Orion #8297 8" F/3.9 User Manual download pdf (Page 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

1 2 3 4 5 6 7 8 9 10 11 12

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Orion #8297 8" F/3.9 User Manual Page 9

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