I think I have a problem with alignment somewhere in the image chain of my f4 Newtonian telescope and I am hoping someone recognises the symptom I have discovered.
Recently I tweaked the primary mirror tilt of my scope while observing the shape of an unfocused star centred in the field and I observed a curious result.
I de-focused the star by moving the focal plane in and then tilted the primary mirror until I obtained a view similar to the left hand ‘sketch’ below. I then refocused on the star and continued to move the focus out until the star was again de-focused but this time with the focal plane on the other side of the focus point. The right-hand ‘sketch’ below shows what I saw.
I expected the defocused star to look the same regardless of which way I moved the focuser but apparently this is not the case.
Has anyone come across this before? Do you know what might be causing it?
( From memory, the movement of the center circle was parallel to the telescope tube. )