Spider vane thumb nuts
Figure 12. 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
Figure 13. To center the secondary mirror radially in the focuser drawtube, make adjustments to the two knurled spider vane thumbnuts that are perpendicular to the focuser.
By “radial position” we mean the position of the secondary mirror along the axis perpendicular to the focuser drawtube, as shown in Figure 13. 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
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 9c, you will need to adjust the tilt of the secondary mirror. Using a small Phillips head screwdriver, first loosen one of the three alignment set screws by, say, one full turn, and then tighten the other two to take up the slack. Do not loosen the center screw during this process. The goal is to center the primary mirror reflection in the secondary mir- ror, as in Figure 9d. When it is centered, you’re done adjust- ing the secondary mirror. Don’t worry that the reflection of the secondary mirror (the dark circle with the four spider vanes adjoining it) is
Aligning the Primary Mirror
The final collimation adjustment is made to the primary mir- ror. It will need adjustment if, as in Figure 9d, the secondary mirror is centered under the focuser and the reflection of the primary mirror is centered in the secondary mirror, but the reflection of the secondary mirror (dark circle containing the light reflective surface and center black “dot” of the collimation cap) is
The tilt of the primary mirror is adjusted with three
To adjust the primary mirror’s tilt, first loosen all three lock bolts by turning the white knobs counterclockwise about one turn each. Now, while looking into the focuser through the collimation cap, turn one of the black collimation knobs a half turn or so in either direction and see if the secondary mirror reflection moves closer to the center of the primary. That is, does the “dot” of the collimation cap appear to move closer to the ring on the center of the primary mirror? If it does, great, keep going until you get it as close as you can. If it doesn’t, try turning the collimation knob in the opposite direction. If turning the one knob does not seem to bring the dot closer to the ring, try using one of the other collimation knobs. It will take some
When you have the dot centered as much as possible in the ring, your primary mirror is collimated. Now lightly tighten the three white lock knobs to secure the primary mirror in place.
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