Astro-physics Photo-Visual Barlow Amplifier (includes part ADA202).

Use this two-part accessory in several configurations to increase the focal length of the objective for high-power visual observation and lunar-planetary photography. We do not recommend the Barlow for deep-sky photos when using a scope that has a fast focal ratio. These photos will exhibit significant field curvature, which will limit the usable portion of the negative.

The 2-element design uses special glasses to preserve the fine color correction of the main objective. The optical elements are hand-corrected and precision centered to insure that no aberrations are introduced into the system. The Barlow focal length is 127 mm which is longer than conventional types. This minimizes the divergence of the edge rays and therefore reduces the apparent vignetting common in many eyepieces (Plossls and Panoptics). The Barlow features a brass locking ring as described for the 2.7" focuser

Note the following positions and resulting magnifications:

1.7x after the diagonal2x before the diagonal or camera adapter