Collimation Help Request - Now with video on later post!

In January of this year, my wife and I purchased a new 15.5” truss-tube Dob from a vendor on Astromart and have since been plagued with problems, mostly collimation related. Wanting to end the madness, we recently bought a Glatter 2” / 1.25” barlowed laser collimator.

This weekend was a washout, so I spent several hours in an indoor, flat surface carefully aligning to what I though was perfect collimation. (I had the secondary perfectly in a sight tube, the laser striking the center of the secondary, down to the exact middle of the collimation doughnut, back up and returning to the same point on the secondary. With the Barlow attachment on, the target made a perfect ring around the laser.) I double checked my results by rotating the laser 360 degrees within the focuser, and racking the focuser in and out; perfect collimation was maintained each time.

When I went to return the scope to its full upright and locked position, I still had the collimator in, and noticed that the collimation went all out of whack. It seems that every time I made a vertical movement, the alignment is lost. When I return it down to the “collimation position” the collimation returns to perfect with no other adjustments.

While I have little idea what might be causing this, I suspect that the primary is too tight and not “floating” on its cells properly. Someone help me!

Dannon,
Make sure your secondary is centered in the UTA and also check to see if the focuser is squared with the secondary. The book "The Dobsonian Telescope" by David Kriege and Richard Barry has a good explanation of collimation. I had a problem with my Obsession last weekend and found that my optics were miss aligned. I spent the latter part of Sunday correcting the problem. Once your secondary and focuser are in the right position, collimation is a snap.

Clear Skies,
Jay

Is your secondary shifting?

Does your collimator beam hit the exact same spot on the secondary regardless of position of telescope?

Does the beam hit the exact center of the primary regardless of position of telescope?

If you still hit the center of the primary regardless of telescope position, your secondary is probably okay.

Alex

Hello,

This may seem too simple, but did you tighten down the truss poles enough so they wouldn't shift. That happened to me once and sure enough, I hadn't tightened one of the poles.

This is sort of like checking to see if something electronic is plugged in before opening it up for repairs.

Hope you fix the problem easily.

If all else fails, contact the guy you bought it from and see if he had a problem with it.

Clear skies.

Tom B.

Dannon
Exactly what do you mean by "went all out of whack". Put the laser in and set the scope at about 45* and center the beam in the donut then raise and lower the scope and note what the beam does. As others have pointed out, it could be a cell or secondary problem or some truss type scopes might have a slight sag from the weight of the UTA. I usually collimate with the scope at a 60* angle which is where most of my viewing is at. You might also snug up every screw and nut in the entire scope before spending too much time trouble shooting.
Just a few more ideas to check out.
Buck