A Barlow lens is an optional accessory which slots in between the focuser (or star-diagonal) and the eyepiece. That is, you put the eyepiece in the barlow, then the barlow in the telescope as if it were an eyepiece. Due to it being a negative lens, it will act to increase the focal length of the telescope, thus increasing magnification.
The most common power for a Barlow is 2x, but they can also be found in 2.5x, 3x, and even 5x varieties, and rarer, more esoteric magnifications can be found as well for certain specialty applications. (Binoviewers, for example).
Barlows are primarily used for two reasons:
- To double the number of magnifications you can use
- To reach higher magnifications with cheaper, simpler eyepiece designs
Many complex eyepiece designs use built-in Barlow lenses as part of their design, but they can usually still be used with separate Barlows should you choose.
A Barlow will multiply the magnification of your telescope by whatever its magnification factor is. That is, a Dobsonian with a 25mm plossl normally provides 48x. Add a 2x Barlow, and that reaches 96x.
When choosing a barlow and eyepieces, it is important that you do not double up on magnifications. For example, a 20mm and 10mm eyepiece are often provided together with beginner telescopes. Getting a 2x Barlow will make your 20mm provide the same magnification as the 10mm eyepiece, so there’s no good reason to get a 2x Barlow instead of a 5mm eyepiece. A 3x Barlow meanwhile, will convert those same two eyepieces into effectively a 6.7mm and 3.3mm eyepieces. These are extremely short focal lengths, but they’re not redundant, so in total you’d have 20mm, 10mm, 6.7mm, and 3.3mm focal lengths to choose from.
It’s also important to remember that Barlows aren’t magic. You still have a maximum useful magnification defined by your aperture and the optical quality of the system. So don’t just get a 5x Barlow because it reaches the highest power.
As discussed in the eyepiece article:
- Magnification = (Telescope Focal Length * Barlow Magnification Factor) / Eyepiece Focal Length
- Maximum Useful Magnification = 2x per mm of aperture = 50x per inch of aperture
What Barlow to get will depend upon the focal ratio of your telescope.
- Focal Ratio = Focal Length in mm / Aperture in mm
Fast telescopes have a lower Focal Ratio, Slow telescopes have a higher Focal Ratio.
If your telescope is at f/10 or f/15, or slower, it may not make much sense to raise the focal ratio even more, so you’ll probably only need a 2x Barlow, if that. However, a telescope at f/5 or slower might actually be able to make use of a 5x Barlow in some rare circumstances. Don’t even consider a 5x Barlow unless your telescope has a fast (low) focal ratio. Generally, the higher the Barlow’s magnification, the less versatile it is, since it’ll only really work with long focal length eyepieces. Stick with magnifications of 2x to 3x.
Barlow Lens Specifications and Brands to Look For
If your telescope came with an included Barlow, odds are it’s very bad. Use it only to convince yourself of its uselessness. The most common type included with cheaper beginner telescopes is a single plastic lens which causes severe chromatic aberration and other distortions in the image.
When searching for a Barlow, you must get one with an achromatic or apochromatic lens(2 lens elements or 3 or more lens elements). It should also be fully multi-coated to prevent internal reflections and preserve high contrast. A Barlow is really not an element to cheap out on, since a bad Barlow can make the image worse. If it’s going to double your eyepiece collection anyway, it seems like you can afford to make sure you don’t end up with one that adds aberrations and false color fringing.
What Barlow to get will depend upon the focal ratio of your telescope. If your telescope is at f/10 or f/15, or slower, it may not make much sense to raise the focal ratio even more, so you’ll probably only need a 2x Barlow, if that. However, a telescope at f/5 or slower might actually be able to make use of a 5x Barlow in some rare circumstances. Don’t even consider a 5x Barlow unless your telescope has a fast focal ratio. Generally, the higher the Barlow’s magnification, the less versatile it is since it’ll only really work with long-focal-length eyepieces. Stick with magnifications of 2x to 3x.
The Barlow I primarily use is a GSO/Apertura 2.5x Apo Barlow. It has good optics yet remains quite affordable. Note that it will make a 25mm/10mm setup common in beginner telescopes redundant. Apertura/GSO also sells a 2x and 3x achromatic Barlow which is quite good.
Celestron, Orion, and others sell a 2x “Omni” or “Shorty” Barlow, which may have slightly more false color in fast telescopes, but is lighter and has a shorter profile than many 2x achromatic Barlows. Shorty Barlows extend the exit pupil out farther than longer Barlows of the same magnification, so they are more likely to mess with long-focus wide-field eyepiece designs. Celestron also carries 2x and 3x Barlows to match some of their other eyepiece lines such as the X-Cel and Luminos brands.
Premium brands like Explore Scientific and TeleVue have their own Barlows, but there’s not a whole lot of difference between a mid-range barlow and a premium barlow, and at that price point, you may want to consider a Focal Extender instead.
Using Barlows in Astrophotography
Barlow lenses are often thought of as halving the focal length of the eyepiece, and when coming up with a collection of eyepieces to use with a Barlow I’d encourage that kind of thinking. (I.e., a 6mm eyepiece with a 2x Barlow performs like a 3mm eyepiece on its own.) However, in truth, the magnification increase is coming from extending the focal length of the telescope as much as it is from shortening the focal length of eyepieces. This means you can, in fact, use a Barlow lens with cameras as well as eyepieces.
(To clarify—when using a DSLR or Astro-camera for astrophotography, the telescope itself is being used as the “lens” of the camera. When imaging with cameras with permanently attached lenses, such as a point-and-shoot camera or a smartphone, you’ll have to use eyepieces as you would when using a telescope visually.)
