Advice On Using A Barlow Lens

barlow lens
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Is a Barlow lens worth it? Why not just use a few favorite eyepieces? Let’s look at why or why not invest in a Barlow lens.

What Does A Barlow Lens Do?

Barlow lenses increase the magnification and virtually extend your eyepiece collection. They are telenegative amplifiers, meaning diverging lenses that magnify.

They effectively decrease the eyepiece’s focal length by the amount of the Barlow’s divergence (or how much it spreads the incoming light).

This increases magnification since magnification equals the telescope’s focal length divided by the eyepiece’s focal length.

The most common Barlow lens included when you purchase a telescope for beginners is the 2x.

What Is A 2x Barlow lens?

A 2x Barlow is a negative lens that doubles the magnification of your eyepieces.

So, if you have three eyepieces, say with focal lengths of 10, 16, and 25 mm, by adding a 2x Barlow lens you get the experience of six, i.e. 5, 8, 12 mm as well as 10, 16 and 25 mm.

Remember the smaller the focal length the higher the magnification.

Similarly, a 3x Barlow triples the magnification, a 4x quadruples it, and so on.

How Do You Fit A Barlow Lens

On a telescope, you fit the Barlow lens into the eyepiece holder immediately before the eyepiece. If you are using an extension you place it between the Barlow lens and the eyepiece.

where to place Barlow lens, telescope
Diagram showing placement of a Barlow lens on a telescope

Where do I place a Barlow lens if using a star diagonal?

You can place the star diagonal ahead or behind the Barlow lens.

Placing the star diagonal ahead of the Barlow lens will not affect the focal length or the double magnification you expect with a Barlow lens.

Placing the star diagonal behind (between the Barlow lens and the eyepiece) will increase the magnification, but you may need to adjust for spherical aberration.

How Do You Choose A Barlow Lens?

Look for quality. Eyepieces can come and go, but it is worth getting a good quality Barlow lens.

It depends on your eyepieces whether you choose a 2x, a 3x, or 5x. Adjustable Barlows are also available where power is increased by adding an extension tube between the Barlow and the eyepiece.

I cover what to expect with magnifications in my article on how to get the best eyepiece collection.

The best is a fully multi-coated lens.

Two types are used, shorty and long versions.

The shorty suits reflectors and the longer Barlow refractors, in general speak.

What Is A Shorty Barlow lens?

A shorty Barlow is a compact lens; hence its name. It has a short barrel of 1.25-inch diameter and 1.6-inches in length.

End to end, its length is 3-inches, which is about half that of the standard Barlow lens.

Can I Use 2 Barlow Lens Together?

You can pair them up. But of course, you need to ensure what you are looking for before venturing deep.

Why Use A Barlow Lens?

Here are four common reasons…

  • Increased magnifications. This gets around the aberrations of having to use short focal length eyepieces that have curved optical surfaces.
  • Increased focal ratio. This improves the focus. Great in cases where eyepieces with short focal-ratio objectives do not work well.
  • A more acute light cone. A higher effective focal ratio of the eyepiece objective means less demand on the eyepiece quality.
    • Cone peripheral rays are more paraxial and less subject to aberration, and
    • There’s a smaller area of the field lens.
  • Better eye relief, since the eye relief of eyepieces typically relates to eyepiece focal length.

The one main disadvantage of using a Barlow lens is slight loss (around 3%) of light.

Jeff Madden

Hi, Jeff Madden here. I'm an enthusiast of the wide-open spaces with over 15 years of experience in using spatial data. My work here is about helping others with outdoor technology.

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