Is perfection on guiding possible ?

[Catanonia]

Is it possible that your polar alignment is so perfect that PHD can't work out calibration due to movements ?

Not that it has happened to me. Just curious.

[themos]

I'd say impossible. PHD sends commands to move the mount and then sees which way the star moves. It deliberately takes the mount out of sidereal tracking.

[Catanonia]

ah, thank you, answered one of those nagging doubts.

[roundycat]

apart from the software you might be using you cannot get perfect polar alignment as atmospheric refraction will affect a stars apparent position dependent on its Dec.

Also perfect polar alignment is less helpful than you might think. With autoguiding you can afford to be 'away' from the pole a bit and not have to worry about field rotation. If you are not perfectly aligned the mount will not have to make possibly alternate corrections in Dec, the drift should only be in one direction. Therefore, even though backlash will not be a problem if you balance properly, it won't become a problem during guiding as corrections will all be in one direction ie, North or South.

Dennis

[brianb]

With autoguiding you can afford to be 'away' from the pole a bit and not have to worry about field rotation.

Depends on how much out you are - if you're several degrees out, field rotation will quickly become an issue (think of an altaz mount as a misaligned equatorial....)

even though backlash will not be a problem if you balance properly, it won't become a problem during guiding as corrections will all be in one direction

Backlash can still be an issue if the balance is "heavy" in the direction of the tracking movement, because the scope will tend to jump & stick - the motor & gears will be regulating the rate at which the scope "falls", rather than working against gravity, when backlash is indeed unimportant, once the slack has been taken up.

[themos]

So, if you're imaging in the east, point your axis slightly west or east of Polaris?

[brianb]

So, if you're imaging in the east, point your axis slightly west or east of Polaris?

I'd try to get it as accurate as possible, but not worry about the last few minutes of arc.

With a GEM, if imaging in the east, the counterweight is rising so it should be "heavy"; if imaging in the west, the counterweight is falling so it should be "light". Not too much though. You'll usually get it right by balancing the scope accurately then applying some friction with the RA axis lock, but many scopes (HEQ5 for a start) don't have the right sort of adjustment to axis friction to make this sensible.

[Whippy]

With a GEM, if imaging in the east, the counterweight is rising so it should be "heavy"; if imaging in the west, the counterweight is falling so it should be "light". Not too much though.

I understood this to be the other way around, if you're imaging in the East, the mount is weighted slightly 'scope heavy' while it's 'weight heavy' imaging West of the meridian.

Tony..

[SteveL]

brian is correct... whatever is on the east side of the mount needs to be heavy, so that the mount is lifting against something.