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Magnification in a telescope is expensive, and you must invest in expensive eyepieces to examine celestial objects closely. But what if you are on a budget? That’s where Barlow lenses come in. Named after its inventor, Peter Barlow, an English mathematician and physicist, the lenses offer a great way to increase the magnification of your existing telescope.
A Barlow lens, also called a focal extender, increases the effective focal length and, as a result, the magnification of your telescope’s eyepiece. It is a diverging (concave) lens that reduces the angle of convergence, increasing the size of the image and making it an excellent asset for stargazers.
As beneficial as a Barlow lens is, finding the right one for your telescope can be tricky. There are so many makes and models on the market that you must know what to look for if you want to get the most out of your purchase. Whether you are an amateur astronomer or a seasoned pro, a Barlow lens can help you take your stargazing experience to the next level.
In this article, we will discuss these fantastic telescope accessories and how to find the best Barlow lenses for your telescope.
What is a Barlow lens?
In the simplest of terms, a Barlow lens is an optical accessory that increases the apparent focal length of your telescope’s eyepiece lens. The increase in the foal length means that you, the observer, will see a magnified image of the object through the eyepiece.
Light enters a telescope and passes through the objective lens or primary mirror, forming an image at the focal point. The eyepiece then magnifies the image and presents it to you. However, the magnification produced will depend on the focal length of your eyepiece lens.
If you need a higher magnification value, you need an eyepiece with a longer focal length. Unfortunately, eyepieces with longer focal lengths are expensive; not everyone might want to invest in multiple lenses. A Barlow lens can come in handy in such situations.
You may also like: Barlow Lenses: Definition, Benefits, and How to Use Them
Barlow lens vs. eyepiece
Many amateur astronomers confuse the Barlow lens with the eyepiece lens. However, they are completely different in terms of their structure and function.
An eyepiece lens is a convex or a converging lens that magnifies the image formed at the focal point of the telescope’s objective lens or primary mirror. The level of magnification produced by an eyepiece lens depends on the focal length. Longer-length lenses will produce more magnified images.
As the f-length of an eyepiece lens is fixed, you can have a fixed magnification with a given objective-eyepiece combination. If you want more magnification options, you must buy eyepieces with different focal lengths, which can be expensive.
A Barlow lens, on the other hand, is a concave lens placed between the eyepiece and the objective. It can increase the apparent focal length of the eyepiece by a factor of 2 or 3. So, with Barlow lenses, you can effectively increase the magnification of your telescope without investing in different eyepieces. For instance, with a 2X Barlow lens, you can double the magnification of your telescope.
Best Barlow lenses for your telescope
Here are some of the best Barlow lenses available on Amazon.
1. Celestron 1.25″ X-Cel LX 3X Barlow Lens – Best Allrounder
The Celestron X-Cel LX is 1.25″ Barlow from Celestron, which will triple the magnification of your telescope. It is made of metal, and you can find 3-element fully multi-coated optics for perfect color correction and contrast.
The outside of the barrel has a rubber grip to avoid accidental dropping. The Celestron X-Cel LX has a standard 1.25″ thread on the end for the filters. It is useful when using different eyepieces, so you don’t have to switch the filter between them. You leave the filter on the Barlow.
A brass compression ring is a neat feature of this X-Cel LX Barlow lens from Celestron. It is very useful because it will grip the eyepiece around the barrel, so it doesn’t hold only on the tip of the screw, like in standard Barlow, protecting the eyepiece barrel from scratches.
The Celestron X-Cel LX’s only downside is that it doesn’t have the thread on the top to attach the O-ring for the DSLR camera. You can use an adapter, but this best 3x Barlow lens is ideal for visual observation only.
2. Celestron Luminous 2″ multi-coated 2.5X Barlow Lens (Silver) – Best 2-inch Barlow lens
If your telescope has a 2″ focuser or diagonal, the Celestron Luminous 2″ Barlow lens is an ideal choice for quality and price. You can slide 1.25″ and 2″ eyepieces inside, thanks to the 1.25″ adapter. It is basically a 2in1 Barlow.
