9
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
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.
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
