Project "Stiff Pole" update and help request.

A couple of weeks ago I had posted for collimation help on a truss dob( http://www.astromart.com/forums/viewpost.asp?forum_post_id=356207&poll_id=&news_id=&page=4 ). The general consensus was weak truss tubes / pole flex.

I've now upgraded to 1.125" diameter, .059" wall poles (as large as the scope will take) from 1.000" diameter, .055 wall. And now the oddest thing is occurring; from approximately 50 degrees vertical to 90 degrees (completely vertical) there is still a collimation shift of about 0.5". (Check out the video's on the prior post to see what I'm talking about). Conversely, from 50 degrees to 0 degrees (horizontal) there is no measurable shift at all, so the stiffer poles have made quite a difference here.

Can anyone help me with why now my problem is only from 50 degrees and up? Here's what's been eliminated so far:

1. It's not the primary; a piece of tape had been laid on top of the mirror box. The laser still moved. (See prior post)

2. All tightening points are tight to overly tight.

3. The collimator is a perfect fit in the focuser and isn't tightened (I've tightened it to check, and the same thing happens.)

Hi Dannon,
Well at least the new poles helped with some of the problem. 8)
Now I would wonder if you could possibly be dealing with the secondary shifting in the holder from it's own weight. If there is not enough packing the mirror could be moving in the shell. That is if your secondary has this type of holder.
If the mirror is not shifting, then it would seem that the secondary holder is shifting from it's weight because of the spider flexing.
We have to assume that the poles can not be flexing now. Is there any chance that the laser could be somehow at fault. Like the laser inside the housing is loose and shifting from it's weight?

I've now upgraded to 1.125" diameter, .059" wall poles (as large as the scope will take) from 1.000" diameter, .055 wall.
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Just a comment on the added stiffness of the larger thicker tubes:

The stiffness of a tube is related to the 4th power of the diameter. The center must be substracted.

In the case you site, you increased the stiffness by 53%, not a big change, this means the deflection of the tubes will be reduced by about a third.

There are two ways you can still use tubes with a 1.125 inch OD and increase the stiffness by large factors, one is to use thicker wall tubing, tubing with 1/8th inch wall would provide nearly a factor of 3 over the original tubes.

The other way is, assuming the current tubes are aluminum, is to use steel tubing, steel is three times stiffer than aluminum.

In both these cases one would need to consider the added weight.

For what its worth...

Jon Isaacs