My 1.25″ and 2″ Barlow Lenses Recommendations (2x to 5x)

The Barlow I primarily use is a GSO/Apertura 2.5x Apo Barlow. I chose it because it has good optics yet remains quite affordable.

When used with any of my telescopes, this 2.5x Barlow will multiply the effective magnification of any eyepiece I possess by 2.5 times. For example, an 8″ Dobsonian with a 1200mm focal length and a 25mm Plossl eyepiece normally provides 48x magnification. Add a 2.5x Barlow, and that reaches 120x. This is because the telescope’s focal length is effectively multiplied by 2.5 times to become 3000mm with the integration of the Barlow lens, which in turn increases the magnification I get at the eyepiece.

My Advices For Those Looking to Buy a Barlow

• What I normally do is avoid duplicating magnifications.

For example, many beginner telescopes come with 20mm and 10mm eyepieces. Getting a 2x Barlow will make your 20mm eyepiece provide the same magnification as the 10mm eyepiece, so in such cases, it’s better to get a 5mm eyepiece instead of the 2x Barlow.

A 3x Barlow, on the other hand, will effectively convert the 20mm and 10mm eyepieces into 6.7mm and 3.3mm eyepieces, respectively. 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.

Likewise, in the case of my previously-mentioned GSO 2.5X Barlow, it will make the 25mm/10mm setup common in many beginner telescopes redundant, and I purchased it with that in mind.

• Don’t overpower your telescope with high magnification Barlows

It’s also important to remember that Barlows aren’t magic when it comes to magnification. Your telescope still has a maximum useful magnification defined by its aperture and optical quality. So don’t just get a 5x Barlow because it gives you the highest power.

• Consider your telescope’s focal ratio before deciding on the Barlow

If your telescope’s focal ratio is at f/10, f/15, or higher, I don’t think it makes 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 less might actually be able to make use of even a 5x Barlow in some rare circumstances. Don’t even consider a 5x Barlow unless your telescope has an extremely low focal ratio.

Generally, the higher the Barlow’s magnification, the less versatile it becomes. High-magnification Barlows often don’t work well with short-focal-length eyepieces because the combination produces extremely high magnifications that can exceed the useful magnification limits of your telescope.

If I were you, I’d stick with magnifications of 2x to 3x. 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 for certain specialty applications. (Binoviewers, for example).

• Be prepared to throw away the Barlow lens that you get while purchasing your telescope

If your telescope came with an included Barlow, odds are it’s very bad. I only use such Barlows to remind myself of their uselessness.

The most common type included with cheaper beginner telescopes is a single plastic lens, which, whenever I’ve used it, has caused severe chromatic aberration and other distortions in the eyepiece image. The dual-element Barlow lens has emerged as the popular choice in the industry.

For optimum results, a Barlow lens should integrate seamlessly without introducing any optical distortions or diminishing the field of view. I believe that the view through a 10mm eyepiece combined with a 2x Barlow should be almost identical to that of a 5mm eyepiece of the same design. If this isn’t the case, it indicates that the Barlow lens is of inferior quality or has design flaws.

• Don’t cheapen out, and get a Barlow with a 2+ lens element that is fully multi-coated

When I’m searching for a Barlow, I always choose one with an achromatic or apochromatic lens (2 lens elements or 3 or more lens elements). I also ensure that it is fully multi-coated to prevent internal reflections and preserve high contrast.

Unlike some others, I don’t consider Barlows to be an element to cheap out on, since a bad Barlow can make the image lose its sharpness and contrast. If it’s going to double your eyepiece collection anyway, I’m guessing you can afford to make sure you don’t end up with one that adds aberrations and false color fringing.

My 1.25″ Barlow Lens Recommendations

It’s likely that you are going to want a 1.25” Barlow over a 2” one. Most people have one or two 2″ eyepieces and the rest are 1.25″. For planetary imaging, 1.25″ short-focal-length eyepieces are all you need.

I have a 2″ 2X GSO Barlow that I like very much, but I don’t use it as much as I thought I would. When I didn’t have wide angle 1.25″ eyepieces, I used it for a while, but as I filled in my eyepiece set with 82 degree 1.25″ eyepieces, I stopped using the 2″ barlow. But, for a while, I used it a lot.

