Orion #8296 10" F/3.9, #8297 8" F/3.9 Adjusting the Secondary Mirror’s Radial Position

Figure 11. To center the secondary mirror axially under the focuser, hold the secondary mirror holder in place with your fingers while adjusting the center screw with a large Phillips-head screwdriver. Later you will adjust the tilt of the secondary mirror by turning the three small set screws that surround the large center screw.

Spider vane thumb nuts

Figure 12. To center the secondary mirror radially in the focuser drawtube, make adjustments to the two knurled spider vane thumbscrews that are perpendicular to the focuser.

for the time being. The secondary mirror itself should be cen- tered in the focuser drawtube. If it is off-center along the axis of the telescope, i.e., positioned too far toward the front open- ing or toward the rear of the telescope, as it is in Figure 8b, you will have to adjust the mirror’s axial position.

To do so, use a small Phillips screwdriver to loosen the three small alignment set screws in the center hub of the 4-vane spider several turns. Now, grasp the mirror holder (the cylinder that is attached to the back of the secondary mirror itself) with

one hand while turning the center screw with a large Phillips head screwdriver with your other hand (Figure 11). Turning the screw clockwise will move the secondary mirror toward the front opening of the optical tube, while turning the screw counter-clockwise will move the secondary mirror toward the primary mirror. When the secondary mirror is centered axially in the focuser drawtube, rotate the secondary mirror holder until the reflection of the primary mirror is as centered in the secondary mirror as possible. It may not be perfectly cen- tered, but that is fine for now. Then, tighten the three small alignment set screws equally to secure the secondary mirror in that position.

Adjusting the Secondary Mirror’s Radial Position

Like the axial position, the secondary mirror’s radial position was set at the factory and will probably not need any adjust- ing, or if it does, you’ll typically need to do it only once.

By “radial position” we mean the position of the secondary mirror along the axis perpendicular to the focuser drawtube, as shown in Figure 12. This position is changed by adjusting two of the spider vane thumb nuts, as shown. Loosen one thumb nut, then tighten the opposite one until the second- ary mirror is centered radially in the drawtube. Do not loosen the thumb nuts too much, to avoid having them completely unthread from the ends of the spider vanes. Also, when mak- ing this adjustment, be careful not to stress the spider vanes or they could bend.

Adjusting the Secondary Mirror’s Rotational Position

The secondary mirror should face the focuser squarely. If the mirror appears to be rotated away from the focuser, the mir- ror’s rotational position will need to be adjusted. Again, this adjustment will rarely, if ever, need to be done.

Grip the sides of the secondary mirror holder with your fin- gers. Then, using a large Phillips screwdriver, loosen the cen- ter screw in the secondary mirror holder about a quarter of a turn only (counterclockwise). That should be enough to free up the secondary mirror to rotate slightly in either direction. Look into the collimation cap and rotate the mirror slightly in each direction to get an idea of how it affects the view of the secondary mirror. Now rotate the mirror as needed so that it precisely faces the focuser. Hold the mirror holder stationary in that position while turning the center screw clockwise until it is just tight (do not over-tighten). Sometimes the mirror may rotate slightly when tightening the screw, so keep at it until the mirror faces the focuser squarely and is secured in place.

Adjusting the Secondary Mirror’s Tilt

Finally, the tilt of the secondary mirror may occasionally require adjustment. If the entire primary mirror reflection is not visible in the secondary mirror when using the collima- tion cap, as in Figure 8c, you will need to adjust the tilt of the secondary mirror. Using a 2mm Allen wrench, first loosen one

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