Wilseus

I always thought this meant that as others have said: the secondary is still an ellipse, but the centre of the field of view from the eyepiece would NOT be in the dead centre of the elliptical mirror if you looked at the mirror straight on. Or something like that.

Thanks all. I think I figured this out. However, when I said the secondary was offset, I meant it was too far to the "east" of my top down view, not too far away from the eyepiece or "south" as @StarDuke82's manual excerpt implies. Interestingly enough, I do have the manual for my scope, but it does not have this paragraph, and in fact it does not mention offsets at all. It's quite different from the one @doublevodka attached. I think mine's a later version of that scope, a "Generation II" What I did was I adjusted the spider vane screws until the secondary was in the middle of the focuser. I got this more accurate by inserting a cheshire eyepiece and then adjusting the focuser so I could match up the outline of the secondary with the outline formed by the end of the cheshire. Then I adjusted the secondary angle so that the reflection of the secondary in itself was centred on the cheshire crosshairs. After that I adjusted the primary so its central dot aligned with the centre of the crosshairs, then I repeated these list two steps until everything lined up.

OK, but whatever I do, I am unable to get the secondary centred in the middle of the focusing tube (pic 1.) Are you saying this doesn't matter?

Hello, To cut a long story short, for a long time I have not been able to get my dobsonian collimated correctly. The best I've been able to get is the spot on the primary aligned with the middle of a collimation cap (dob1.jpg), but the whole lot is off centre of the crosshairs when I insert a cheshire eyepiece (dob2.jpg) I contacted Explore Scientific's tech support who suggested adjusting the secondary mirror, which of course I have tried, but to no success. After quite a bit of fiddling, I discovered that the assembly that holds the secondary is clearly nowhere near perpendicular (dob3.jpg). It can clearly be seen that an imaginary line perpendicular the plane of the secondary mirror assembly points way off to the top right. It must be several degrees out. I experimented with tightening the bolts at top right and bottom left and loosening the other two, to try and increase the tension bottom left to top right to rotate it anti-clockwise, but this was unsuccessful. Can anyone shed any light on what is going on here? Am I doing something wrong? Is the scope faulty, bad QC perhaps?