A Barlow lens is an intermediate optical device that goes between the eyepiece and the objective lens, or primary mirror. It is named after Peter Barlow, so you will often see it capitalized.
The photo shows a typical Barlow lens. The eyepiece is slipped into the top, and the Barlow lens is slipped into the focuser, or diagonal, where the eyepiece would normally go.
How does a Barlow Lens Work?
To increase the magnification that any given eyepiece offers, we place a Barlow lens in the optical path. In this way, a single Barlow lens can increase the effective number of magnification choices you have in your eyepiece set. It does so economically and can simplify your eyepiece needs.
While we often associate the Barlow Lens with the eyepiece, it is really more correctly associated with the optical tube. A Barlow lens is a divergent lens, which means that it moves the focal point out, effectively giving the telescope a longer focal length. So, if your telescope has a 400 mm focal length, a 2X Barlow inserted between the eyepiece and the objective lens or primary mirror will effectively make it behave like it would in a telescope with an 800 mm focal length. In this way, it gives each eyepiece two magnifications, one with and one without the Barlow.
In practice, we more often talk about Barlow lenses in the context of the eyepiece, as it seems more convenient to think of them that way. You will read that a 2X Barlow will make a 10 mm eyepiece deliver the magnification of a 5 mm eyepiece. The net effect is the same, so it really doesn’t matter, unless you are really interested in how the barlow lens works.
For visual use, Barlow lenses from 1.5X to 3X are common. Generally, Barlow lenses of greater than 3X are considered applicable to astrophotography, though there is nothing to prevent you from using one with your eyepiece.
Barlow lenses come in diverse design configurations. In a conventional model, one inserts the eyepiece into the Barlow lens, and then places the entire assembly into the diagonal, or focuser. However, many Barlow designs enable the lens housing assembly to be screwed directly onto the eyepiece, often termed ‘Barlow elements’. You can either attach the eyepiece to the Barlow unit or directly affix the lens housing to the eyepiece or to the bottom of your camera/adapter. This often results in varied magnification levels: the full assembly might provide 2x magnification, whereas the element alone may yield 1.5x. This is caused by the fact that when the lens is closer to the focal plane, the rays cannot diverge as much, leading to a weaker amplification effect. Telecentric Barlows and “focal extenders” do not exhibit this feature, offering a fixed amplification no matter where they are placed in relation to the eyepiece or camera.
However, there are also Barlow lenses that are screwed onto the eyepiece. These are sometimes referred to as Barlow elements. An example is shown in the photo. The top shows an eyepiece and the Barlow element separately. The bottom shows the same eyepiece with the Barlow element screwed on like a filter.
Commonly, Barlows come labeled with magnification indicators like 2x. If an eyepiece yields a 100x magnification, coupling it with a 2x Barlow would enhance it to approximately 200x.
The lens composition of Barlow lenses can be varied, ranging from a single lens element to designs that incorporate up to four or five lens elements. Higher-end models integrate exotic ED glass types to reduce chromatic aberration and other optical defects.
Similar to eyepieces, the lens elements of Barlow lenses can have edge blackening and various coating grades, with “fully multi-coated” ranking as the top tier. These coatings and blackening techniques are engineered to reduce light loss and internal reflections.
How to Use a Barlow Lens
Begin by inserting the Barlow lens into the focuser or the diagonal of your telescope. For those new to the world of telescopes, the focuser is the mechanism that adjusts the telescope’s focus, and the diagonal is a secondary mirror or prism that deflects the light path by 90 degrees, commonly found in refractors and Cassegrain telescopes. Once the Barlow is securely in place, introduce the eyepiece into the Barlow lens. This eyepiece is what you will look through.
For users of refractors, Schmidt-Cassegrains, and Maksutov-Cassegrain telescopes that traditionally incorporate a diagonal in the optical path, there exists an alternative setup to use a barlow lens. Here, the Barlow lens is inserted directly into the focuser, or visual back. Subsequently, the diagonal can be placed within the Barlow lens. As always, the eyepiece is then placed on the diagonal, allowing you to observe as usual. This configuration may alter the magnification factor depicted on the Barlow lens, however, and may not be able to reach focus at all, depending on how much additional travel your focuser can provide.
One reason to use a Barlow lens is to take advantage of the longer eye relief typically found in longer focal length, lower power eyepieces. Eye relief defines the distance you have to place your eyeball from the top lens in order to see the full field of view.
Short eye relief can make eyepieces uncomfortable to use. In many eyepiece designs, as the focal length of the eyepiece gets shorter, the eye relief gets smaller. And people who wear eyeglasses while observing may not be able to get their eyes close enough to the lens to see the image fully. So longer eye relief can be helpful to them.
To illustrate, a 30 mm Plossl eyepiece might have 22 mm of eye relief. Anything less than about 18 mm may be no good for glasses wearers. Anything less than 10 mm may be uncomfortable for some people, even if they don’t wear glasses.
If you wanted a 10 mm Plossl to give you the magnification you wanted, you would have to deal with an eyepiece with about 7 mm of eye relief. That would be no good for the eyeglass wearer, and others would have their eyes almost on the lens.
But place that 30 mm Plossl in a 3X Barlow, and you would have the equivalent magnification of a 10 mm Plossl but would retain the 22 mm of eye relief of the 30 mm eyepiece.
Where can a Barlow lens be helpful?
- When you don’t have an eyepiece at the magnification you want to use.
- When you want to retain the long eye relief of a low-power eyepiece at high power.
- When you want to stretch your eyepiece budget, effectively doubling your eyepiece set.
I usually recommend that new people on a tight budget get the best Barlow they can get their hands on to provide maximum flexibility at the lowest cost. You can always add more eyepieces later, but if you have a Barlow, each of those eyepieces will provide two magnifications.