Collimating Vixen VC200L / VMC200L Optical Tubes
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b.
If you use a laser collimator, first be sure that the laser itself is collimated. Using an extension
tube, insert the laser making sure it is flush up against the drawtube extension and tighten the focuser
tension knob. Snug down the set screws gently and rotate the collimator. If the beam moves at all, the
collimator will need to be adjusted before proceeding. If laser passes collimation test: place a piece of
transparent tape over the central hole in the spider or use a holographic laser. Adjust the push/pull
screws around the focuser (fig. 1-2) until the beam goes through the center of the hole in the spider.
Just getting the beam to shine through the hole is not good enough; it must go through the exact
center. It may still be valuable to recheck collimation visually as it is most important to be sure the
end of the baffle tube is centered on the spider.
C.
Collimating the secondary mirror
1)
When you install the secondary mirror and holder, make sure to replace the washers as described in
section A.2. Attach the mirror holder to the spider with only the center ‘pull’ Phillips screw by
tightening until the holder is roughly stable. Then retighten the 3 ‘push’ collimation set screws
nearly one turn each or until they are all just snug.
2)
Point the telescope at some bright, uniformly lit object, such as a white wall or daytime sky (well
away from the sun). Attach the extension tube to the focuser drawtube and insert the centering
eyepiece into the eyepiece holder. Rack in the drawtube and look into the centering eyepiece. Look
for the dark ring in the center of view. This is formed by the reflection of the inside of the drawtube.
It may be well off center if the secondary mirror is badly out of collimation (Fig. 3). It should be
exactly centered.
3)
Follow either (a) or (b) below to collimate the secondary. The visual method (a) is preferred.
a.
As you extend the drawtube outward, you will see the edge of the primary mirror (surrounded by
a ring of darkness) go out of the field of view. The dark ring outside the primary must disappear
evenly inside the end of the baffle tube (which will not appear quite as dark, it is very subtle) as the
drawtube is racked out (Fig. 4). Some older scopes have three mirror clips which make it easier to see
the edge of the primary. Adjust the tilt of the secondary mirror by adjusting the three ‘push’
collimation screws (fig. 1-1). Loosen at least one screw before tightening another. Do not over-tighten
the three collimation screws as this may distort the secondary mirror.
b.
To use a Cheshire eyepiece with crosshairs, simply adjust the secondary mirror to align the
reflection of the crosshairs in the bright center of the central black donut with the crosshairs
themselves. Keep in mind though, that the procedure above (3.a.) may be more sensitive. If using a
laser, adjust the secondary so that the reflected beam goes directly back to its source. Laser
collimators with a “rear view” port are useful for this application.
