Are you using a Barlow lens to get more value and versatility from your eyepieces?
As backyarder astronomers, we have a big interest in equipment that makes our stargazing better without burning a hole in our pockets! The humble Barlow lens is a great example of such a piece of kit.
It’s an essential piece of equipment every astronomer should have but, if you don’t know much about it, then this article will explain everything you need to know.
Best Barlow Lens – Quick Comparison
What is a Barlow Lens?
To put it simply, Barlow lenses are a cost-effective way to increase the magnification of your eyepieces.
The Barlow lens was invented in the 19th century by Peter Barlow, A mathematician, optician, astronomer and a member of the Royal Society of London.
They contain a concave lens that is placed between your telescope’s objective lens or mirror and eyepiece. (Actually, they contain more than one glass element to reduce chromatic aberration).
Their effect is to increase the magnification of any eyepiece used with them, usually 2 or 3 times. As you’d expect, a 2x Barlow doubles your eyepiece magnification, whilst a 3x trebles it.
Let’s work through an example:
Using a 20mm eyepiece on a telescope with a 1000mm focal length gives 50x magnification (1000mm / 20mm). Attach a 2x Barlow lens to that same eyepiece and you double its magnification to 100x!
How do Barlow Lenses Work?
You might know that the magnification of a telescope is the focal length of objective divided by the focal length of the eyepiece.
Therefore, to increase the overall magnification, we can do either (or both) of the following:
- Increase the focal length of the objective
- Decrease the focal length of the eyepiece
Barlow lenses do the first of these, i.e. they increase the effective focal length of the telescope.
The diagram shows an effective increase in telescope focal length – the yellow light cone’s point is further along from the ‘diverging lens’ than the gray light cone’s point. The longer focal length of the yellow light cone is due to the impact of the Barlow lens.
Choosing and Using a Barlow
One thing to keep in mind while buying a Barlow lens is that if it fits the tube of your eyepiece. Most tubes come in one of two standard sizes, either 1.25 or 2 inch, so finding the one that fits won’t be a problem. You just need to make sure to get the right one.
Barlow lenses come in different magnifications. The most common are 2x, but lenses that offer 3x or 5x are also available.
We recommend going with 2x for most users (see why later).
When you’ve picked one (using our Best Barlow Lens guide) you’ll be pleased to find they are very easy to use.
Instead of inserting an eyepiece into the focuser, you place in the Barlow lens. Your eyepiece is then inserted into the Barlow, like the example, left, from Celestron.
This video by David Fuller of Eyes on the Sky explains Barlow lenses in detail, including how to use them, and the types of Barlow which are available:
It’s that simple to enjoy the beauty of the sky with more magnification!
How do Barlows Differ from Zoom Lenses?
Barlow lenses are often confused with zoom lenses – and for a good reason; they pretty much do the same thing.
But, there are a few differences:
Zoom lenses provide a variable amount of magnification, whereas Barlow lenses have a fixed magnification level. Also, Barlow lenses generally have a smaller field of view than zoom eyepieces.
The lenses used in SLR and DSLR cameras to vary the focal length (and hence zoom) are zoom lenses. Given below is an image taken while varying the focal length of a camera using a zoom lens:
Their changing focal length makes it harder to correct for aberrations, so zoom lenses have a combination of at least 4 glass elements. This makes them bigger and heavier than Barlow lenses.
The convenience of a zoom lens variable focal length comes with a compromise on either image quality or price.
We believe you’ll be better off with a decent Barlow lens most of the time, especially early on in your stargazing career, when simplicity and cost are big concerns.
Barlow Lens Pros and Cons
A Barlow lens is a tremendously useful instrument to have in your collection. In fact, we think they are an essential piece of equipment for every backyard astronomer!
However, they do have shortcomings you should be aware of.
Lower-priced Barlows have fewer or lower quality lenses. These can show minor astigmatism and spurious color at the field edges with fast (smaller than f/6) focal ratio scopes.
But, since a Barlow is normally used to look at objects at high power in the center of the field (where the image is unaffected), this is not a dealbreaker.
Earlier in the article, we recommended you choose a 2x Barlow.
The reason we do is because more powerful Barlows don’t work well with smaller telescopes. Small scopes don’t pull in much light, so powerful Barlows deliver dim, disappointing images.
All that said, if you have a large scope, say 8″ plus, then a 3x Barlow might be a good investment for you.
Our how telescopes work article explains the detail of why this happens.
Recommended Barlow Lenses
With your Barlow lens, a vast visual feast awaits you!
Having a Barlow lens not only doubles the magnification, it doubles your collection of eyepieces as well!
For instance, if you have a 30mm, 24mm and 10mm eyepiece, having a Barlow in your collection effectively adds a 15mm, 12mm and 5mm eyepiece to your collection. Since one decent Barlow lens is much cheaper than three new eyepieces, it is really cost effective!
One final advantage is that Barlows do not change the longer eye relief typical of lower power eyepieces (in fact, they actually increase it some in some cases). This allows eyeglass wearers to see a full field of view at high power without removing their glasses.
If you want to discover more about Barlow Lenses, check out these helpful links:
- Cloudy Nights forum discussion on Barlow lenses v small eyepieces
- Cloudy Nights user review of a Barlow lens
- Technical reading on how Barlows work
- Wikipedia Barlow lens page
Last update on 2020-11-24 / Affiliate links / Images from Amazon Product Advertising API