17
aligning the secondary Mirror
With the collimation cap in place, look through the hole in the
cap at the secondary (diagonal) mirror. Ignore the reflections
for the time being. The secondary mirror itself should be cen-
tered in the focuser drawtube. If it isn’t, as in
Figure 28b, its
position must be adjusted. This positional adjustment of the
secondary mirror will rarely, if ever, need to be done.
To adjust the secondary mirror left-to-right in the focuser draw-
tube, use the included 2mm hex key to loosen the three small
alignment setscrews in the center hub of the 4-vaned spider
several turns. Now, grasp the mirror to prevent it from rotating
(be careful not to touch the surface of the mirror), while turn-
ing the center screw with a Phillips screwdriver
(Figure 30).
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 cen-
tered left-to-right in the focuser drawtube, rotate the second-
ary mirror holder until the reflection of the primary mirror is as
centered in the secondary mirror as possible. It may not be
perfectly centered, but that is OK for now. Tighten the three
small alignment setscrews equally to secure the secondary
mirror in that position.
Note: When making these adjustments, be careful not to
stress the spider vanes or they may bend.
The secondary mirror should now be centered in the focuser
drawtube. Now we will shift our attention to the reflections
within the secondary mirror in order to properly adjust the tilt
of the secondary mirror. Adjusting the tilt of the secondary
mirror and the tilt of the primary mirror are the two collimation
adjustments that will be done most often.
If the entire primary mirror reflection is not visible in the sec-
ondary mirror, as in
Figure 28c, you will need to adjust the tilt
of the secondary mirror. This is done by alternately loosening
one of the three secondary mirror alignment setscrews while
tightening the other two
(Figure 31). Do not make excessive
turns of these setscrews or force them past their normal trav-
el. A simple 1/2 turn of the screw can dramatically change the
tilt of the secondary mirror. The goal is to center the primary
mirror reflection in the secondary mirror, as in
Figure 28d.
Don’t worry that the reflection of the secondary mirror (the
smallest circle, with the collimation cap “dot” in the center) is
off-center. You will fix that in the next step.
aligning the Primary Mirror
The final adjustment is made to the tilt of the primary mirror.
It will need adjustment if, as in
Figure 28d, the secondary
mirror is centered under the focuser and the reflection of the
primary mirror is centered in the secondary mirror, but the
small reflection of the secondary mirror (with the “dot” of the
collimation cap) is off-center.
Figure 29.
The SkyQuest optical tube properly set up for
collimation.
Figure 30.
To center the secondary mirror under the focuser, hold
the mirror holder in place with one hand while adjusting the center
bolt with a Phillips screwdriver. Do not touch the mirror’s surface!
Figure 31.
Adjust the tilt of the secondary mirror by adjusting one
or more of the three alignment setscrews with a 2mm hex key.
Collimation cap
