Simply, it's a concave (negative) lens placed between a telescope objective and eyepiece to increase the magnification, usually by two or three times. The negative lens reduces the angle of convergence of the light cone, effectively making it appear to the eyepiece that the primary has a longer focal length. It was invented in the early1800s by English physicist Peter Barlow (1776–1872).
Why do you need a Barlow lens:
Adding a Barlow lens and a focal reducer to your equipment box will effectively triple the number of eyepieces at your disposal. These two accessories are remarkably simple and fairly low cost. They have the effect of also changing the character of your telescope. A focal reducer effectively shortens the focal ratio of the telescope creating a lower magnification and wide field of view. The Barlow lens does exactly the opposite. It will double or even triple the magnification provided by a typical eyepiece.
The Barlow lens utilizes a single element concave lens that has the effect of doubling the magnification of the eyepiece–telescope combination.Adding one Barlow effectively doubles the number of eyepieces at your disposal. Barlows are usually inexpensive to add to a telescope collection making it a great addition for any scope. They are however a bit on the long side and that means there will be a dramatic change in focus from where the focuser of the telescope was originally set. Barlows also are available in a triple magnification design. Some manufacturers also offer a short Barlow lens that will not affect focus position as severely as more traditional designs.
Another variant on the Barlow theme is the zoom lens that introduces a movable negative element into the light path. By rotating a knob, the magnification of the telescope can be varied over a wide range allowing the user to work at different magnifications without changing eyepieces. Once you start handling expensive oculars with numb hands in freezing cold, you will quickly come to appreciate the value of a zoom eyepiece. In nearly all cases the Barlow lens or zoom lens slips into the drawtube of your focuser or visual back.The focal reducer is a positive lens that does the opposite of the typical Barlow.Instead of doubling the magnification, the focal reducer shortens the focal length of the telescope and reduces the magnification and creates a wider field of view. In addition to the wide field of view, low f-ratios mean brighter photography and shorter exposure times, both issues of great importance to astrophotographers. The telecompressor offered by Celestron reduces the focal ratio of my Schmidt-Cassegrain from f/10 to f/6.3. That means that my 26-mm Plossl's magnification is reduced from 77× to 48× while the field of view increases from less than three-quarters of a degree to better than 1.2 degrees. The telecompressor has disadvantages as well. Its large lens size makes it unsuitable for a draw tube insertion. Mine attaches directly to the Celestron's rear cell, then the visual back screws onto the back of the telecompressor.
The telecompressor also requires a lot of back focus. Some observers may run out of focus travel before the telecompressor–eyepiece stack can come to a sharp focus. Observers who wish to turn their telescopes into real wide field instruments may opt for ultra-low f-ratio reducers such as the model designed by Optec. This reducer takes an f/10 telescope down to f/3.3. That would, using the eyepiece discussed previously with my Celestron,reduce magnification from 77× to 26× and take that three-quarter degree field of view up to over 2.1 degrees. The Optec reducer does introduce severe field curvature and Optec does not advise using their product for visual observing. It is strictly a photographic accessory.Users of Barlows and telecompressor also must remember one other thing.Anytime you introduce a piece of glass into the light path, not all the light that enters that lens will come out the other side. Introducing any additional lens into the light path will reduce the brightness of the image at final focus. Given a choice of using a 20-mm ocular or a 40-mm with a Barlow to produce 100×, I would prefer to use the 20mm. Telecompressor users should also be aware that if they are doing astrophotography or CCD imaging that using a shorter f-ratio also means shortening the range of critical focus, the precise range in which the focuser must be placed to produce maximum image sharpness on film or the CCD chip. This range, measured in microns, can be cut in half by a telecompressor. These two accessories can add great flexibility to your equipment box but just as you would not use a screwdriver on a nail, make sure you use the right tool for the right job. A Barlow costs around about $40. Telecompressors are more expensive, usually around $140.
A focal reducer is a group of lenses with a positive power, which decreases the primary focal length of the telescope. As in the case of a Barlow lens, its reduction factor depends on its focal length and its distance from the focal plane. When this distance increases, the reduction is more pronounced.Because of aberration correction problems, it is advised to use a focal reducer at a reduction factor very close to its nominal factor. The drawing above shows that a focal reducer moves the focal plane closer to the telescope. On some instruments, the focusing range may not be sufficient to reach the point with a focal reducer. Furthermore, specially on SCTs, vignetting problems are usually more critical with a reducer than without.
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