Refractor Collimation

CAUTION: The Class IIIa laser in most collimators can cause eye damage if the laser beam is focused on the same area of the retina for as little as 0.25 second. Direct eye exposure should be avoided! With a refractor, the laser beam will always exit the front of the telescope, so run a wide strip of masking tape across the diameter of the lens cell opening to act as a safety beam stop to prevent direct eye exposure.

For optimized collimation of a refractor, the focuser drawtube must first be aligned so that the laser beam passes through the center of the objective lens. Place the collimator directly in the drawtube, without using a diagonal prism or mirror and turn it on. The laser beam will be visible as a red spot on the masking tape safety stop. Check the alignment of the drawtube axis by seeing if the beam passes through the center of the objective lens. With a single beam collimator you can cut a paper circle the same size as the front cell opening, and punch a small hole in its center. Place the paper circle over the lens cell and see if the red dot of the laser beam on the masking tape falls directly under the hole in the paper. Centering is also easily checked by holding a ruler across the front cell opening, so you can measure the distances from the red spot on the masking tape to the edge of the lens cell. Check the centering both horizontally and vertically.

With a holographic collimator, you can project the grid pattern on a wall. The centering of the beam can be checked directly by the symmetry of the pattern at the edges of the objective. If the drawtube alignment is off, it must be corrected before proceeding with collimation. Very few refractors have adjustments for this, so usually drawtube misalignment must be fixed by telescope mechanic work on the drawtube, focuser, or tailpiece. Shimming, filing, or machining may be required.

The next step is to adjust the angular alignment of the objective so that the beam from a single beam collimator, which is reflected from the exact center of the rear surface of the objective, is returned to the collimator face and centered on the laser aperture. The holographic collimator should be put in single beam mode for this, so that the diffracted pattern does not annoyingly interfere with viewing the reflected beam impact on the collimator face when viewed off-axis through the objective. Removing the hologram lens will also make the central beam brighter, which will be helpful. Enough light is reflected even from an anti-reflection coated lens surface to perform this adjustment.

Some light will be reflected from each lens surface within the objective, although the reflection from the rear surface may be brightest. If the elements of the objective are accurately centered and collimated with respect to each other and have no wedge, all the reflections will coincide, forming one reflected spot. If there are multiple reflected spots, it is an indication of alignment problems within the objective. The collimator could then be used to diagnose and correct the inter-element misalignment using the lens cell’s collimation screws.