If you are looking to buy a Barlow lens, you may come to find a similar optical tool called Powermate or focal extender. They look the same, but there is a difference between Powermate and Barlow lens.
The Barlow lens consists of 2 or 3 optical elements(negative doublet). This optical element configuration results in diverging rays that are leaving the Barlow. It moves the exit pupil further out and extends the eye relief, which may cause vignetting in some eyepieces. The Powermate is made of 4 optical elements(negative doublet and positive doublet) correcting this issue and providing pure magnification without diverging the rays.
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They both provide magnification, and the only difference is in the optical elements and price. But that doesn’t mean that I don’t recommend Barlow lenses and you have to buy the Powermate instead. So let me explain it and give you advice on what you should buy for your telescope. The first thing to understand is what is the Barlow lens.
What Is Barlow Lens?
Barlow lens is a tool that extends your telescope’s focal length to achieve higher magnification with the given eyepiece. It is a goto tool for planetary and moon observing. Barlow lenses are also used to achieve the prime focus with DSRL for astrophotography.
You can choose from many options. 2x, 2.5x, 3x, and 5x Barlow. What it means is that it is multiplying your focal length; for example, 2x Barlow will double the telescope’s focal length. To choose the right multiplier, check my article about the best Barlow lenses where I go deeper into this topic.
The optical elements inside will not only magnify the image but also diverge the rays coming out. Moving the exit pupil out is not an issue for some people. But the vignetting with long focal length eyepieces is the problem. This problem is almost invisible with short focal length eyepieces. So keep this in mind.
Another problem for some people is that you have to refocus each time you change the eyepiece in the Barlow lens. However, some high-quality eyepieces require only a tiny bit of refocusing after changing the eyepiece.
The advantage of the Barlow lens is that it is shorter than Powermate, and it is not extending from your focuser as much as Powermate.
Recommended Barlow Lens
What Is Powermate?
Powermate is basically a Barlow lens with the additional optical element, positive doublet, to correct the pupil exit. What it means is that the Powermate is “invisible” in the light train of the telescope. Putting the eyepiece in the Powermate is like putting it directly into your focuser.
The only thing you notice is the magnification. There is no light ray diverging and no vignetting. The exit pupil is the same as without the Powermate.
Using the Powermate means that you don’t have to refocus each time you change the eyepiece. Also, some people say that the image quality is better compared to the Barlow lens.
However, the Powermate is usually tall, so it extends a bit more than the Barlow lens. Powermate is more expensive too, and sometimes it is not worth the money for the particular telescope and setup.
If you are deciding between the Barlow lens and Powermate, my recommendation is to consider the price. If you are on the budget, buy the Barlow lens first; it is a great tool to increase your telescope’s magnification. If you are a beginner, I guarantee that you will be happy. Just one thing; buy a better quality one and not the cheap one element Barlow.
You have to realize that this kind of tool is a lifetime item. You can use it with any telescope, and it will probably stay with you forever. So you can think about investing more to get a high-quality one. And you can also buy both a Powermate and a Barlow lens because you may use them in different situations. You will be changing telescopes and eyepieces but never a good Barlow or Powermate.