Barlowed laser collimation is a technique to very precisely adjust the angular alignment or collimation of the Newtonian primary mirror. Normally, a telescope takes parallel light rays from a distant star and converges them to a point at the eyepiece focus. Barlowed laser collimation takes advantage of the fact that a telescope will work in reverse.
Placing a laser collimator into a Barlow lens in the focuser will cause the parallel rays of the laser beam to diverge, apparently from a point just behind the Barlow lens. The diverging rays projected from the laser/Barlow combination are turned into a beam of all-parallel rays when they are reflected back from the primary mirror, except for where a center mark on the primary prevents the mirror from reflecting. This reflected beam, containing a superimposed shadow of the primary’s center mark, is projected up to the diagonal mirror, and then reflected out to the laser collimator in the focuser.
The tilt of the primary mirror is aligned by adjusting the primary mirror’s collimation screws to center the shadow of the primary mirror mark around the central laser aperture in the face of the collimator. The adjustment is insensitive to inaccuracy or “slop” in the fit of the collimator in the focuser. The position of the shadow on the screen is effected very little by motion of the illuminating beam. It is almost startling to see the shadow remain stationary as you “bend” the collimator and Barlow around in the focuser.
This unique and inexpensive Barlow lens screws into the laser aperture of the Glatter holographic laser collimator for making the primary mirror adjustment. The Barlow attachment is a disc with a small Barlow lens mounted in its center hole, and a flat white front surface as a screen. It makes Barlowed laser collimation procedure quicker and more convenient than any other collimation method. The Barlow cannot be used for visual observing.
The primary mirror mark referred to above is a self-adhesive “donut” placed on the center of your primary as an aid to precise beam centering. Putting a donut in the center of the mirror does not affect the optical performance of the scope, as the center of the mirror is not in use because it is in the shadow of the diagonal mirror. Self-adhesive collimation “donuts” are supplied with all Glatter collimators, along with instructions for their safe placement.
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