Does my guide scope need to be perfectly aligned for the PA to work?

Using SharpCap's Polar Alignment feature
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Does my guide scope need to be perfectly aligned for the PA to work?

Post by admin » Sat Feb 25, 2017 12:31 pm

This is a question that seems to be asked pretty frequently and there is a misconception among some people that SharpCap Polar Alignment will somehow polar align the axis of the scope you are using with the pole rather than aligning the mount to the pole. Let's see if I can explain why that isn't an issue...

As long as the alignment of the guide scope (or, if you aren't using a guide scope, whatever you are using for the PA process) isn't way out, then as you rotate the mount about the RA axis there will be some point within the guide scope image that is the point the rest of the stars appear to rotate around - that point will stay still. That point in the sky is where the RA axis is currently pointing.

You can demonstrate this yourself - sit on a swivel chair under something on the ceiling that gives you a point of reference - maybe a light. Turn your smartphone camera on and point it upwards, but not straight up, so you can see the light or whatever you are sitting under but it's not in the middle of the screen. Now spin on your chair (without letting it roll). You'll see that the thing you are sitting under stays in the same place on the screen - everything else appears to move around it. Actually the thing that stays perfectly still in this example will be the point on the ceiling that the swivel axis of your chair is pointing at. The important thing is that the point staying still as you spun is not affected by the fact that your smart phone wasn't pointing straight up - it's only affected by the axis you spin around. (Obviously this experiment isn't 100% accurate since you are unlikely to get the axis of your swivel chair exactly under the light and you will probably let it move about a bit as you spin the chair and so on, but for a firmly mounted guidescope it is accurate.)

So, SharpCap knows where the RA axis is pointing, since that is where the stars appear to spin around when you rotate the RA axis and it knows where the pole is from plate solving, so it can easily work out how much you need to move the scope to make the two line up and guides you through the correction process.

Any offset of the guidescope from the axis of your mount doesn't matter, since the stars are at infinity, so being a foot or two sideways doesn't change your view. It even works if the center of rotation is out of view as SharpCap can still work out how far out of view and in which direction it is (ie it would be at pixel x=-127,y=330 for being just out of view to the left of frame). It can't be too far out of view though otherwise the errors in working out where it is become too large and the accuracy of the whole process breaks down. That's why you have to do it within a few degrees of the pole.

hope that explains it!

Robin

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