|Hall's Harbour Observatory, Hall's Harbour, NS|
How Important is Level?
|08-Feb-2020 13:19 AST||08-Feb-2020 17:19 UTC|
It is not important!
Let me state my position clearly up front. Leveling the mount is NOT essential for polar alignment. It is a convenience, nothing more. You can get a perfectly accurate polar alignment with the base of the mount completely cock-eyed. Leveling just makes it easier to achieve.
What is the problem?
You have seen numerous photos of observatory piers. The builder has put a great deal of effort into pouring a solid concrete footing, making sure the pier is rigid, isolating the floor from the pier to ensure no vibration, etc. Then they go and attach the mount to the pier with three or four SPRINGS!
By "springs", I am referring to the leveling bolts that are so often positioned between the mount and the pier. They are made of steel, and steel is springy. That's why we make springs out of it! Okay, fine, the bolts are not quite as flexible as if they were coil springs. But make no mistake, they are flexible. They allow the mount head to vibrate laterally in any direction as well as rotationally. This makes no sense. You want the mount attached rigidly to that big concrete footing in the ground.
Why do people do it?
Where does this obsession with level come from, and why is it so strong that people will deliberately sacrifice the rigidity of their pier to satisfy it? It is because we are trying unsuccessfully to solve in our heads a problem that we believe involves three-dimensional spherical trigonometry. We are not very good at it. Because we are trying to visualize the problem from outside, we make it more complex than it needs to be.
Instead, look at polar alignment from inside the mount: from the view through the polar scope.
The first thing to notice about polar alignment viewed through the polar scope is that it is a two-dimensional problem, not a three dimensional problem. The objective is to place the crosshairs in the middle of the reticle onto the correct spot in the sky. That is it! I repeat: that is the entireity of polar alignment. Put the X on the right spot, and you are polar-aligned. Nothing else matters. The tripod could be upside-down, but if the X is on the right spot, you are polar aligned and the scope will track perfectly.
With the mount level, the azimuth and elevation controls move the X towards the pole in horizontal and vertical movements respectively.
What happens if the mount is out of level by being north-low? Well, Polaris and the NCP simply start out being lower in the polar scope. You have farther to move the elevation adjustment. No big deal, right?
What happens if the mount is out of level by being east-low? Now the azimuth and elevation controls move the X in a fashion that is slanted with respect to horizontal and vertical. Does this prevent you from getting the X onto the NCP? Of course not! Being out of level may require more effort to get the small Polaris circle correctly oriented. But once you establish the correct orientation of the reticle, there is nothing preventing you from placing the centre mark where it belongs.
Even if the mount were at a 45° angle, you could still get it polar aligned. However, we are not talking about such gross out-of-level conditions. The top of your pier is going to be within a degree of level. So you will have no difficulty getting it aligned, even without leveling the mount. You should perform a drift alignment when you place the mount on the pier. If your azimuth and elevation movements are a degree out of level, it will have negligible effect on the drift alignment. You might not get quite as close on the first iteration as you might have, but the difference will be so tiny that the second iteration will fix it.
So do not design your pier with springs to hold the mount, and do not obsess about level. It isn't that important!
Copyright © 2019 by Kathleen Walker
Last modified: 01-Feb-2019