The first thing you notice holding this Barlow lens is how excellent the finish and build quality are. It has a hard-anodized aluminum barrel with a rubber ring for a better grip. The oversized thumbscrews are a nice little feature that helps in cold weather when wearing gloves.
This Barlow lens uses a brass compression ring to hold your eyepiece securely and is also on the 1.25″ adapter.
The Celestron Luminous features 4-element multi-coated apochromatic optics that provide excellent viewing quality and a flat field for sharp images free of extraneous color. The filter thread is only for 2″ filters, and there is no thread on top for the O-ring to attach to your DSLR, but for the price, it is a phenomenal Barlow lens both optically and aesthetically.
3. Televue Powermate 2-1.25 inch – Best For Imaging
This is technically not a Barlow lens. It is a Powermate but does the same thing as a Barlow lens and more. If you are looking for the best of the best, then you need to buy Powermate.
Televue Powermate has 4-element parfocal multi-coated optics consisting of negative achromatic doublet and positive doublet. It is a high-end optical instrument that increases the magnification of your telescope fourfold.
Its fully multi-coated optics make it perfect for visual observations and planetary imaging. Powermate is used for some of the finest planetary imaging by amateur astronomers.
4. Orion shorty 1.25″ 2X Barlow lens – Best 2X Barlow lens for beginners
If you are not a seasoned pro looking for the best budget Barlow, the Orion Shorty 2X Barlow lens is a great choice. It comes in a fully metallic build and accepts 1.25-inch eyepieces. It is one of the most compact Barlows on the market.
The Shorty 2X Barlow lens comes with two-element, fully multi-coated optics. It also features lens elements with their edges blackened to reduce internal reflections.
It is the best 1.25 Barlow lens you can buy to take excellent shots of the moon’s surface and planets. You can also use it as an extension for a camera T-adapter.
5. Astromania 1.25″ 4-Elements 5X Barlow Lens – Best 5X
With a magnification of 5, the Astromania Barlow is one of the best high-magnification lenses out there. You can use the lens with any 1.25” eyepiece to get fantastic close-up shots of planets and their moons.
One of the best features of this Barlow is the 4-element optic design. Thanks to the fully multi-coated optics, the Astromania guarantees sharp images with high brilliance and low chromatic aberrations.
Another huge advantage of the multi-coated lens is that due to its good-quality optics, the color fringes are virtually absent with this lens. The Barlow also features a compression ring to provide optimal grip on the eyepiece lens and protection from scratches.
Best budget Barlow lens
Barlow lenses are available for almost every budget. However, if you are an amateur star-gazer, you might be unable to justify spending several hundred dollars on an accessory. In that case, you can opt for an entry-level Barlow lens.
One of the best Barlow lenses for beginners that won’t break the bank is the SVBONY 2X Barlow Lens. It is a 1.25-inch lens with multi-coated optics for distortion-free images. We like how compact this lens is and that it is made from 100% anodized aluminum.
How does Barlow’s lens work?
A Barlow is a diverging or concave lens placed between the telescope’s eyepiece and the objective lens or primary mirror.
The lens diverges the light, causing it to spread out and create a virtual focal point farther away from the telescope than the original focal point. This results in an apparent increase in the telescope’s focal length and a magnified image.
Barlow lenses are available in different configurations with effective magnifications of 2X, 3X, and even 5X. In other words, Barlow lenses can magnify the image twice, thrice, or even five times.
Seems incredible, right? But there is a caveat.
As 2X Barlow lenses introduce additional glass surfaces for the light to travel through, they can cause distortions or chromatic aberrations. The distortions are more pronounced with low-quality Barlow lenses with poor optics and inferior materials.
However, if you choose the right Barlow lens with high-quality astronomy optics, you can enjoy a bigger, more detailed image of celestial bodies like nebulae, galaxies, stars, and planets without losing the image quality.
You may also like to read: Telescope Magnification: Beginners Guide W/ Examples.
How to use a Barlow lens?
