The self-Barlow attachment is a disc with a white screen on the front and a small built-in Barlow lens in the center of the disc. It is then screwed into the Howie Glatter laser collimator (sold separately) aperture and provides a highly compact method for Barlow-collimating.
As with regular Barlowed laser collimation, the shadow is projected onto the screen at the front of the disc at the focuser end so adjusting the mirror at the back of the scope can be difficult in some cases, especially with reflectors having a long focal length.
This is the new version ("NV") with coarse thread. The attachment fits all Howie Glatter/Starlight Instruments laser collimators, which also have a new thread ("NV" in the item number). If you own an older version of the laser (before 2020), please contact us prior to your order!