Quote:
Originally Posted by g__day
I ponder is there any polar misalignment or mirror shift that can manifest solely as an error in RA rate rather than DEC?
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In a nutshell, no. Polar misalignment and mirror shift will bring about apparent
shifts in both RA and Dec.
There are other error terms that will be predominantly in the RA axis,
notably Collimation in Hour Angle (CH) and Non Perpendicular Axis (NP)
error terms. However, since your mount is polar aligned and since
you are imaging a star with a constant Dec, these will not be involved
in this instance.
Quote:
This stumps me a bit - the shift is always along the RA axis - which to me reads like a tiny tracking rate error (like its one part in a thousand faster than it should be).
I track a star for 30 minutes and see maybe a 1-2 pixel drift in DEC (so say an 1-3 arc seconds or 25% of a star diameter out of position) but in RA it is likely to be 4 star diameters out of position in RA.
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Do you see the same magnitude of error in both RA and Dec for all zenith distances?
I see further below the answer to this may be that you are unsure at this stage
since you have insufficient data.
The reason I ask is that even if your pointing computer is performing refraction
calculations, it won't be adjusting the actual elevation of the mount to account for
refraction. All it can do is change the tracking rates in both RA and Dec.
Are you auto guiding?
Best Regards
Gary