Polar alignment when guiding?

[Kaptain Klevtsov]

It has been nagging at me now for a couple of days and I can't find the time to sort it out myself, so I thought that it would be a good idea to ask a question. My dad told me that it is ALWAYS a good idea, so here goes.

Autoguiding software is freely available these days, allowing us to use a humble webcam to guide with an accuracy that would have been impossible even twenty years ago, but are we doing it as well as we could?

The idea that came to me involves the backlash in the Dec. drive. This is something that can't be zero and therefore will always (well almost always) give problems as far as guiding is concerned. Each reversal of the guide drive must take into consideration the amount of rotation of the worm drive that has to occur before the mount actually moves at all. The mechanical clearances that are necessary to allow the mount to move freely mean that some of the rotation of the worm don't actually move the mount at all, but just take up the slack in the drive train.

Moving the mount ever so slightly away from accurate polar alignment would ensure that any Dec. corrections would always be in the same direction, and would remove the backlash problem entirely. There would never be a time when the Dec. drive had a reversal, so the need for the backlash allowance would go away.

So the bottom line question is, do you get better guiding if the polar alignment is slightly out, and by how much should it be out if this is the case?

Captain Chaos

[blinky]

I like the theory, sounds like it would work as well. I think you have set yourself a challenge there

[centroid]

As you say, 'backlash' will always be present to a small degree in any 'gear chain', even with the 'best' of mounts.

This is usually compensated for, in RA by ensuring that the mount's RA axis is not neautrally balanced, but is a little heavier on it's Eastern side.

With the Dec axis, if the mount is accurately Polar Aligned, then there is no real need to 'guide' in Dec at all. If the one end of the scope's fore & aft axis, is slightly heavier, then this will keep t against one side of the a gear 'tooth' face.

This of course assumes that the scope is equatorially mounted.

In my experience, the best polar alignment is achieved with the 'Drift Method'. If done accurately, it is very time consuming, and as such really only suitable for permanently mounted scopes.

However, if done with a high power Reticule eyepiece, or even better, a CCD camera, it can give very accurate Polar Aligment.

With my LX200GPS, I only on guide in the RA axis, and because I have spent time getting the Polar Alignment as close as I can, and also refining the Periodic Error correction, the guiding software has very little work to do.

Dave