Alignment (Collimation) of the Newtonian Reflector Optical System
The optical systems of Newtonian Reflector telescopes include the following parts: pri- mary mirror (1, Fig. 34); secondary mirror (2, Fig. 34); secondary mirror-holder (3, Fig. 34); secondary mirror-vanes (4, Fig. 34) and (1, Fig. 35); primary mirror-tilt screws (5, Fig. 34). The telescope’s image is brought to a focus at (6, Fig. 34).
1.Confirm alignment - To confirm optical alignment look down the focuser draw- tube (1, Fig. 37) with the eyepiece removed. The edge of the focuser drawtube frames reflections of the primary mirror (2, Fig. 37), the secondary mirror (3, Fig. 37), the three (“spider”) vanes (4, Fig. 37) holding the secondary mirror, and the observer’s eye (5, Fig. 37). With the optics properly aligned, all of these reflec- tions appear concentric (centered), as shown in Fig. 37. Any deviation from con- centricity of any of these telescope parts with the eye requires adjustments to the secondary mirror-holder (Fig. 35) and/or the primary mirror cell (Fig. 36, as described below.
2.Secondary mirror-vane adjustments: If the secondary mirror (1, Fig. 38) is left or right of center within the drawtube (2, Fig. 38), slightly loosen the 3-vane adjust- ment/lock knobs (1, Fig. 35) located on the outside surface of the main tube and slide the entire secondary mirror-holder system up or down in the slotted holes of the main tube until the secondary mirror is centered in the drawtube. If the secondary mirror (1, Fig. 38) is above- or below-center within the drawtube, thread inward one of the adjust- ment/lock knobs (1, Fig. 35) while unthreading another of these knobs. Only make adjustments to two knobs at a time until the secondary mirror appears as in Fig. 39.
3.Secondary mirror-holder adjustments: If the secondary mirror (1, Fig. 39) is cen- tered in the focuser drawtube (2, Fig. 39), but the primary mirror is only partially vis- ible in the reflection (3, Fig. 39), the three “+” (Phillips head) secondary mirror-tilt screws (2, Fig. 35) should be slightly unthreaded to the point where the secondary mirror-holder (3, Fig. 35) can rotate about its axis parallel to the main tube. Grasp the secondary mirror-holder (avoid touching the mirror surface!) with your hand and rotate it until, looking through the drawtube, you can see the primary mirror centered as well as possible in the reflection of the secondary mirror. With the rotation of the secondary mirror-holder at this best-possible position, thread in the three Phillips head screws (2, Fig. 35) to lock the rotational position. Then, if necessary, make adjustments to these three Phillips head screws to refine the tilt-angle of the second- ary mirror, until the entire primary mirror can be seen centered within the secondary mirror’s reflection. With the secondary mirror thus aligned the image through the drawtube appears as in Fig. 40.
4.Primary mirror adjustments: If the secondary mirror (1, Fig. 40) and the reflection of the primary mirror (2, Fig. 40) appear centered within the drawtube (3, Fig. 40), but the reflection of your eye and the reflection of the secondary mirror (4, Fig. 40) appear off-center, then the primary mirror tilt requires adjusting, using the Phillips head screws of the primary mirror cell (3, Fig. 36). These primary mirror-tilt screws are located behind the primary mirror, at the lower end of the main tube. See Fig. 36. Before adjusting the primary mirror-tilt screws, first unscrew by several turns (use either a hex wrench or pliers) the three hex-head primary mirror lock screws (2, Fig. 36) which are also located on the rear surface of the primary mirror cell and which alternate around the cell’s circumference with the three Phillips head screws. Then by trial and error turn the primary mirror Phillips head tilt screws (3, Fig. 36) until you develop a feel for which way to turn each screw to center the reflection of your eye in the drawtube. (An assistant is helpful in this operation.) With your eye centered as shown in Fig. 37, turn the three hex head primary mirror lock screws (2, Fig. 36) to re-lock the tilt-angle of the primary mirror.
The telescope’s optical system is now aligned, or collimated. This collimation should be re-checked from time to time, with small adjustments (per steps 1, 2, and/or 3, above) effected as required to keep the optics well-aligned.
