Strange tracking error

[Carl M]

Hi, I'm curious as to what is causing this strange occurance on my subs. Just normal unguided CG5 mount with dual axis drives. It seems that if I use 30 seconds I am getting weird tracking error but using 15 seconds it's normally fine give or take a few subs. First I thought it was polar alignment but it is the same every night. The stars dont trail as such but they are duplicating. Any ideas?

Here is an example of it:

Carl

[steelfixer]

Have you checked the balance of your scope.

It looks like it is creating some sort of friction and then is releasing it which will cause the scope to jump hence the double image.

Just a thought.

graham

[Shibby]

Yep, what Graham is referring to is known as backlash. This can be reduced by balancing the scope slightly differently (you want the lifting side to be slightly heavier)

The other possibility is PE (Periodic Error).

[Carl M]

Thanks, so I need the counter-weight side to be slightly heavier and the top end of the scope with the camera on to be slightly heavier?

[steelfixer]

That is about the long and the short of it.

It just means there will be a constant weight on the gear train thus preventing the backlash.

Graham

[Shibby]

If your pier side is west, pointing east, then the counterweight will be rising and that side will need weighting. On the other side of the meridian, the scope is rising and that end will need weighting... I think that's right, perhaps someone can sanity check that for me...