"I read further through posts, and some people are saying that for recent DOBs you don't need secondary shadow offset, as this has been already taken into account when designing and/or fixing the secondary mirror on its base."
Offset has nothing to do with collimation involving alignment of optical axes (eyepiece and primary). The flat secondary has no optical axis of it's own. So, it's possible to achieve great collimation without offset of any kind. Offset is designed to bring the fully illuminated field into the center of the eyepiece FOV. This has to do with the geometry of the converging light cone from the primary mirror being intercepted at an angle. Hence we use an elliptical secondary to slice the primary cone at an angle. Offset simply ensures the elliptical diagonal intercepts the entire light cone correctly. The reason you see the dark offset being wider toward the primary is because the light cone is a little larger closer to the primary mirror and a little more narrow farther from the primary mirror. These are the points where the light cone strikes the secondary, and by the way, allows you to see the entire primary from the focuser peep hole. If you can see the entire primary mirror reflection from top dead center of the focuser, the diagonal flat is large enough to capture the on axis light cone form the primary and fully illuminate the field at that point in the focuser travel. If it's well collimated and fully illuminated, yes, you can leave that alone if you want.
There are two models of collimation using offset. The classic model requires bidirectional offset of the secondary to intercept the light cone. The diagonal is offset toward the primary and away from the focuser so that it corresponds to the incoming light cone's varying dimensions. When it's offset this way, you will see the reflection of the focuser to one side of the secondary shadow. If there was no offset, the bottom of the light cone would miss the lower part of the secondary altogether and you would not be receiving all the light from the primary mirror. In other words, vignetting the primary and reducing the effective aperture by reducing some of the volume of the light cone and area of the primary mirror reflected to the eyepiece.
Bi directional offset ensures the geometric center of the elliptical secondary and the optical axes of the primary light cone coincide. (Edit: Ooops, misspoke. Got confused visually slicing a cone in my head). The faster the primary, the more steeply the light cone converges and the more offset is needed to capture it.
The new model most folks are talking about these days only relies on only one offset direction toward the primary. I believe folks are trying to coin this offset as unidirectional offset. This happens automatically when you ensure the secondary is concentric under the bottom of the focuser or draw tube. You do not need the offset away from the focuser. However, the optical axes and the secondary geometric center no longer coincide. That's okay. A simple adjustment in secondary and primary tilt will capture the light cone and reflect it toward the focal plane entirely (just not right at at 90 degrees like bi directional offset). The nice thing about it is, we only need concern ourselves with the secondary position directly under the focuser. This is why this is the first step in collimation. So, yes, in this case unidirectional offset is automatically achieved and you will receive full illumination regardless. Aligning the axes at this point, then, is achieving collimation with the secondary properly placed for full illumination at the center of the FOV.
In unidirectional offset, and since the secondary still needs to intercept some of the wider light cone closer to the fast primary, you should see some offset toward the primary as indicated by the (opposite) offset reflection of the focuser inside the secondary shadow. If the focuser reflection is not offset, but you can see the entire primary reflection through focuser travel, then you are still fully illuminated on axis somewhere in the field of view, anyway. To fix this, you may want to ensure the secondary edge is actually concentric under the focuser. My f/6 uses bi directional offset, so I cannot look at it to see how unidirectional offset appears. Your scope may well have the diagonal flat mounted on the stalk with offset built in, too. You can tell because the lower lip of the diagonal mirror is much larger than the upper lip (which is what we see as an offset secondary shadow). The longitudinal axis of the diagonal stalk does not go through the geometric center of the diagonal's reflective aluminium face - because the diagonal mirror is offset away from the focuser when it was mounted. If so, then you still center it under the focuser as before and you should still see an offset focuser reflection. It's more pronounced at f/4.7 than slower focal ratios.
Two images of classical and the new model collimation you might find interesting:
The thread: https://www.cloudyni...rror-attaching/
And this: https://www.cloudyni...xt10/?p=8168213
Edited by Asbytec, 13 September 2019 - 10:16 AM.