When doing lunar or planetary imaging with a DSLR, you’ll often want to use a Barlow, perhaps even a very strong one like a 3x or 5x, to reach an image scale where each pixel corresponds to the limit of the telescope’s resolution.
Often, Dobsonian reflectors intended for visual use are incapable of coming into focus with a DSLR, because the sensor is too far back in the camera body. Often, a Barlow lens can be added to move the light path outward enough, so most Dobsonian users must use a Barlow when doing astrophotography with their DSLR. This basically restricts them to the Moon at high power and the planets, since the focal ratio becomes too slow to image dim objects.
Barlows vs Zoom Eyepieces: Which to Get?
In the eyepieces article, we discussed Zoom Eyepieces, which, like the Barlow, is a scheme to use a single accessory to vastly increase the number of available magnifications. I’ll spare a brief thought for them here, too. Zooms use a complicated optical path and mechanism to smoothly change the focal length of the eyepiece to access any magnification within a certain range. Zooms have several drawbacks compared to regular eyepieces: they often have narrower fields of view, they can be softer at the edges, and they can have mechanical problems. But whereas Barlows will double your range of magnification, Zooms give you an unlimited number of possible magnifications to choose from.
I’d rather have a set of eyepieces and a good Barlow. Barlows tend not to degrade the quality of eyepieces. (Though we’ll see that some low-power luxury eyepieces can be affected), whereas Zooms are often not as good as individual eyepieces.
Because Zooms have a limited range (something like the high power being 2x-3x times the low power) and usually top out at mid-powers, a Barlow can often still be useful in faster telescopes to reach higher powers with a Zoom. A good kit for a travel telescope working at f/5 (a Short Tube 80 refractor, or perhaps a 5” or 6” f/5 Newtonian) might be a 32mm Plossl for wide field and low power, an 8-24mm Zoom eyepiece for mid powers, and a 3x Barlow to turn the 8-24 into a 2.7-8mm.
Barlow vs. “Powermates” or “Focal Extenders”
TeleVue, a premium/luxury eyepiece manufacturer, sells both Barlow lenses and a series of accessories called “Powermates.” The generic term is “Focal Extender.” The reason being that a simple, single-lens-group Barlow lens causes a few changes to the properties of eyepieces as a result of pushing the exit pupil outwards. For example, it can increase their eye relief, cause kidney-bean blackouts when you move your eye to look around, and certain Barlows may add small amounts of false color fringing. They may also vignette the light path of certain long-focal-length eyepieces, providing a dimmer, narrower field of view. Focal Extenders use a more complicated optical system to remove all the effects of the extended eye relief in long-focal-length luxury eyepieces, allowing you to treat the telescope as if it were a regular telescope with a longer focal length. These are rather niche devices, and are only really important if you have long-focal-length, wide-field-of-view luxury eyepieces. For example, my generic 32mm Plossl and my Explore Scientific 11mm 82-degree work fine with a 2.5x regular Barlow, but my Televue 16mm 82-degree Nagler seems to be wrecked by the Barlow.
Another crucial difference between Barlows and Focal Extenders is that the exact magnification factor of a Barlow will be slightly different depending upon the spacing between the lens and the eyepiece. Just as every eyepiece will have a slightly different position when focusing the telescope, every eyepiece will have a slightly different magnification factor from the Barlow. When imaging with a camera, the distance between the Barlow and the camera sensor will also change precisely how much the Barlow modifies the focal length. Focal extenders, however, fully correct for this, and will always magnify exactly by the magnification factor, no matter the spacing between the sensor or eyepiece and the telescope optics.
Explore Scientific carries its own brand of Focal Extenders, which are highly comparable to TeleVue’s Powermates.
Unscrewable Lens Cells
Most eyepieces have threads on the end of the barrel to attach filters. Though Barlows generally don’t, they are often designed in such a way that the Barlow Lens itself can be unscrewed from the barrel and then screwed onto the eyepiece directly. This results in a lower magnification than you would get from using the lens with the spacer tube, often something in the order of 50% or 75% of the normal factor. However, messing with the spacing too much will result in changing the correction of the Barlow, meaning there could be added Spherical Aberration, causing images to be too fuzzy, so you generally don’t want to extend the tube.
There are some Barlows specifically intended to be threaded onto the ends of eyepieces. This would be lighter and have a lower profile, but it’d be generally less convenient because you’d have to unscrew it and screw it constantly. Additionally, some screw-on Barlows are used with specialty equipment like Binoviewers and DSLR cameras to shunt the light path out enough to be used in visual Dobsonians and other Newtonians, where there might not be enough inward travel in the focuser for those devices.
Is Stacking Barlows Recommended?
It is possible to stack Barlows together. However, the actual magnification factor you get is not easy to calculate. You might think a 2x Barlow on a 5x Barlow would result in a 10x Barlow. But because the exact magnification factor depends upon the spacing of the Barlow versus the eyepiece, there is no simple equation for figuring out the magnification of a stacked Barlow. For example, a 2x on a 5x would have a very different magnification than a 5x on a 2x. (If memory serves, the latter had a far higher power.)
Because of the maximum useful magnification of a telescope, you will almost never find a situation that requires stacking Barlows. But if the telescope has a very fast focal ratio, the seeing is incredibly steady, the optics of the telescope and barlow lenses and eyepieces are all very good, and you want to try to squeeze as much detail as possible out of a bright planet or the Moon, you could try stacking more than one Barlow and just seeing what happens. That said, I don’t recommend planning an eyepiece collection based around stacking Barlows. There are too many unknowns, and you’ll never know for sure what magnification you’re actually using.