Barlow lenses and Focal Reducers

Barlow lenses:

Barlows, sometimes called telenegative lenses, are a relatively low cost way to multiply your range of magnification with a given set of eyepieces. Barlow lenses most commonly provide a magnification factor of 2, but range from in magnification from 1.8x to 5x. To obtain an equivalent eyepiece focal length you divide the actual eyepiece focal length by the magnification factor of the barlow. Thus a 26mm eyepiece with a 2x barlow would result in the same magnification as a 13mm eyepiece. There are both positive and negative benefits to using a barlow. On the positive side you can make two eyepieces perform like four with the addition on one piece of hardware. Also, for eyeglass wearers, although magnification is increased there is no reduction in the eye relief of a given eyepiece, whereas switching to a short focal length eyepiece will usually reduce the eye relief. On the negative side, you must choose a barlow and your lenses carefully. If you have a 25mm and 12.5 mm eyepiece and you purchase a 2x barlow you haven't gained much more than buying a new eyepiece, and generally a barlow-eyepiece combination will have more optical aberrations than an eyepiece alone.

On the other hand barlow lenses reduce the brightness of an object since the magnification is increasing substantially. Although this is the same with an eyepiece giving equivalent magnification, the extra lenses with their subsequent light losses can result in faint objects not being visible at all when you add a barlow, especially one with a small aperture or without well coated lenses. So, as with many things, barlows are a tradeoff. If you have a telescope that is more than minimal in diameter and a few low power eyepieces a barlow can be a very good thing. If you have a 60mm refractor and try to use a .965” diameter 3x barlow, the result will be very disappointing. The prices below reflect a range of better barlow lenses, with the top end of the 1 ¼” line reserved for high quality, high magnification barlows such as the Televue PowerMate.

1 ¼”	2”
$20-120	$50-100

Focal reducers:

These lenses provide a second way to increase the range of your eyepieces. They reduce the magnification and increase the field of view. They are especially handy if you have a long focus refractor, Schmidt Cassegrain or Maksutov telescope. Focal reducers are also an important accessory for astrophotography, since they also reduce exposure times. Generally the reduction in focal length is from one third (0.65) to one half (0.5). Similar to the barlows described above, a given eyepiece focal length is divided by the magnification factor to arrive at an equivalent eyepiece focal length. Thus the 26mm eyepiece with a 0.5 focal reducer would result in an image similar to that of a 52mm eyepiece. Although most units are designed for SCTs, there are a few reducers available for threading into 2" and 1 1/4" eyepieces as well. The price range depends on the quality of the coatings, and whether the lenses are also designed to flatten the field. The latter units are the more pricey.

SCT	1 ¼”	2”
$40-80	$30-60	$40-80