Using a Barlow lens is quite simple, and it involves the following steps:
- Pick the right Barlow lens depending on your magnification needs. I suggest using a 2X Barlow lens as a good starting point. If you need higher magnifications, use a 3X Barlow lens.
- Insert the 2X Barlow lens into the telescope’s focuser. Attach the eyepiece to the Barlow lens, ensuring it is appropriately aligned.
- Focus your telescope normally, adjusting it to get a clear and sharp image. That’s it.
The Barlow lens’s magnification depends on its magnification factor and the focal length of your telescope’s objective lens or primary mirror.
Experiment with different eyepieces or adjust the position of the Barlow lens in the focuser until you get the desired result. Once you have adjusted the magnification, take a moment to observe and enjoy the view through your telescope. Experiment with different objects in the sky, such as planets, stars, and galaxies, and enjoy the increased detail and magnification the Barlow lens provides.
Magnification of Barlow lens
The telescope’s magnification is calculated by dividing the optical tube’s focal length by the eyepiece’s focal length. An optical tube with a 1000mm focal length and 10mm eyepiece will give you 100x magnification.
Barlow lenses come in different multipliers (2x, 3x, 5x, etc.). This number tells you how much the Barlow will increase the focal length of your telescope, hence increasing the magnification.
So in your example, the telescope’s focal length using a 2x Barlow lens will be 2000mm, doubling the magnification.
So as you can see, having a Barlow lens in your inventory is like having a second telescope with a different f-length. But it is not as simple as that because it also depends on what eyepiece you are using.
If you also have a 5mm eyepiece, using a 2x Barlow lens is pointless. You can get the same magnification with a 5mm eyepiece in a 1000mm focal length telescope, and you don’t need to use a 2x Barlow lens.
However, you will do it sometimes because using an eyepiece with a longer focal length is more convenient. But the general rule is to buy a Barlow lens with a multiplier that will fill the magnification gaps between your telescope eyepieces.
The highest useful magnification with the Barlow lens
You must be careful when buying a Barlow with a high multiplier, like 5x. You may think: I have a 1000mm telescope, and with 5x Barlow, it will be 5000mm! Boom, unlimited magnification power. No, you can’t do that.
The magnification is limited not only by atmospheric conditions where you can’t go beyond a certain point (usually 200x), but your telescope also limits it with its aperture and focal length. It is called the highest useful magnification.
The maximum useful magnification is 50x per inch of aperture. Or take your aperture in millimeters and multiply it by 2. A telescope aperture of 150mm has the maximum useful magnification of 300. You will have issues focusing the telescope beyond that number. So keep this in mind when choosing the Barlow lens for a particular telescope.
This applies to visual observation with the eyepiece in the Barlow lens. The only situation where you can go beyond this is when you attach a camera to the Barlow lens. This lens is used in planetary imaging, which I will mention later.
Changing the focal and ratio with a Barlow lens
The focal ratio is the “F” number in the telescope specification. It is calculated by dividing the tube’s focal length by its aperture. For example, a 1000mm telescope with a 100mm aperture is F10.
Increasing the telescope’s f-length with the Barlow lens will also increase the ratio. So, what are the consequences?
We need to understand what the ratio does. The ratio is also known as how “fast” your telescope is. Fast telescopes are usually F5 and lower, and slow telescopes are F6 and higher.
I will not go into details, but the general rule is that the fast telescope image is brighter than the slow telescope.
If you go too high with the ratio by increasing the f-length with the Barlow lens, the image can be very dark, and it will be hard to see any details of the object. It is not a big issue with the planets and the moon because they are bright but don’t go to extremes. Just keep this in mind.
The physical size of the Barlow lens
You can buy Barlow lenses in two sizes, just like eyepieces – 1.25″ and 2″. Here, the decision is simple because it depends on your focuser size and the size of the eyepieces you want to use.
As with the eyepieces, the 2″ Barlow will give you better views, but it is also more expansive. It is really up to your preference, budget, and the setup you are using.
You can choose from long ones and short ones. The short eyepiece is usually called “shorty,” They are almost invisible in the image train. They are good if you plan to attach the camera to the Barlow. The long Barlow extends too far out of the focuser, disturbing the telescope’s balance if you use a heavy DSLR.
