Telescope Reviews: Another Collimation Question

A little explanation of concepts, here:
If the secondary is off-center in the tube (let's say severely), then collimating would tilt the primary toward one side of the tube, and the focuser would be aimed at the secondary (wherever it is), and the intersect angle for the optical axis at the secondary could be 90 degrees or another angle.
All of this won't matter for collimation. Collimation is making the focuser axis coincident with the optical axis, adjusting the secondary to have the optical axis hit the center of the primary, and adjusting the primary to make its optical axis coincide with the secondary so light rays go straight up the focuser.
All the rest is an attempt to have the light evenly illuminate the edge of the field all the way around.
We center the secondary under the focuser so its edge evenly illuminates the focal plane of the eyepiece. We try to get the primary optical axis close to that of the mechanical centerline of the tube so that the tube opening evenly illuminates the primary mirror all the way around.
But collimating doesn't require even illumination of the field (unless you consider that part of collimation, which I do--I call it mechanical collimation rather than optical collimation, and both are important, but for different reasons).

So if the shadow of the secondary moves as you move the focuser, it is probably (not always) because the focuser axis is not aligned with the optical axis.

Your original post showed that there are several ways to interpret what you see in the focuser. You chose to change the alignment of the focuser to point it at the secondary. Fine, that should have had no effect on collimation. I'll assume the secondary is centered and the laser hits dead center in the primary and the primary's reflected shadow of the centermark was centered on the Blug's bottom. OK, so far.

The optical axis may not coincide with the mechanical axis of the scope at this point, but it is collimated optically. An out-of-focus star image will show a tiny amount of offsetting to the secondary. It might not even be noticeable because the amount is only a tiny fraction of the secondary's diameter.

But you see it offset a lot? That shouldn't be unless something is grossly wrong. I did a thought experiment in which I moved the secondary halfway to the side of the tube, and the out of focus shadow of the secondary was oval instead of round, but it still appeared roughly centered.

So I think you haven't achieved collimation. Let's review the steps:

1) Center the secondary in the tube using a cardboard template and adjusting the spider vanes. Make certain all the collimation screws have equal amounts of threads showing.
2) Make sure the primary's centermark IS centered. Make sure the primary is centered in its cell.
3) Look down the focuser and adjust the rotation of the secondary until it appears round to the eye.
4) If close to centered in the focuser (using a sight tube),
you can center using the collimation screws. If not fairly close to already centered, you can adjust the tilt of the focuser to achieve the same. This may pose a conceptual difficulty, but as far as the optical path is concerned, both achieve the same thing.
5) align the secondary tilt using the laser. If the mirror moves off center in the focuser to do this, try adjusting the tilt of the focuser again. Repeat. Each time you repeat, the tilt adjustment of the secondary and the resultant realignment of the focuser will be less until you get to where both are good at the same time. You may have to choose between a centered secondary and a good laser beam adjustment of secondary tilt. CHOOSE THE TILT. Having the secondary perfectly centered is a recommendation but is not critical.
6) align the primary using the BLUG (or TUBLUG, or simple barlow).
7) Re-align the secondary. You may have to move it a bit.
8) Realign the primary.
9) Go back and forth as often as you need until both show collimation simultaneously. When both show perfect collimation at the same time, in my experience, even an autocollimator shows only very tiny errors.
10) Move the focuser in and out. The laser beam should hit the primary exactly through all the travel of the focuser.
If it does not, then the laser is being tilted in your focuser when it is being used to align the focuser axis to the secondary tilt.
A. You are tigtening it down with a setscrew? The tops of some focusers are not superlatively square.
B. You are using a 2" laser in a 2" focuser? If you're using a 1.25" laser in a 2" focuser, with an adapter, loosen the adapter while the laser is tight in the adapter and rotate the entire thing in the focuser. Laser beam move? Your adapter is not "square" and you cannot achieve collimation with it. Replace it.

So how will the shadow of the secondary appear in the out-of-focus star image? It will appear nearly centered, or centered exactly. The offset you've achieved is tiny (let's say 1/8" unless the scope is 20" and up), so the inexact nature of gauging the centering of the shadow will make it appear pretty much centered. And, more importantly, racking the focuser in and out through focus will not change the centering of that shadow. Though technically it will appear offset in opposite directions on either side of focus, the amount of offset really won't be easily seen. I don't notice it in an f/4 scope, so you probably won't either.

After going through the steps above, repost your results here. I'll be curious to see what happens.
Don

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Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie