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Image visible in the sight-tube
If the view does not match this description and illustration, then repeat the three adjustment steps until the view is
correct.
The Second Step in Collimating—Use of the Cheshire Eyepiece
A Cheshire eyepiece is extremely useful for the next step in collimating, although the auto-collimator or star test can
accomplish the same thing. Once the secondary mirror is adjusted by means of the sight-tube, insert the Cheshire
eyepiece in the focuser. Do not push it all the way: the cutaway in the eyepiece must be exposed to a light source.
(A red flashlight works well in the field at night.)
If you look through the eyepiece you will see the following (aside from the bull's-eye on the primary mirror):
The reflection of the primary mirror in the diagonal.
A generally dark field.
A brightly lit annulus, or ring, in the center of the field.
A dark spot, or bull's-eye, in the center of the annulus.
Detail of the image visible in the Cheshire eyepiece
Your goal is to bring the bull's-eye of the primary mirror into alignment with the central dot imaged by the Cheshire.
Accomplish this by using the following steps to adjust the three collimating bolts of the primary mirror cell. Adjust in
small increments while rotating the nose assembly and checking the alignment at various positions of the rotation.
First, loosen the jam-nut on each of the three collimating screws to make it possible to turn the collimating bolts.
(Tighten the nuts again when collimation is complete.) Begin with the nose assembly at any position and adjust the
bull's-eye toward the center of the illuminated annulus.
Then rotate the nose assembly 180° and repeat the
Summary of Contents for NGT-12.5
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