Optics of the best Barlow lenses
Maybe the most essential thing in the Barlow lens is the quality of the optical elements. If you have a really expensive telescope with high optical quality, using a crapy Barlow will degrade the image you see.
The Barlow lens is a lifetime accessory that will stay with you forever. You can use it on every telescope, so don’t cheap out on the Barlow.
The Barlow quality is determined by the number of optical elements and the coating of the telescope optics.
Never buy one element, Barlow; replace it if you have one that came with your telescope.
Barlow lenses can have 2, 3, and sometimes 4 optical elements. The more elements, the better, as the image will be crisper.
The top end of the Barlow family is called Powermate. These are expensive, high-quality Barlow lenses; to learn more about them, read my article Powermate vs. Barlow lens.
Using a Barlow lens for astrophotography
We are using a Barlow lens for astrophotography for several reasons. Some people use it to achieve the focus of the cameras. But not every telescope is made for astrophotography. When a camera is attached to the telescope, it can be impossible to achieve focus.
There is not enough back focus to do that. It is called prime-focus astrophotography; to fix that, you have to modify your telescope as I did. A decent-quality Barlow lens worth a few dollars can fix that by attaching the camera to it.
However, I don’t recommend using it for deep-sky astrophotography because it will increase the telescope’s focal ratio, and you will need much longer exposure time to get enough light on the camera’s chip.
Planetary and lunar imaging with the Barlow
A Barlow is widely used in planetary imaging. It is the only way to increase the magnification because you do not use an eyepiece unless you make an eyepiece projection. The planets are tiny, and to get a good image by recording a video and then stacking the frames, you need more magnification to see more details. A Barlow is perfect for that.
Planets are much brighter than deep-sky objects like nebulae and galaxies, so the high focal ratio and light loss are not problems. It is also an excellent choice to get close-ups of the moon’s surface. For example, here is a video with and without the Barlow and DSLR camera:
FAQs about Barlow lenses
Barlow lenses are a great way to expand your telescope’s magnification capabilities. These accessories can give you more details in your images at a fraction of the cost of buying a new eyepiece. Here are a few frequently asked questions that might help you better understand these great telescope accessories.
What does a Barlow lens do on a telescope?
Barlows increase the apparent maximum focal length of your telescope’s eyepiece, acting as focal extenders. It means that when you use this accessory in your telescope, you get a higher magnification with the same eyepiece-objective combination.
Where does the Barlow lens go on a telescope?
You should place your Barlow lens between the eyepiece and your telescope’s objective lens (or mirror). Remember that the quality of the image with a Barlow depends on its position in your telescope. When setting up your telescope, take time to align the lens and follow the manufacturer’s instructions.
What is a Barlow lens used for?
The primary role of a Barlow is to help increase your telescope’s magnifying power. With the right Barlow lens, you can double, triple, or even quintuple your telescope’s maximum magnification. If you use the right combination of Barlow and eyepiece lenses, you can create a wide range of magnification options without purchasing multiple expensive eyepieces.
What is the best Barlow lens for astrophotography?
The best Barlow lens for astrophotography has good optical performance, such as low chromatic optical aberrations, minimal distortion, and good contrast. You should ensure that the lens is compatible with your telescope or camera before you purchase it. The best Barlow lens for astrophotography is Tele Vue 2″ 4x Powermate.
Takeaway – Choose a Barlow lens to make your telescope versatile and more powerful
A Barlow lens is an excellent accessory for making your telescope more versatile and powerful. It allows you to achieve higher magnification and observe celestial objects in greater detail. If you pick the right Barlow lens, you can enhance your viewing experience and expand your astrophotography capabilities.
My last piece of advice is, again, please don’t go for a cheap Barlow lens. You won’t need to upgrade it with the new telescope. You can keep it forever, so buy the best quality one your budget allows. And if you don’t have enough money now, wait until you can afford a good Barlow lens or Powermate. As a beginner, I made the same mistake of buying cheap Barlow and searching for a better one second later.