A Barlow lens is an intermediate optical device that goes between the eyepiece and the objective lens or primary mirror. It is named after Peter Barlow, so you will often see it capitalized.
The photo shows a typical Barlow lens. The eyepiece is slipped into the top and the Barlow lens is slipped into the focuser or diagonal, where the eyepiece would normally go.
We insert a Barlow lens in the optical path to increase the magnification provided by any given eyepiece. In this way, a single Barlow lens can increase the effective number of magnification choices you have in your eyepiece set. It does so economically and can simplify your eyepiece needs.
While we often associate the Barlow Lens with the eyepiece, it is really more correctly associated with the optical tube. A Barlow lens is a divergent lens which means that it moves the focal point out, effectively giving the telescope a longer focal length. So, if your telescope has a 400 mm focal length, a 2X Barlow inserted between the eyepiece and the objective lens or primary mirror, will effectively make it behave like it is in a telescope with an 800 mm focal length. In this way, it gives each eyepiece two magnifications, one with and one without the Barlow.
In practice, we more often talk about Barlow lenses in the context of the eyepiece as it seems more convenient to think of it that way. You will read that a 2X Barlow will make a 10 mm eyepiece deliver the magnification of a 5 mm eyepiece. The net effect is the same so it really doesn’t matter, unless you are really interested in how the optics work.
For visual use, Barlow lenses from 1.5X to 3X are common. Generally, Barlow lenses of greater than 3X are considered applicable to astrophotography though there is nothing to prevent you from using one with your eyepiece.
Barlow lenses come in a variety of designs. The typical design is shown above where you place the eyepiece into the Barlow and then place the Barlow into the focuser or the diagonal. However, there are also Barlow lenses that are screwed onto the eyepiece. These are sometimes referred to as Barlow elements. An example is shown in the photo. The top shows an eyepiece and the Barlow element separately. The bottom shows the same eyepiece with the Barlow element screwed on like a filter.
Some Barlows can be used both ways. You would place the eyepiece into the device, or you can remove the element and screw it directly onto the eyepiece. When used this way you will get two different magnification factors. The combined Barlow unit might provide 2X while the element alone on the eyepiece might provide 1.5X.
As mentioned they are usually labeled with a multiplication factor, such as 2X. If your eyepiece produces 100X and you put it in a 2X Barlow it will deliver 200X.
Barlow lenses can be based on a single lens element, two lens elements, three lens elements, and a few incorporate four lens elements. Some incorporate exotic glass in the design.
And, like eyepieces, there can be lens element edge blackening. And there can be various levels of coatings on the lenses, with “fully multi-coated” being the best. The coatings and blackening are there to minimize light loss and internal reflections.
Without going into a deep optical evaluation, the 2 element Barlow lens is more or less the industry standard. But counting lens elements won’t tell you which is best. You would have to compare them side by side in the same scope with the same eyepiece to know if one is better than the other in your application.
I would not recommend buying a single lens element Barlow. These are usually packaged with low-end telescopes. While they work, they often introduce optical aberrations. If you have one of these, use it until you can afford to replace it with something better.
A Barlow should disappear in use. That is to say, it should not introduce optical aberrations. It should not cause a loss of field of view, cutting off the edges of a comparable eyepiece at the implied focal length. If you took a 10 mm eyepiece and a 5 mm eyepiece of the same design when putting the 10 mm in a 2X Barlow you should get an image that is very close to or equivalent to the image produced by the 5 mm eyepiece. If you don’t, the Barlow is of poor design and quality.
Naturally the more glass elements you add in the light path the more light that is lost as it passes through each element. Some people object to the use of a Barlow lens, for this reason. In the past, this may have been a valid argument. However, modern design and optical coatings can reduce this to the point that any light losses in a quality Barlow are minimal and likely not noticeable in the eyepiece.
One reason to use a Barlow lens is to take advantage of the longer eye relief typically found in longer focal length, lower power eyepieces. Eye relief defines the distance you have to place your eyeball from the top lens in order to see the full field of view.
Short eye relief can make eyepieces uncomfortable to use. In many eyepiece designs, as the focal length of the eyepiece gets shorter the eye relief gets smaller. And people who wear eyeglasses while observing may not be able to get their eye close enough to the lens to see the image fully. So longer eye relief can be helpful to them.
To illustrate, a 30 mm Plossl eyepiece might have 22 mm of eye relief. Anything less than about 18 mm may be no good for glasses wearers. Anything less than 10 mm may be uncomfortable for some people even if they don’t wear glasses.
If you wanted a 10 mm Plossl to give you the magnification you wanted, you would have to deal with an eyepiece with about 7 mm of eye relief. That would be no good for the eyeglasses wearer and others would have their eye almost on the lens.
But place that 30 mm Plossl in a 3X Barlow and you would have the equivalent magnification of a 10 mm Plossl but would retain the 22 mm of eye relief of the 30 mm eyepiece.
How to Use a Barlow?
The use of a Barlow lens is quite simple. You place the Barlow in the focuser or the diagonal and insert the eyepiece into the Barlow.
For refractor, SCTs and MCTs, which typically have a diagonal in the path, you do have the option of putting the diagonal in the Barlow and putting the Barlow into the focuser. Then you would place the eyepiece in the diagonal as usual.
This will result in a different magnification factor than what is shown on the Barlow. In some cases, this might create an awkward set-up with the diagonal, but it is a valid way of using a Barlow.
Extending Your Eyepiece Set
Typically each eyepiece delivers one magnification. If you want more choices you need more eyepieces. But this can become expensive and require a larger storage case.
If you buy your eyepieces with the use of a Barlow in mind, a Barlow can greatly enhance your choices at a minimal cost. For example, say you have eyepieces of 30, 12, 8 and 5 mm focal lengths. That would give you four magnification choices. But if you add a 2X Barlow into the mix you would be adding the equivalent of having 16 mm, 6 mm, 4 mm and 2.5 mm eyepieces giving you 8 magnification choices. And a single Barlow would typically be much less expensive than four eyepieces.
Or use the Barlow for those magnifications that you don’t think you will be able to use very often. Many nights I can’t get above 200X on with my scope due to atmospheric conditions. An economic plan would be to buy eyepieces for 200x and below and to use a Barlow to provide those magnifications above 200x that I may not be able to use very often. If I find that 250X can be used often then I might add an eyepiece for that later, when I know it is justified. Or I can continue to use the Barlow.
I have three Barlows, 1.5X, 2X, and 2.5X. These allow me to use the same eyepieces in all 6 of my scopes which have focal lengths of 400 mm to 1900 mm. The Barlows let me optimize those eyepieces according to the focal length of each scope. I don’t use them all the time, but they are there to help me extend my reach when I need it or to fill in a gap when conditions make that advantageous.
Where can a Barlow lens be helpful?
- When you don’t have an eyepiece at the magnification you want to use.
- When you want to retain the long eye relief of a low power eyepiece at high power.
- When you want to stretch your eyepiece budget, effectively doubling your eyepiece set.
I usually recommend that new people on a tight budget incorporate a Barlow into their eyepiece strategy to provide maximum flexibility at the lowest cost. You can always add more eyepieces later, but if you have a Barlow, each of those eyepieces will provide two magnifications.