Classical Cassegrain Collimation

The general procedure to optimize a Cassegrain’s collimation is to check that the back eyepiece holder or drawtube axis coincides with the main optical tube axis, and then put the secondary, primary, and corrector plate (if present) successively into alignment with that axis. The single beam collimator allows eyepiece axis and secondary mirror alignment, but a holographic collimator is necessary for primary and corrector alignment.

The laser beam impact on lens and mirror surfaces is seen only by light scattered from tiny particles of dust and dirt, and optical roughness. In very clean, sealed systems, such as Schmidt and Maksutov Cassegrains, it may be difficult to see these impacts. In such cases, visibility can be improved by lowering the ambient lighting.

First, the centering of the eyepiece or drawtube fitting at the back of the telescope should be checked by slightly withdrawing the collimator and laying a yardstick on the outside of the tube. With a ruler or caliper, measure the distance between the collimator body’s outside diameter and the telescope tube’s outside diameter (as determined by the yardstick laid on the outside of the tube). Measure at 90° intervals around the tube. The measured distances should all be the same if the drawtube is centered in the optical tube. If it is not, some mechanical adjustment will be required (if possible with your scope). Consult your telescope’s manufacturer for instructions if such an adjustment is needed.

The alignment of the back axis can also be checked by measuring the beam centering within the tube. Cut a strip of cardboard the exact length needed to span the tube’s inside diameter and mark the exact center of the strip. Place it in the tube at the front cell and note if the laser beam falls on the center mark on the strip. Check with the strip placed both horizontally across the tube opening and vertically. The beam should fall on the center mark in both orientations if the optical axis is centered in the tube.

Next, the secondary should be similarly checked for centrality within the tube with a ruler or caliper. Once the secondary’s centering is confirmed or adjusted, the back can be checked for squareness by seeing if the laser beam strikes the center of the secondary. If you are using a single beam collimator with a Cassegrain that has a corrector plate, beam centering on the secondary must be judged by eye (viewed by reflection in the primary). This is not the best practice, but by moving your gaze in a circle around the secondary while checking centering, accuracy can be improved. With the holographic collimator you can see if the target pattern is centered on the secondary.

The next step is angular adjustment of the secondary so that the beam reflected from the center of the secondary is centered on the laser aperture on the face of the collimator. This sets the secondary square with the optical axis.

The holographic lens should be threaded into the collimator for the next step. The secondary will now reflect and project the holographic pattern upon the primary, and the primary should be centered within the pattern. The pattern is now reflected by the primary and projected in a collimated beam out the front of the telescope. The angular alignment of the primary is adjusted by projecting the pattern onto a surface or screen in front of the telescope and adjusting the primary to center the pattern on the shadow of the secondary. In the case of a corrector plate, if the pattern projected from the scope is undistorted, the corrector alignment is fine. If not, adjust the corrector to remove pattern distortion.