When it comes to the use of Barlow lenses, telescope users are divided. Some say they’re great to use, others say they’re unnecessary. Here are the pros and cons of using them for your telescope.
What is a Barlow Lens?
A Barlow lens is not an eyepiece, but it is a special lens that when placed in between the optics and the eyepiece, it increases the magnification of that eyepiece. Depending on the lens, it can double, triple, and even quadruple or quintuple the magnification!
How it works is they are inserted into the eyepiece barrel first, and then the eyepiece itself is placed on top of it into a special barrel.
Pro – They Essentially Double Your Eyepiece Collection
Telescopes that accept standard 1.25″ eyepieces can accept any 1.25″ Barlow! Any eyepiece in your collection can be paired with the Barlow, which means your eyepiece collection has doubled.
Barlows virtually change the focal length of a telescope, thus a 1000 mm telescope is now virtually a 2000 mm focal length scope. That means you can say you have “two telescopes in one.”
Con – They Won’t Be Needed As Often
It just depends on what you’re observing.
Barlows can be a treat to use to get high magnification views of the Moon and planets, and perhaps used to resolve a few double stars, but they will more than likely not be used when viewing dimmer deep sky objects.
Another factor is of course the atmosphere, which when the seeing conditions are bad, then using them for higher magnification becomes pointless due to the images being distorted.
Pro – They Can Give High Magnification Views in a Lower Power Eyepiece
Remember that the wider the mm of the eyepiece, the lower the magnification.
A 2x Barlow lens will double the magnification of any eyepiece you use, so if you’re using a 20 mm eyepiece, it makes the eyepiece have the same power as a 10 mm eyepiece. If your 20 mm eyepiece gives you 50x magnification, then using the Barlow will give it 100x.
This comes in handy especially for people who wear glasses, as it’s much easier to look into a wider eyepiece than it is into a narrow, and the wearer still gets the views they desire.
Con – The Same Views Can Simply Be Achieved with the Correct Eyepiece
It is easier to insert a single eyepiece into the barrel than to insert a Barlow into the barrel and then the eyepiece on top of it. A lot of users don’t like messing around with changing magnification too much.
While a Barlow can increase the magnification, so can simply switching to a higher powered eyepiece. On a 1000 mm focal length telescope, a 10 mm gets you 100x, an 8 mm gets you 125x, and a 6 mm gets you 150x. But when you’re using a refractor or an SCT that has longer focal lengths well over 1000 mm, then those same eyepieces are already giving you higher magnification views and you don’t really need a Barlow lens to double them.
Pro – Barlow Lenses Can be Stacked
Let’s say you happen to have more than one Barlow, whether it being by separate purchase or from buying kits that included them. You might have a couple 2x Barlow lenses on your hands, but you see that attractive looking expensive 4x lens on the market. Good news, you can stack two 2x Barlow lenses and get the same 4x power!
Stacking Barlows doesn’t add the factors, it multiples them – thus stacking a 2x with a 3x gets you 6x. Now, on your 1000 mm focal length scope, with a 25 mm eyepiece (40x) paired with that 6x stack, you’re getting a 250x magnification view!
I myself have stacked two 2x’s and a 3x to get a factor of 12x, which pushed my telescope to the limit!
Con – They Add Too Much Bulk and Have Narrow Views
Even when just using a 1.25″ shorty 2x Barlow, you’ll notice that it makes the eyepiece you’re using appear to stick out of the tube like a cancerous tumor. 3x’s and higher usually have longer barrels, and thus stick out more. When you’re stacking them, or simply using larger 2″ eyepieces, then they are even longer!
This can not only throw the telescope off balance and put more strain on your focuser, but the views through the longer barrels are narrow, and your eye needs to be near perfectly aligned to see the object, otherwise all you’re seeing is blank.
When viewing the Moon, the longer barrels can reflect the bright moonlight, giving a tough glare to view around.
These are the main pros and cons of using a Barlow. If you ever do decide to get one, make sure it’s not a cheap plastic Barlow. The 4 element Plossl Barlows are a good way to go and they’re not too expensive.
The main reason why I personally recommend them is that in my own telescope, I would not get these views and these following images without the use of Barlow lenses. These were both taken using a 3x Barlow and high powered eyepiece.