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Choosing a Barlow lens for your telescope
A Barlow lens sits between the telescope and the eyepiece and multiplies the magnification, effectively doubling your eyepiece collection with a single accessory.
What a Barlow does
A Barlow is a diverging lens that extends the effective focal length of the telescope. A 2x Barlow doubles the magnification an eyepiece would otherwise give, and a 3x triples it. Because it multiplies rather than replaces, a 2x Barlow paired with your existing eyepieces adds a second high-power set without buying new short eyepieces.
For example, on a 1200 millimetre telescope a 10 millimetre eyepiece gives 120 times; a 2x Barlow in front of it gives 240 times, the same as a 5 millimetre eyepiece would.
Barrel size and fit
Barlows come in 1.25 inch and 2 inch barrels to match your focuser and eyepieces. A 1.25 inch Barlow accepts 1.25 inch eyepieces; a 2 inch Barlow suits a 2 inch focuser and can take a 1.25 inch eyepiece through an adapter. Match the Barlow to the barrel size you already run.
Some Barlows let you unthread the lens cell and screw it directly onto an eyepiece, which reduces the multiplication factor and shortens the light path.
Respecting the magnification limit
A Barlow does not raise the useful magnification ceiling, which is set by aperture and is roughly two times the aperture in millimetres. Barlowing a short eyepiece past that limit gives a larger but dimmer, softer image rather than more detail. Use a Barlow to reach the top of the useful range, not to exceed it.
A quality Barlow adds little to no visible aberration; a poor one degrades the view, so with a fast telescope in particular the optical quality of the Barlow matters.
When a Barlow is worth it
A single good 2x Barlow is one of the most cost-effective accessories, because it fills the high-power gaps in a modest eyepiece set. It is especially useful for planetary and lunar viewing, where high magnification on nights of steady air reveals fine detail.
Match a Barlow to your telescope and eyepieces
Open the Fit My Telescope gear finderPick your telescope to see suitable Barlows and the powers they reach.Find 2x barlow lens telescope on eBayaffiliateRelated
Sources: Celestron Barlow lens documentation ยท Tele Vue Barlow and Powermate reference