What specifications should you look for in a Barlow lens?

There are only a few specifications you need to keep an eye on when it comes to Barlow lenses in order to differentiate the better ones from the rest. Here’s what the specifications mean.

• Barrel size. This can be 1.25″ or 2″. You will need to select the one that fits your telescope. If you don’t know which one you have, your eyepieces probably have it on their label. It is the same.
• Power. This is the magnification multiplier you will get from using the Barlow. It is generally in the name and it can be 1.25x, 2x, 3x, 5x, etc.
• Multicoating. Just like in glasses or any other type of lenses, multiple coatings reduce light reflections.
• Blackened edges. This will darken the edges around the image, increasing the contrast.
• Achromatic. This means the lens is designed to reduce the effects of color aberrations/filters. This means your image will not look green/yellow/blue-ish. This is usually not listed in the specifications as it should be a given, but some Barlows are better at it than others.

 

How to choose a Barlow lens

Here’s the recommended process to follow when you are shopping for a Barlow lens.

If you already have a telescope, make a list of the eyepieces you already own. If you are only planning to buy one, find out the size of the eyepieces that come bundled with it.

Next, calculate the focal lengths and magnification values you would have access to with Barlow lenses of different rates and compare them to the maximum and minimum magnification your telescope supports. These are generally listed in the telescope’s specifications.

 

Let’s take a look at an example so all this is easier to understand.

Example: Let’s say you bought a mid-range Orion 130 telescope that comes with two eyepieces: 25mm and 10mm. The highest useful magnification for this telescope is 260x, and the minimum is 19x according to the manufacturer. The focal length of the telescope is 650mm. This means we have access to the following magnifications with the included eyepieces

If you remember from earlier, the formula for magnification is as follows:

magnification = telescope focal length / eyepiece focal length

25mm eyepiece	26x magnification
10mm eyepiece	65x magnification

Since we know we can go between 19x and 260x, we can see there’s a lot of room to go up in magnification while our low magnification needs are already met as we are close enough to the minimum with the 26x we get from the 25mm eyepiece. This is a perfect case of when a Barlow lens can add lots of value.

Adding a single 2x Barlow lens to the mix, we now would have access to the following magnifications:

25mm eyepiece	26x magnification
25mm eyepiece + 2x Barlow	52x magnification
10mm eyepiece	65x magnification
10mm eyepiece + 2x Barlow	130x magnification

That covers a lot more ranges, but for lunar and planetary viewing we can probably still go higher. Let’s try a 3x Barlow.

25mm eyepiece	26x magnification
25mm eyepiece + 2x Barlow	52x magnification
25mm eyepiece + 3x Barlow	78x magnification
10mm eyepiece	65x magnification
10mm eyepiece + 2x Barlow	130x magnification
10mm eyepiece + 3x Barlow	195x magnification

Hmm…the 78x magnification is a bit too close to the 65x we are getting from the 10mm eyepiece alone. It wouldn’t make much of a difference there, so it seems like we would only purchase this to use it with the 10mm which doesn’t feel good.

Instead, let’s scratch that 3x Barlow and let’s consider adding another eyepiece, but since we are planning on getting the 2x Barlow, remember the value of every additional eyepieces we buy doubles as we get access to two new magnification values.

Let’s try a 6mm eyepiece

25mm eyepiece	26x magnification
25mm eyepiece + 2x Barlow	52x magnification
10mm eyepiece	65x magnification
10mm eyepiece + 2x Barlow	130x magnification
6mm eyepiece	108x magnification
6mm eyepiece + 2x Barlow	216x magnification

Now we are talking. we have covered that large gap between the 65x and 130x magnification and we are much closer to our highest useful magnification of 260x.

So, in conclusion, for this telescope, it would be recommended to get a 2x Barlow lens and add a 6mm or even a 5mm eyepiece.