For those of you still confused about secondary offset but too lazy
to follow the link in my post above to the Sky and Telescope website's page on collimating Newtonians and read through the article, here is a relevant excerpt:
Quote:
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When Step 3 is done, the optical axis is accurately centered in the focuser, and collimation is complete. However, if you look carefully you will notice that the Cheshire eyepiece does not appear exactly centered inside the shadow of the secondary. Don't worry; this is in fact how things should look because the secondary mirror is slightly offset.
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For an f/4 you could change "slightly" to "considerably".