Perfect polar alignment AND field rotation?

[Jessun]

Reading latest Astronomy mag, regarding guiding Mr Hallas states that even with perfect polar alignment you will get field rotation unless the guide star is in the centre of the field.

I'm jumping in the deep end here perhaps but I suggest that this is not the case.

I would argue that with perfect polar alignment there will be no field rotation regardless of how you mount and aim your scope - and guide scope, but if your alignment is off, then rotation will set in and appear worse the greater the distance between target and guide star.

Am I missing something?

[ollypenrice]

Do we care? How long do your subs have to be to show such an effect, and at what kind of focal length? A bit of drift and rotation help your Sigma stacking. There is more theory than practice in some circles and I'm of the practice school. Off the top of my head I can't see where this rotation would come from but if it existed it might marginally improve my images. All sorts of things will muck up your pictures but this isn't one of them.

Olly

[Jessun]

Some good questions Olly - and in practice we don't care an awful lot, but boiling it down to pure theory, if the polar alignment is perfect then how could things possibly rotate - it forms the basis of unguided imaging doesn't it?

I totally agree with you that rumbling about a bit is good when sigma stacking, but Mr Hallas statement is quite to the point on guiding, and I fail to see that it's correct.

[andrew s]

Jesper - There is no such thing as perfect polar alignment. The apparent position of the pole varies with the altitude you image at. On professional wide-filed telescope they actually vary the polar axis to compensate when they are imaging different altitudes.

This is due to differential refraction which cause stars to travel across the sky at varying apparent sidereal rates

But, I don't see how using a non central guide star will make much difference unless your field is several degrees in which case the image scale will be too small for it to matter anyway.

Andrew

[Jessun]

This is due to differential refraction which cause stars to travel across the sky at varying apparent sidereal rates

Andrew I thought long and hard about this before posting but concluded that a trailing/leading guide star due to atmospheric refraction could not in itself cause rotation, but merely trails for us amateurs imaging at kindergarden scale.

Interesting to hear how the big boys correct for polar alignment depending on altitude...

[andrew s]

Andrew I thought long and hard about this before posting but concluded that a trailing/leading guide star due to atmospheric refraction could not in itself cause rotation, but merely trails for us amateurs imaging at kindergarden scale.

QUOTE]

I think it can in theory but no more or less than a central guide star as it is stars at different altitudes that have different refracted poles - but for a typical amateur I think the effect are negligible.

Andrew