Collimating Vixen VC200L / VMC200L Optical Tubes
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D.
Collimating the primary mirror
1)
Remove all sight tubes and extensions from the drawtube and rack it all the way in. Look along the
inside edge of the drawtube (not the center) and look at the outer edge of the central black donut.
Just outside that donut you will see a reflection of the edge of the secondary baffle as a section of a
thin dark ring with light on either side (Fig. 5). If collimation is good, that ring will be evenly spaced
from the central donut as you move your eye all the way around the inside edge of the drawtube.
This is more sensitive than it sounds, and can be made most sensitive by racking the drawtube out
a little and/or adding an eyepiece adapter so that you can just see a gap of light all the way around.
If the light gap is not even or cannot be seen on one side, adjust the primary mirror push/pull
collimation screws (fig. 1-2) to center the pattern. This should get the collimation very close. Other
methods such as trying to align the spiders with Cheshire crosshairs or using a holographic laser are
not as reliably accurate, but can be used.
E.
Star Test
1)
Once the coarse collimation adjustments are made, take the telescope out at night and check star
images.
2)
Under steady seeing conditions, aim the telescope at a second-magnitude star more than 45
degrees above the horizon at moderate magnification. (Do not use a star diagonal.)
3)
Place the star in the center of the field and make the star image slightly out of focus to see whether
the rings surrounded the star (diffraction rings) are concentric. If the optical axis is not aligned
correctly, the off-centered diffraction rings will appear as shown in figure 6-1.
4)
Move the optical tube to displace the star image a little from the center of the field toward the
direction of off-centered rings. (Fig. 6-2)
5)
Adjust the collimation screws of the primary mirror (Fig. 1-2) to bring the out-of-focus star image
back toward the center of the field. Repeat steps 3 and 4 above until the diffraction pattern
becomes concentric. (Fig. 6-3)
6)
Changing to an eyepiece with high magnification will allow you to make a more accurate optical
collimation.
