Lockleys Observatory

[Bart]

None of this explains why it doesn't need levelling or what is meant by frequencies of the pier and mount head.

[bojan]

Levelling of the pier is not so important because the mount itself has all provisions for alignment (N-S. Up-Down screws).
The frequencies are different matter altogether.. but also not very important - the important bit is damping of oscillations of any frequency at which the whole setup may oscillate, when bumped (accidentally or intentionally), and their amplitude.
Usually, the more massive and sturdier the pier, the smaller the amplitude and the higher that frequency is (because then only mount itself and the load (telescope + accessories) are the factors), and the duration of oscillation is shorter - a very desirable situation.

[Bart]

So in other words, it is all bollocks.

As long as the pier is sturdy and is not an obviously flimsy setup which is susceptible to wobble and vibration, then there is no reason not to use a levelling setup.

Also, if levelling helps reduce the number of polar alignment iterations, then that has to be a good thing as well.

[bojan]

Originally Posted by Bart

So in other words, it is all bollocks.

As long as the pier is sturdy and is not an obviously flimsy setup which is susceptible to wobble and vibration, then there is no reason not to use a levelling setup.

Also, if levelling helps reduce the number of polar alignment iterations, then that has to be a good thing as well.

Exactly right.

However, if you crunch some numbers, you will see that if you are levelled your pier within 2-3° (very easily achieved during pier construction phase), the number of required iterations does not really increase (it can be said that the alignment error of each iteration will be proportional to sin(x) where x is the departure from ideal horizontality of the pier base.. so sin(2°)=0.035.. 3.5%.. not really significant.
After all, who guaranties that the base of the mount itself is really aligned with the horizontal axis of the polar axis?
EQ6 base is just aluminium cast, no machining.. so go figure

[cfranks (Charles)]

I read Peter's comment as being more concerned with the 3 long leveling screws. Compared to the pier, they would act like a wobbly spring.

[troypiggo (Troy)]

It's as simple as this. You have a 3 tripod legs connected to a plate that holds them together. Rock solid. Alternatively you have a column type pier that is suitably sized to be rock solid. The plates on top of these can be levelled to pretty close to horizontal. Any other accurate levelling is taken care of when you polar align your mount.

You have a mount and telescope sitting on top of that. Rock solid.

If you then put 3 slender bolts with inadequate stiffness between the 2 rock solid parts, you've created a potential zone of weakness. Critical in that statement are the words "slender", "stiffness" and "potential." So it's not to say that you can't do it, as there are obviously people who have done this and used with success.

Slenderness is a function of the bolt diameter and length. Stiffness is a function of bolt diameter, length, and material properties. So there's going to be some point where you've got a short enough and/or large enough diameter bolt where the effect of using them is negligible. The fact that they're there means they will always be a little more potential for weakness, just a matter of how much.

But at some point you will be either too long or too small a diameter bolt for that length, and it will noticeably be a weak point in your train.

So all I'm wondering is, as long as your pier/tripod is close to level, and your mount can be polar aligned on top of that, what's the point of introducing a potential zone of weakness. When I'm using the term "weakness", I'm not saying the whole thing will collapse (although if you had some huge/heavy PME and massive RCOS on top etc, I would certainly be checking the bolt length), but I mean movement, vibration etc that are critical for astroimaging etc.