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Originally Posted by OneOfOne
most guides seem to indicate that "the secondary mirror should appear to be round and in the centre of the field". But how well can the eye determine if something is "round". Is there a way of checking this alignment.
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In practical terms, You want to make the secondary appear round and centred under the focuser while blocking the view of the primary with a sheet of paper. Then check again after removing the paper whilst taking note of centering the reflected view of the primary mirror.
The most important thing is to adjust the tilt of the secondary so that the optical axis is aligned with the focuser tube axis. If it is not, then the focal plane at the eyepiece will also not be at right angles, but rather tilted.
This may cause the focus point for different points in the field to differ.
You can check that the optical axis are lined up with a laser, or by checking with a combo cheshire tool that the cross hairs line up with their reflections whilst centred on the primarys centre spot.
If your optical axis is aligned as above, any physical offset of the secondary from optimum will merely result in the illuminated field being offset from centre.
Before i discovered that my secondary wasnt centred properly ( it was offset in the 90 degree plane to the focuser), I could never get the crosshair reflections to stack. This is more important than centering the secondary under the focuser. Ultimately, you are aiming for both.