TLDR: 

Everyone who uses simple eyepieces needs a Barlow, not just for magnification but for field flattening, longer eye relief, and less optical distortion with their Plossl lenses.

Astronomers use Barlow lenses to diversify their eyepiece sets and increase the overall magnification of their scopes.

But these aren’t the only uses of these wonderful negative elements being in the light stream of a telescope.

In this article we’ll talk about lesser known qualities that make Barlows nearly essential to any kit, especially kits where most of the eyepieces are simpler, such as RKEs and Plossls, or even Erfle eyepieces.

Magnification of Barlow Lenses

This is the number one use, so let’s get it out of the way.

Barlow lenses multiply the magnification of your scope and eyepiece combination by the number stated on the Barlow.

Many people think of this as changing the focal length of the eyepiece, which makes for easy calculations, but this isn’t exactly what is happening.

The cone of light entering your focuser has a certain pitch based on how fast the scope is, or the ratio of length to the size of the primary optic. The Barlow lens intercepts this cone of light and extends it before it can reach your eyepiece.

It does nothing to the eyepiece, and everything to the scope. Barlows effectively double the focal length of your telescope.

The effect is the same, they double the magnification for a chosen eyepiece, but we’ll see why this distinction is important. Read on.

Turning a Fast Scope into a Slow Scope

If you have a short tube of any kind, you may have a focal ratio of f/5, or even f/4. The light cones in these scopes can be very acute, which causes all kinds of problems with simple eyepieces like Plossls.

You will probably hear other astronomers in your club comment frequently that Plossl eyepieces don’t work well in fast scopes. They simply can’t correct that sharp cone of light without introducing unwanted optical elements, such as coma near the edge of the view.

You may have even experienced this yourself.

A 2x Barlow will effectively turn a wild f/5 telescope into a much more tame f/10 that will readily use less expensive eyepieces without issue.

This does come at a cost, however. By stretching the light cone, you will also be stretching any color inconsistencies of the scope itself. They can add some color fringing. In some cases can also vignette the image when using a longer focal length eyepiece like a 38mm.

Overall, however, they can be a saving grace for your “budget” Plossl eyepieces, and make them functional in very fast scopes.

In fact, there are a few very short telescopes floating around out there that have a telenegative element permanently affixed in the focuser to correct the issues associated with short scopes and lengthen the overall focal length of the tube.

Eye Relief

Another advantage to Barlows is that they will preserve, and in some cases extend, the eye relief of your eyepieces.

Nobody likes swabbing the lens with their eyelashes, as you sometimes must do on short focal length Plossl eyepieces.

As the focal length of the eyepiece shortens, so does the eye relief. This is an unfortunate rule of physics, though some better modern eyepieces resolve this issue in various ways (sometimes by putting a negative element similar to a Barlow right at the end of the eyepiece itself).

Anyone with experience knows that as you get below 10mm on a Plossl, the viewing window gets annoying small and the eye relief can have you pressing your eyeball right against the glass. That’s never fun. The effect is magnified when you’re freezing in your scope chair.

Consider, instead of using that annoying 6mm Plossl, simply plop a 12mm into a 2x barlow (or an 18mm into a 3x) and get the same view with much less strain on the eyes, or the nerves for that matter.

Nothing says “junk eyepiece” like a terribly short eye relief. A 3mm Plossl is nearly unusable.

The Poor Man’s Zoom Lens

Did you know your Barlow can be used as a zoom lens?

Personally, I hate fiddling with the thing for it to serve this purpose for me, but the distance from the Barlow to the eyepiece glass will actually affect the magnifying potential of the Barlow. Experiment by sliding the eyepiece in or out of the Barlow tube a little before clamping it down.

So, if you are using a 90-degree diagonal on your scope and you want some extra magnification, try putting a 2x Barlow ahead of the diagonal on your Cassegrain or refractor telescope, and you should have around 3x magnification!

The reasoning for this is a little complicated, but it’s something that most people never learn about. Consider yourself one of the few.

Less Light Transmission

Barlows aren’t all sunshine and buttercups, there are some drawbacks.

The biggest argument against them is that they rob a little light from the optical system.

This is certainly true, and to minimize the effect, you should consider getting a Barlow that is fully-multicoated, or at the very least full coated. Every glass element should have some anti-reflection coating on both sides to maximize the amount of light getting through.

In general, I would say this isn’t something to worry about, unless you have a really cheap Barlow with uncoated lenses, and more so if you can actually see dimming between a barlowed 12mm and a regular 6mm.

The most we are probably talking with good glass is around 5-10%, which will be unnoticeable to the observer except on targets that are just at the edge of optical reach for your system.

My personal counterpoint to these issues will remain that Barlows are most commonly used at magnifications that give an exit pupil less than 2mm, and the image is already significantly dimmed due to the magnification, not the optics.

Reducing the Barlow Hassle

Many astronomers out there don’t like Barlows because of all the extra eyepiece swapping that must be done. Barlow with this eyepiece, then take it out for a new eyepiece, then Barlow the new eyepiece, etc.

The whole process can actually be simplified quite easily if you have a set of eyepieces with a good range of magnifications. Use your low magnification to scan for and identify the target, then put the Barlow in and don’t remove it until you change targets.

Simply swap out your eyepieces in the Barlow.

Of course, if you only have 2 or 3 eyepieces on hand, it may be better to play the Barlow shuffle to help you frame your target for the most optimal view, but this is one area where those eyepiece kits can really come in handy.

Conclusion

Eyepieces tend to get most of the attention when it comes to getting the most from your telescope, but we shouldn’t forget about the humble Barlow.

Barlow lenses are not just for magnification. They can do so much more when someone knows how to utilize them well. They can turn a “cheap” set of Plossl eyepieces into a wonderful accessory kit for just about any telescope, even the fast ones.

High-end eyepiece and scope manufacturers know this, and make use of telenegatives (of which Barlows are one type) to enhance their fancy eyepieces to give incredible views. Take a closer look at the optics inside expensive eyepieces. In addition to the plethora of oddly shaped lenses that make up the focusing portion, there is typically a leading element ahead of them made from only one or two pieces of glass, just like sticking a Barlow in the eyepiece tube.

While these designs typically also use their telenegative to reduce other optical effects caused by the rest of the unit, they are essentially putting a Barlow ahead of a simpler eyepiece to get wonderful effects.