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Drift Alignment Question

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#1 canondslr

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Posted 19 September 2019 - 06:24 PM

Hello everyone.

 

For a few years, I have been using the drift align method with a Canon DSLR, Bogen Tripod, Astrotrac, and  Astrotrac wedge.  Its always worked very well , but something has me a little confused.  When I first line up close to the Celestial Equator, and Meridian, I typically run 8 minute exposures with a Canon 200MM. lens. Then after adjusting the wedge to the East or West to reduce Declination drift, I'm always left with a bit of East West trailing, which I had always been taught to ignore. Next, I adjust the mounts North South orientation, while still imaging on the Celestial equator, but now near the Eastern horizon. Strangely, after again correcting for Declination drift, I'm left with mostly round stars.  The East West component of drift I was left with near the Meridian, has all but dissappeared near the horizon. 

 

My question is. Since both adjustments are made along the celestial equator, why would the East West drift almost dissappear completely near the horizon, even before making any further adjustments, but remain obvious at or near the meridian? And this seems to happen every time I go through the procedure. My guess was some sort of flexure in the tripod, ballhead, or camera attachment, when lined up near the Meridian?  Or am I missing something in the geometry, where East West trailing at the Meridian would typically be greater than near the Horizon?  I didn't want to go through to much work finding the weak link if this is typical result?

 

Thank you in advance for any insights.

 

Tom



#2 Jerry Lodriguss

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Posted 19 September 2019 - 07:16 PM

Hello everyone.

 

For a few years, I have been using the drift align method with a Canon DSLR, Bogen Tripod, Astrotrac, and  Astrotrac wedge.  Its always worked very well , but something has me a little confused.  When I first line up close to the Celestial Equator, and Meridian, I typically run 8 minute exposures with a Canon 200MM. lens. Then after adjusting the wedge to the East or West to reduce Declination drift, I'm always left with a bit of East West trailing, which I had always been taught to ignore. Next, I adjust the mounts North South orientation, while still imaging on the Celestial equator, but now near the Eastern horizon. Strangely, after again correcting for Declination drift, I'm left with mostly round stars.  The East West component of drift I was left with near the Meridian, has all but dissappeared near the horizon. 

 

My question is. Since both adjustments are made along the celestial equator, why would the East West drift almost dissappear completely near the horizon, even before making any further adjustments, but remain obvious at or near the meridian? And this seems to happen every time I go through the procedure. My guess was some sort of flexure in the tripod, ballhead, or camera attachment, when lined up near the Meridian?  Or am I missing something in the geometry, where East West trailing at the Meridian would typically be greater than near the Horizon?  I didn't want to go through to much work finding the weak link if this is typical result?

Hi Tom,

 

East-west trailing is generally periodic error in your RA gears and isn't related to polar alignment.

 

Jerry



#3 canondslr

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Posted 20 September 2019 - 12:02 PM

Thank you for the response Jerry

 

Good to know Polar alignment isn't the issue.

 

I had originally figured it was just Periodic Error, but I'm wondering if the forces from the tripod, or ballhead are contributing to the issue? If so, time for better equipment. 

 

For example, once I've finished polar alignment, a 5 minute exposure with a 200mm lens will show noticeable East West drift along the Equator near the Meridian. But the same exposure taken along the equator near the horizon will have round stars with virtually no East West trailing? The Astrotrac typically has very low PE. When shooting along the Celsestial equator, would one expect the East West trailing  at the Meridian to be similar to the trailing at the Horizon? 

 

Thanks again for any insights.

 

Tom




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