Orion 8956, 8958 instruction manual 9b 9c, Optical Axis Adjustment, Star Testing

Secondary
mirror holder	Optical
Reflective surface of
	axis
collimating eyepiece

Secondary mirror
center mark	Border

Collimation eyepiece mark

Spider vanes

9a

9b

9c

Figure 9a-c:The view through the collimating eyepiece (not to scale). Figure 9a shows the Ritchey-Chrétien aligned with all components identified. Figure 9b shows the secondary out of alignment. Figure 9c shows the optical axis out of alignment.

Optical Axis Adjustment

These adjustments will require a 3mm and 2.5mm hex key. There are three sets of “push-pull” screws on the back-end of the optical tube where the focuser attaches (Figure 8). The optical axis is denoted by a thin outline of light around the perimeter of the view through the collimating eyepiece. Each set consists of a smaller black screw and a larger chrome screw; these must be adjusted in tandem. Loosen one and tighten the other to make changes to the optical axis. This will adjust the tilt of the optical axis in relation to the second- ary mirror. If the optical axis is not aligned, the entire image through the collimating eyepiece will appear as if it is tilted to one side (Figure 9c). When properly aligned you will see a very thin concentric outer white circle around the perimeter of your view through the collimation eyepiece. This procedure will require only micro-adjustments, if any.

Star Testing

An optional star test can be performed to confirm the collima- tion accuracy of the telescope. The adjustment procedure on the telescope is the same as using the Collimation eyepiece; testing, however, will be done in the night sky. This method is more difficult as you will need to keep your target star cen- tered in your field of view. Furthermore, seeing conditions will affect the end result. Keeping the star precisely centered in the field of view is critical to avoid false negatives. It is sug-

Out of collimation Collimated

Figure 10: An unfocused view of a bright star through the eyepiece. Proper collimation will show a symmetrical diffraction pattern such as the one on the right. If the circles are unsymmetrical the scope needs collimation.

gested that you choose a star close to the zenith (straight overhead) rather than at the horizon to minimize atmospheric distortions. Using Polaris as your target star can be helpful as minimal drift adjustments will be required.

Center a bright star in your field of view using moderate to high magnification. Place your eyepiece directly into the fo- cuser. Do not use a star diagonal while performing this proce- dure. In a well collimated telescope, a defocused star image should appear symmetrical, with the dark central obstruction centered in the star’s ring pattern (Figure 10).

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