The problem here is that you are *not* collimated. The area of the 'sweet spot' in this case is really a line (if you assumed flat image planes). I think the confusion stems from using the term 'squaring-on' for both optical alignment and mechanical alignment. Ideally, both should coincide, but *at least* the optical squaring-on is required.
> Note that at the beginning I said that the focuser was perfectly squared
> on to the tube. That however is not what you are after. The focuser should be
> squared on to the position of the diagonal. This doesn't even have to be a 45
> degree angle. Wherever the diagonal is, that is where the focuser must be
> pointed. More correctly, the mechanical/optical axis of the focuser must point
> at the OPTICAL center of the diagonal.
Actually, what is wrong here is that the *diagonal* is mispositioned. It is the diagonal which must be positioned properly with respect to the (mechanically) squared-on focuser. If you must compensate for diagonal mis-position by changing its angle, then it is no longer at 45 degrees (mechanically) and is obstructing more light and giving a non-circular illuminated field (though this is probably of no consequence as the fully illuminated field is impossible to detect visually).
Complete collimation (to me) means both optical and mechanical - for optical this means the eyepiece (focuser) is centered on the optical axis and parallel to the optical axis. Mechanically, this means that the focuser is square to the tube's axis and pointing at it's axis, and that the diagonal mirror is properly located with respect to the focuser and the tube (centered 'side to side' and offset the required amount away from the focuser and towards the primary).
You can have optical collimation without mechanical alignment and will still have good images, but the diagonal won't be at 45 degrees. Given the amount of adjustability in secondaries/spiders/focusers, there is no reason why you can't achieve mechanical alignment also (though, it is precisely this adjustability that results in most scopes not being properly adjusted). I contend that a laser makes this adjustment more doable.
> I emphasize the optical center, rather than the geometrical center. If you
> are using a laser collimation tool, you must spot the secondary, and this spot
> should be at the appropriate location, taking into consideration the offset of
> the secondary. If this is getting a little confusing, you now understand why I
> hesitate to recommend using a laser collimator, unless you fully understand
> the principles of collimation, and can do a satisfactory job with standard
> tools. Then a laser collimator can make the job a snap. (And if you have a
> closed tube and can't see the laser's spot on the secondary, the laser
> collimator is practically useless. I've heard that some have drilled a hole
> next to the focuser so they can see the diagonal. Hokey, but I guess it's a
> solution.
> Sorry if I've muddied the waters. Gosh, it seems clear to me. *>}. RICH
>
Yes, by all means the secondary should be spotted (or use removeable crossed threads - what I do). In order to properly locate the diagonal, this should be done - how else can you accurately determine location if offsetting is desired? Spotting is easy to do.
You make a good point regarding the ability to see the laser spot on the diagonal in a closed tube. The drilled hole approach seems like an easy/simple (not hokey to me) way to do this and a small price to pay for the easy collimation the laser provides. I would caution, though, there is a risk in this approach - Make Sure that the laser beam is not exiting this hole before looking into it!!!
The standard set of tools (sight tube, cheshire, autocollimator) if anything (to me) are what makes this all seem so difficult. A properly set up laser (beam adequately centered/parallel to its mechanical axis) makes focuser/diagonal positioning and collimation a very obvious process.
Bob L (hey, I'm friendly too!)