A Barlow is a great tool for those interested in planetary astroimaging with a Newtonian telescope and a DSLR. Typically, the T-adapter needed to mount a DSLR in a Newtonian telescope focuser will space the camera too far out, and the focuser lacks the inward travel needed to focus the camera. Adding a Barlow, such as a Celestron Barlow T-adapter, moves the focus point outward, allowing the camera to reach focus.
Today’s advanced planetary photographers use a 1.25-inch barrel planetary camera, specifically designed to be inserted into a 1.25-inch telescope focuser or Barlow lens. A dedicated planetary camera captures a several-thousand-frame video, which is then processed using the free RegiStax or AutoStakkert programs to stack the best video frames into a high-resolution single image.
Lunar and planetary photography achieves high magnification solely using the telescope's focal length, not an eyepiece. An 8-inch Schmidt-Cassegrain has a native focal length of 2,000 millimeters. At this focal length, the inherent small imaging sensor in planetary cameras produces a pleasing lunar closeup that covers the area of a typical lunar mare, or the dark lunar “seas” that create the caricature of the Man-in-the-Moon.
For detailed close-ups of lunar craters or any of the planets, increased focal length is needed. A 2x Barlow will increase the 8-inch Schmidt-Cassegrain focal length to 4,000 millimeters, and a 3x Barlow will yield a 6,000 millimeter focal length, sufficient to reveal good planetary detail if the atmosphere is steady. The longer focal lengths will require a correspondingly longer exposure, but the high sensitivity of today’s planetary cameras allows recording videos at up to 60 frames per second.