The Davidson Optronics D-691 Laser Alignment Autocollimator is designed to be an economical alignment autocollimator that can be mounted permanently or for portable use. With its Wide 1-degree field and the ability to null up to 5 seconds of arc, as well as its 90-degree eyepiece this compact telescope fills many niches. It is a Laser Alignment Autocollimator due to its ability to autocollimate on small reflectors (such as those in Laser Tubes). The barrel diameter is 1.500 inches, and the aperture is 0.9".
I believe the NSN for this is: 4931-01-150-9565