A Barlow lens is an optical instrument used to increase the magnification of an eyepiece for visual astronomy or extend the focal length of a telescope for astrophotography. It is named after Peter Barlow, an English physicist and mathematician who invented it in the 19th century. A Barlow lens is a diverging lens that increases the effective focal length of an optical system as perceived by all components that are after it in the system. This results in magnification of the image. A real Barlow lens is not a single glass element, because that would generate chromatic aberration and spherical aberration if the lens is not aspheric. More common configurations use three or more elements for achromatic correction or apochromatic correction and higher image quality.
A standard Barlow lens is housed in a tube that is one Barlow focal-length long, so that a focusing lens inserted into the end of the tube will be separated from the Barlow lens at the other end by one Barlow focal-length, and hence produce a 2x magnification over and above what the eyepiece would have produced alone. Barlow lenses come in different magnifications, with the most common being 2x, but lenses that offer 3x or 5x are also available.
Barlow lenses have several benefits, including:
- Increased magnification: A Barlow lens can double the magnification of a view.
- Maintained eye relief: Many high-powered eyepieces do not offer a great deal of eye relief, requiring the user to place their eye very close to the eyepiece. A Barlow, paired with a lower magnification eyepiece, will allow for greater eye relief for the user, which can be particularly beneficial for glasses wearers or children.
- Cost-effective: A Barlow lens is a cost-effective way to increase the magnification of a telescope.
When choosing a Barlow lens, it is important to ensure that it fits the tube of the eyepiece. Most tubes come in one of two standard sizes, either 1.25 or 2 inches.
