star diagonal, or mounted in the star diagonal of a catadioptric or refractor scope. Generally, if the Barlow is installed between the visual back (or focuser drawtube) and the star diagonal of a refractor or catadioptric (in other words in front of the diagonal), the projection distance from the Barlow's lens to the eyepiece will be increased and the resulting magnification can be as much as 50% higher than that specified on the Barlow. However, some Barlows are inconvenient to use in a star diagonal because of their length, or are physically incompatible, and are so noted in the individual Barlow descriptions.
Barlows are optimized for one specific lens to focal point spacing. Using them at other spacings, such as in front of a diagonal as mentioned above, can reduce the performance slightly at these other spacings. Variable Barlows, that change magnifications by varying the lens/focal point spacing, tend to be optical underperformers for this reason. Also, variable Barlows require a layer of lubrication between their lens cell and the tube it rides in to allow free movement of the cell. At the least, this can introduce internal reflections that may degrade the image, and the possible creep of lubrication onto the optics due to summer heat at the worst. It is for these reasons that we don't carry variable power Barlows.
The Barlow lens was invented in 1834 by English physicist and mathematician Peter Barlow (1776-1862).