Do you want to get the biggest possible level of versatility from your telescope and its eyepieces? If that is the case, then a Barlow lens may be just what you need. People who are into backyard astronomy constantly have to make compromises between their need to obtain better stargazing equipment, and being financially sound and not burning through your savings.
To that end, Barlow lenses are often recommended. But their function and mathematics can be confusing for new stargazers. So, to start with, just what is a telescope Barlow lens?
What a Barlow lens is
A Barlow lens is a concave lens placed between your telescope’s eyepiece (or mirror) and its objective lens. Generally, a Barlow can double or triple the magnification you observe with the eyepiece. However, it doesn’t directly affect the eyepiece itself, but actually increases magnification by increasing the overall focal length of the telescope.
Modern Barlows often have not just one piece of glass, but 3-4 elements to reduce chromatic aberration.
For experienced astronomers, this is pretty much an essential and standard piece of equipment. But, if you’re not familiar with it — we’ll get you up to speed below!
Barlow lenses: design & history
There are plenty of devices out there that we use to see objects and targets that are otherwise too small or too far away. From telephoto lenses, telescopes, binoculars, all the way to superzoom smartphone cameras — there are plenty of tools at your disposal.
But apart from these main pieces of technology, there are plenty of secondary tools and devices that can help you increase your magnification even more. And if you’re using a telescope to look at far-away celestial objects — a Barlow lens is one such tool. Sure, you can increase telescope magnification via short focal eyepieces — but a Barlow lens allows you to zoom even closer.
This type of lens is pretty much the most affordable way to improve the magnification that you can achieve with your astronomy setup. With that in mind, before you invest in another lens or a more expensive telescope to see farther — check out Barlow lenses first.
The lens was invented by the eponymous Peter Barlow — a scientist from the 19th century. As you might imagine, the man was a scholar — astronomer, optician, and mathematician. Thus, he was perfectly equipped with the knowledge needed to improve the optics of astronomy.
How do Barlow lenses work?
So, considering the basics that we’ve described above —- how do these lenses actually work? You may already know how we determine a telescope’s magnification level. Usually, this is a division between the focal lengths of the telescope’s objective and the eyepiece that you’re using.
This also shows us how you can increase the level of magnification. Logically, you can either lower the eyepiece’s focal length or raise the objective’s focal length. The Barlow lens can help you with the latter: it allows you to raise the telescope’s focal length. And as a result, you have higher magnification levels.
Choosing & using a Barlow Lens
Considering all of the above — how do you actually choose the best possible Barlow lens for your telescope? Well, there are a couple of things that you need to keep in mind. But primarily — you need to make sure that it will be a good fit with your eyepiece’s tube. Generally, there aren’t many different diameter sizes of eyepiece tubes — it’s either going to be two inches or 1.25. If you remember that, you won’t have any issues with finding a lens that fits your telescope.
As we’ve mentioned above, there are a couple of different levels of magnifications you can achieve — depending on which Barlow lens you purchase. You can encounter the 2x lens most often — but there are 5x and 3x out there as well. However, at least for starters, we recommend you pick the 2x one.
Once you choose the Barlow lens that you want, you’ll proceed to install it; luckily, it’s really a no-fuss addition to your telescope. So, instead of putting your eyepiece inside the focuser — you insert it into your Barlow lens. And you insert the eyepiece into that combo. Sooner than later, you’ll be able to enjoy all of the advantages of seeing the night sky closer than ever before!
What difference does a Barlow lens make?
While people often make the mistake of confusing zoom lenses and Barlow lenses — they achieve the same effect, but they’re not exactly the same thing. Namely, you’ll find that there are a couple of differences. For one — your Barlow lens has a fixed level of magnification; you know just how much you’re increasing your telescope’s magnification as soon as you’ve bought the lens.
Conversely, zoom lenses can give you variable magnification enhancements. Also, they have a bigger field of view compared to Barlow lenses. For instance, zoom lenses are the ones that DSLR and SLR cameras have — seeing as they can raise or lower the focal length as needed.
So, why do we need Barlow lenses in telescopes in the first place? Well, since zoom lenses can change their focal length — making aberration corrections is far more difficult and requires more parts. As a consequence, zoom lenses have more glass elements — making them heavier and bigger than any Barlow lens.
And sure, while a zoom lens gives you the convenience of being able to switch up your focal length — you will probably have to make other compromises for that zoom, such as paying a higher price or settling for lower image quality. If we’re specifically talking about stargazing, expenses and simplicity of use are always big concerns — which is why a high-quality Barlow lens is always an excellent choice for telescopes.
Benefits and common issues with a Barlow lens
As you might have realized by now, it’s quite useful to have a Barlow lens as a part of your astronomy instrument collection. We might go as far as to say that it’s pretty much essential for any kind of backyard astronomer.
That being said, there are certain issues related to Barlow lenses that shouldn’t necessarily turn you away — but you need to be aware of them. First of all, there are different quality levels for different Barlows. And the lower-priced ones, logically, tend to have issues with quality. So, you might get bad colors at the edges with some of them, especially if you’re using fast scopes.
However, the entire point of a Barlow is to observe magnified views of objects that are located in the center of your field of view — and seeing as the quality of the image is not affected there, we don’t consider this to be a deal-breaker; just something you need to know before buying.
Also, you might remember how we’ve recommended that you go with a 2x lens when choosing your first Barlow. Well, there’s a reason for that — if you’re getting a Barlow for the first time, chances are that you’re a novice astronomer. As a consequence, you’re likely handling a small telescope. These devices aren’t capable of drawing too much light; meaning that heftier Barlow models will deliver dimly lit images. So, don’t go with a larger Barlow if you’re using a telescope that’s 8″ and below.
When to use a Barlow
Barlow lenses are often used for astrophotography, but only of nearby planets and celestial bodies. Those are situations where you want the image to be darker and provide you with more details of the bodies’ surfaces.
If you’re looking to perform deep-sky astrophotography, you should definitely opt for focal reducers; in this scenario, you’ll want better magnification and brighter images.
Can you use a Barlow with a zoom eyepiece?
Yes, it’s possible — many people use the combination of a Barlow lens and a zoom eyepiece when they want more flexible options for high-power observations; mostly for double stars, planets, and the Moon. In rarer cases, it can also be applied to globular clusters and planetary nebulae.