One reason we check telescopes before shipping is

Product Description
One reason we inspect telescopes before shipping is that the telescope's focusers (regardless of whether they're refractor or Newtonian) often tilt. This tilts the optical plane, causing the eyepiece to exhibit off-center optical aberrations, and the image isn't as good as if the eyepiece were on the optical axis.

There are usually two screws for this purpose, which can be used to guide the focuser tube.

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