What is a barlow?

Monday, 12 November 2018

Quick answer? A Barlow lens fits between your telescope and eyepiece. It increases the magnification by its power.  E.g. a 2x Barlow will double the magnification of an eyepiece.

Barlow’s are handy addition to your kit as they double up the usefulness of each eyepiece you have.

Typical beginner telescopes come with 10mm and 25mm eyepieces, with a Barlow fitted these become 5mm and 12.5mm giving you four magnification options to choose between.

Interested to know more, than read on.

How do they work?

Lets get technical… Barlow’s contain a diverging lens which spreads the incoming light rays, effectively increasing the focal length of the optics up to that point which in turn has the effect of increasing the magnification.

Magnification = Telescope Focal Length / Eyepiece Focal Length

So doubling a telescope’s focal length with a Barlow, doubles the magnification.

This sounds to good to be true, what’s the catch?

If you use a good quality Barlow then the only real downside is a slight loss of light transmission – around 3% – due to the additional optical elements the light must now pass through.

Is more magnification good?

It depends what you want to look at!

For planetary and lunar observing more magnification can show finer details but often magnification is not the limiting factor – many astronomical objects are big but dim. Increasing magnification will dim the view even more what you really need is more aperture (or better skies).

The quality of your telescopes optics is also important,  over magnifying poor quality (dare I say, department store plastic!) optics with a Barlow will even make the image worse.

Local seeing conditions on a given night will also affect your views – if the seeing is turbulent then more magnification will only highlight this more.

How do I choose?

Determine the Barlow size you need by checking the size of your telescope focuser and eyepieces. Typically beginner scopes will have a 1.25″ focuser and can only use 1.25″ eyepiece and accessories, so you will need a 1.25″ Barlow.

If you have a 2″ focuser but use 1.25″ eyepieces then you can choose either size. A 1.25″ Barlow will be cheaper but a 2″ version can also be used with 1.25″ eyepieces so will future proof you should you decide to use 2″ eyepieces later.

If you use 2″ eyepieces then you will need a 2″ Barlow.

Tip: most 2″ Barlow’s come with a 2″ to 1.25″ adapter but if not they are readily available.

Once you know the size you need, decide on the level of power you want – typically 2x is a good place to start but 2.5x, 3x and even 5x Barlow’s are common. Anything over 2.5x is more commonly used for high magnification, Lunar, Solar or Planetary imaging.

Also consider the quality of the Barlow’s optics and its mechanical characteristics.  ED glass elements can help maintain brightness and contrast. Brass compression rings offer a secure eyepiece fitting without marring the eyepiece nose-piece through over tightening grub screws.

Other uses

As mentioned above, Barlow’s are often used for imaging and some Barlow’s come with T threads at the top to enable them to be directly attached to DSLR (with a suitable T ring) or CCD / CMOS cameras.

Some Barlow’s also have the ability to unscrew the optical elements which in some cases can fit directly to the bottom of eyepieces where a filter would usually attach, providing a lesser amount of magnification.

Once removed, the rest of the Barlow body can be used as an extension tube which can be useful for achieving focus with cameras in some situations.

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