Polar alignment and RA drift

May 22, 2006



#29869 May 22, 2006

Hi Everyone,

I've been following the thread talking about the RA drift - about 2.5

arc seconds per minute retrograde drift, from what I could see in the

posted graphs. First, it seems strange that this is precisely the solar

tracking rate (I think). Can anyone comment on that aspect?

I went and reviewed some PEMPro records and sure enough I also see some

RA drift although not to that extent - maybe 0.6 arc seconds per minute.

Here is my question for the group.

Why, with a pretty solid but not perfect polar alignment, would one

experience ANY RA drift due to polar alignment error? I've been

whacking the geometry of the thing around in my head and the RA drift

always seems to be a function of the DEC drift times the SIN of the

polar alignment error (depends on where you are pointing as to how

altitude and azimuth error would each influence the drift). And the SIN

of a 5 arc minute polar alignment error is a very very small number.

I just can't see how RA drift due to polar alignment error would be

anything but a very small number, if the scope is even roughly aligned.

I routinely have my scope polar aligned to within 5 arc minutes and I

still see this RA drift.

In fact I see a pretty consistent RA drift of about 0.7 arc seconds per

minute no matter how well polar aligned I am. At the start of a session

the DEC drift might be very high - 12 arc seconds per minute - and 20

minutes later I have made some alignment adjustments and the DEC drift

is down to .4 arc seconds per minute (indicating I think a 2 arc minute

polar alignment). But the RA drift is still there.

I am going back over some old data and might do some experiments if I

get a night clear enough to test but not clear enough to image.

The more I look at this the more I am convinced that the RA drift is

something to do with tracking rates, and the geometry seems to back

this up.


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