Barlow Lens: How It Works and When It's Worth It
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A Barlow can double your eyepiece collection, as long as you pair it with the right eyepieces. Here's how it works, per Celestron and Tele Vue. Product details are from the listings; we haven't tested them.
How it works
Celestron's glossary describes a Barlow as "a diverging lens, which, when used with an eyepiece, can double or triple a telescope's focal length." Since magnification is focal length divided by eyepiece focal length, doubling the effective focal length doubles the power.
The catch
Tele Vue's Al Nagler warns that Barlows "allow even a short-focal-length telescope to achieve absurdly high magnifications," and points to the "600-power, 2.4-inch 'department store' telescope" as the classic example. Celestron's own example: a 4 mm eyepiece plus a 2x Barlow on a 650 mm scope gives 325x, past that scope's 307x useful limit.
Rule of thumb: Barlow your 25 mm and 15 mm, not your 6 mm.
A worked example
On an 8 inch Dobsonian at 1200 mm with the usual 25 mm and 10 mm eyepieces:
| Eyepiece | Alone | With 2x Barlow |
|---|---|---|
| 25 mm | 48x | 96x |
| 10 mm | 120x | 240x |
Our arithmetic. An 8 inch scope's ceiling is about 480x by Celestron's 60x-per-inch rule, so both are fine on paper; on most nights, the atmosphere will set the real limit.
2x Barlows compared
Celestron X-Cel LX 2x
Three-element apochromatic, fully multi-coated, with a brass compression ring and a filter-threaded barrel, per the listing.
Celestron X-Cel LX 2x Barlow (1.25 in.)
- Doubles the magnification of any 1.25 in. eyepiece, per the listing
- 3-element apochromatic, fully multi-coated
- Threaded for 1.25 in. filters
Celestron Omni 2x
Fully multi-coated, threaded barrel, and the listing says it keeps your eyepieces' eye relief.
Celestron Omni 2x Barlow (1.25 in.)
- Doubles each eyepiece's magnification, per the listing
- Keeps the eyepiece's eye relief
- Fully multi-coated; threaded barrel
SVBONY 2x
A metal 2x Barlow with an M42 thread that can double as a camera T-adapter with a separate T-ring, per the listing.
SVBONY 2x Barlow (1.25 in.)
- 2x Barlow for 1.25 in. eyepieces, per the listing
- Metal body with M42 thread for a camera T-ring (sold separately)
Or get one in a kit
Celestron's 1.25 inch accessory kit includes a 2x Barlow plus five eyepieces and filters. See telescope eyepieces.
Celestron Eyepiece, Barlow & Filter Kit (1.25 in.)
- 5 Plossl eyepieces: 32, 17, 13, 8 and 6 mm, per the listing
- 2x Barlow lens
- Moon filter plus colored planetary filters
- Foam-lined hard case
FAQ
What does a Barlow do?
Doubles or triples the effective focal length.
Can it give too much power?
Yes; keep under ~60x per inch.
2x or 3x?
Do the math for your eyepieces.