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Barlow lens is a fantastic tool and should be in every amateur astronomer inventory. You will probably end up with a few of those. But with so many Barlow lenses to choose from, you need to know and understand what Barlow lens is and what it does to make the right purchase.
I will explain what the Barlow lens is and how to choose the right one in this article. I will also give you a list of 3 best Barlow lenses you can’t go wrong with if you buy them right now.
1. Celestron 1.25″ X-Cel LX 3x Barlow Lens – Best Allrounder
This is 1.25″ Barlow from Celestron that will triple the magnification of your telescope. It is made of metal, and you can find 3-element fully multicoated optics for perfect color correction and contrast.
The outside of the barrel has a rubber grip to avoid accidental dropping. It has a standard 1.25″ thread on 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 lens.
The neat feature of this Barlow lens from Celestron is a brass compression ring.
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 only downside of this Barlow lens is that it doesn’t have the thread on the top to attach the O-ring for the DSLR camera. You can go around using an adapter, but this Barlow lens is ideal for visual observation only.
2. Celestron Luminous 2″ 2.5x Barlow Lens (Silver) – Best 2″
If you have a 2″ focuser or diagonal on your telescope than the Celestron Luminous 2″ Barlow lens is an ideal choice when it comes to quality and price. You can slide 1.25″ and 2″ eyepiece inside thanks to the 1.25″ adapter. It is basically a 2in1 Barlow lens.
The first thing you notice holding this Barlow lens is how excellent the finish and build quality is. It is a hard-anodized aluminum barrel with a rubber ring for a better holding grip. And oversized thumbscrews are a nice little feature that helps in the cold weather when wearing glows.
This Barlow lens uses a compression ring to hold your eyepiece securely, and it is also present on the 1.25″ adapter.
The Celestron Luminous features 4-element multicoated apochromatic optics providing excellent image quality and flat field for sharp images that are 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 your DSLR, but it is a phenomenal Barlow lens both optically and aesthetically for the price.
3. Televue 2″ 4x Powermate – Best For Imaging
This is technically not a Barlow lens. It is a Powermate, but it 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. It is superior to the standard Barlow lens, and you can learn more about the differences in my article Powermate vs. Barlow lens.
Televue Powermate has a 4-element parfocal multicoated optics that consists of negative achromatic doublet and positive doublet. It is a high-end optical instrument increasing the magnification of your telescope 4 times.
It is perfect not only for visual observations but also for planetary imaging. Some of the finest planetary imaging by amateur astronomers is done with Powermate.
It has a 2″ barrel, so you need 2″ focuser or diagonal. It comes with 1.25″ adapter for 1.25″ eyepieces.
What Is a Barlow Lens And How To Choose The Best One?
Barlow lens is a diverging lens used in the telescopes and other optical instruments to increase the effective focal length of the device. The result of using a Barlow lens is increased magnification.
Barlow lens is installed in the eyepiece holder(focuser) or diagonal before the eyepiece. Then, the eyepiece is placed in the Barlow lens.
Magnification Of Barlow Lens
Magnification of the telescope is calculated by dividing the focal length of the optical tube by the focal length of the eyepiece. Optical tube with 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 lens will increase the focal length of your telescope, hence increasing the magnification. So in your example, the focal length of the telescope using 2x Barlow 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 focal 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 5mm eyepiece in 1000mm focal length telescope, and you don’t need to use a 2x Barlow lens.
However, you will do it in some cases because using an eyepiece with 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 eyepieces.
The Highest Useful Magnification With Barlow Lens
You have to be careful when you are looking to buy a Barlow lens with a high multiplier like 5x. You maybe think: I have 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. It is called the highest useful magnification.
The highest useful telescope magnification is 50x per inch of aperture. Or take your aperture in millimeters and multiply it by 2. Telescope aperture of 150mm has the highest useful magnification of 300. You will have issues to focus the telescope beyond that number. So keep this in mind when choosing the Barlow lens for a particular telescope.
This applies to visual observing with the eyepiece in the Barlow lens. The only situation you can go beyond is when you attach a camera to the Barlow lens. It is used in planetary imaging that I will mention later.
Changing The Focal Ratio With Barlow Lens
The focal ratio is the “F” number in the specification of the telescope. It is calculated by dividing the focal length of the optical tube by its aperture. 1000mm telescope with 100mm aperture is F10.
Increasing the focal length of the telescope with the Barlow lens will increase the focal ratio as well. So what are the consequences? We need to understand what the focal ratio does in the first place.
Focal ratio is also known as how “fast” is your telescope. 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 in the slow telescope.
If you go too high with the focal ratio by increasing the focal 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.
Physical Size Of The Barlow Lens
You can buy Barlow lenses in two sizes, 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 setup you are using.
You can choose from long ones and short ones. The short eyepiece is usually called “shorty,” and 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 much out of the focuser, disturbing the balance of the telescope if using heavy DSLR.
Optics Of The Best Barlow Lens
Maybe the most important thing in the Barlow lens is, of course, the quality of the optical elements. If you have a really expensive telescope with high-quality optics, using a crapy Barlow lens 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 lens quality is determined by the number of the optical elements and the coating of the optics. Never buy one element Barlow lens, and if you have one that came with your telescope, just replace it.
Barlow lenses can have 2, 3, and sometimes 4 optical elements. The more elements, the better because the image will be crispier. The top end of the Barlow family is called Powermate. These are expensive high-quality Barlow lenses, and if you want to know more about them, read my article Powermate vs. Barlow lens.
Using Barlow Lens For Astrophotography
We are using a Barlow lens for astrophotography for a few reasons. Some people are using it to achieve the focus of the cameras. Not every telescope is made for astrophotography, and when you attach the camera to the telescope, it can be impossible to achieve focus.
There is simply 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. You can find it in my article about budget astrophotography, or you can use the Barlow lens.
Barlow lens can fix that by attaching the camera on it. But I don’t recommend using it for deep sky astrophotography because it will increase the focal ratio of the telescope, and you will need much longer exposure time to get enough light on the chip of the camera.
Planetary And Lunar Imaging With The Barlow Lens
Barlow lens is vastly used in planetary imaging. It is the only way to increase the magnification because you are not using any eyepiece unless you are making 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. Barlow lens is perfect for that.
Planets are much brighter than the deep-sky objects like nebulae and galaxies, so there is no problem with the high focal ratio and loss of light.
It is also an excellent choice to get close-ups of the moon surface. For example, here is a video made by me with and without the Barlow lens and DSLR camera:
My last advice for you is again; please don’t go for a cheap Barlow lens. It is not an item that you will need to upgrade with the new telescope. You can keep it forever, so buy the best quality one that your budget alow. And if you don’t have enough money at the moment, just wait till you can afford a good Barlow lens or Powermate.
Believe me, as a beginner, I made the same mistake of buying cheap Barlow and then searching for a better one second later.