How do I collimate my Newtonian – aligning the mirrors in daylight?
October 8, 2008
Collimation of a Newtonian
The optical performance of most Newtonian reflecting telescopes can be optimized by re-collimating (aligning) the telescope's optics, as needed. Before collimating your telescope, take time to familiarize yourself with all its components. The primary mirror is the large mirror at the back end of the telescope tube. This mirror is adjusted by loosening and tightening the three screws, placed 120 degrees apart, at the end of the telescope tube. The secondary mirror (the small, elliptical mirror under the focuser, in the front of the tube) also has three adjustment screws. You will need an optional tool to perform collimation. To determine if your telescope needs collimation first point your telescope toward a bright wall or blue sky outside. Look on the back end of your scope (away from the eyepiece) where the main mirror is located. The collimation screws are paired with locking screws (which act to hold the mirror in place to maintain collimation) for a total of six screws.
The most (clockwise) screw for each pair (as seen from behind the mirror) is the collimation screw. You can verify this by looking into the tube from the front. The exposed screws around the mirror cell will turn as you turn them from behind the mirror with a screwdriver.
Always loosen the locking screws first before turning any of the collimation screws. You may need to back out the collimation screws if turning one screw doesn’t give you enough travel to properly center the image of your scope’s secondary mirror.
When you are finished adjusting the collimation screws, tighten the locking screws to hold the mirror in place.
Aligning the Secondary Mirror
If you have an eyepiece in the focuser, remove it. Obtain a collimating tool for Newtonians, often called a collimating cap. Rack the focuser tube in completely, using the focusing knobs, until its silver tube is no longer visible. You will be looking through the focuser at a reflection of the secondary mirror, projected from the primary mirror. During this step, ignore the silhouetted reflection from the primary mirror. Insert the collimating cap into the focuser and look through it. With the focus pulled in all the way, you should be able to see the entire primary mirror reflected in the secondary mirror. If the primary mirror is not centered in the secondary mirror, adjust the secondary mirror screws by alternately tightening and loosening them until the periphery of the primary mirror is centered in your view. Do not loosen or tighten the center screw in the secondary mirror support, because it maintains proper mirror position.
Aligning the Primary Mirror
Now adjust the primary mirror screws to re-center the reflection of the small secondary mirror, so it’s silhouetted against the view of the primary. As you look into the focuser, silhouettes of the mirrors should look concentric. Repeat steps one and two until you have achieved this. Remove the collimating cap and look into the focuser, where you should see the reflection of your eye in the secondary mirror.