A 1.25” Barlow is also cheaper and much lighter in weight than a 2” unit.

For just above $100, I’d recommend a Tele-Vue Barlow

The Tele-Vue Barlow lenses are relics of an era when Barlows featured weaker lenses, allowing for greater tolerance of optical defects but having clearance issues on some diagonals or focusers. Nonetheless, they remain an excellent, if not exactly cheap, choice. Tele-Vue presently offers 2x and 3x units, but I’ve also found 1.8x and 2.5x Barlows on the used market.

When it comes to Barlows from premium brands like Explore Scientific and TeleVue, I don’t see a whole lot of difference between a mid-range barlow and a premium barlow. I think at the price point of $200 or more, you may want to consider a Focal Extender instead.

Recommendations for Barlowing 2″ Focuser/Eyepiece

A 2” Barlow is unnecessary for most users, as you’re more likely to use 1.25″ eyepieces for higher magnifications. If you’re using a 1.25″ eyepiece, you only need a 1.25″ Barlow to extend magnifications higher than your eyepieces allow. Likewise, for planetary imaging, any suitable camera sensor does not need a 2” Barlow; 1.25″ will suffice.

The exception may be those people who have very long focal length scopes, say over 3000 mm, who would like to leverage their 2″ eyepieces and their wide apparent field of views. 2″ eyepieces tend to be quite expensive so being able to Barlow them can be a very reasonable approach if your eyepiece budget is limited.

One of the scopes I use is a 14″ Meade LX200 with a 3500 mm focal length. I have two 2″ eyepieces. I could use my 2″ barlow with this one, but I still tend to go to my 1.25″ 82 degree eyepieces rather than barlow the 2″ eyepieces. Most people don’t Barlow 2″ eyepieces.

Also, combining a 2” Barlow lens with a 2” eyepiece often adds significant weight to telescope setups. This combination introduces balance issues in telescopes with cheap focusers, affecting the stability and orientation of the telescope. I’d warn you to always ensure that your telescope can comfortably support the combined weight of the Barlow lens and the eyepiece to avoid any mishaps.

“Powermates” or “Focal Extenders” For Extreme Quality

TeleVue, a premium/luxury eyepiece manufacturer, sells both Barlow lenses and a series of accessories called “Powermates.” The generic term is “Focal Extender.” If you must have the highest-quality image to support your top-of-the line eyepieces, I believe this is the one to get.

Explore Scientific carries its own brand of Focal Extenders, which are highly comparable to TeleVue’s Powermates. These devices used to be priced lower than their Tele-Vue equivalent, but after COVID, I’ve noticed that the price difference is so thin that I would urge you to just get the higher-quality Powermate.

All of these are rather niche devices, and are only really important if you have long-focal-length, wide-field-of-view luxury eyepieces. For instance, 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.

The reason being that a simple true 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
• Vignette the light path of certain long-focal-length eyepieces, providing a dimmer, narrower field of view.
• Certain Barlows may add small amounts of false color fringing

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 us to treat the telescope as if it were a regular telescope with a longer focal length.

Another crucial difference between Barlows and Focal Extenders is that the exact magnification factor of a Barlow will be slightly different depending on the spacing between the Barlow lens and the eyepiece or camera sensor. Focal extenders, however, always magnify exactly by the magnification factor, no matter the spacing between the camera sensor or eyepiece and the telescope optics.

Opinion: I Prefer Barlows Over Zoom Eyepieces

I’ve written a standalone article on 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 mechanism to smoothly change the focal length of the Zoom eyepiece to access any magnification within a certain range. For example, a Celestron 8-24 Zoom eyepiece can change its focal length to any number between 8mm and 24mm to get an unlimited number of possible magnifications.

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.

Zoom eyepieces have a limited range, with the higher focal length being about 2x-3x the lower focal length, like in the case of Celestron’s 8mm to 24mm Zoom, and usually top out at mid-powers. For example, the Celestron 8-24mm Zoom eyepiece tops out at 8mm, a medium power focal length for many telescopes. If you already have a Zoom eyepiece, a Barlow can still be useful for low-focal-ratio telescopes to reach higher powers.

For example, a good kit for a f/5 travel telescope (could be 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.