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Star Drift Polar Alignment

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#1 MMBARS

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Posted 20 May 2019 - 10:57 PM

I am interested in learning how to polar align by manually adjusting the mount using the star drift method.  I have read several post and summaries of the method used but I remain unclear on how to orient the reticle in the finder.  I seems that Y axis should be tangential to the celestial equator.  So if a star is picked due south near the celestial equator the Y axis should be parallel  to the horizon and the X axis should be near the Meridian pointing north.  The same principle should apply when picking a star in the east, but orienting the Y axis of the reticle tangential to the celestial axis is more difficult to estimate.  If the reticle is not oriented correctly, adjustments in azimuth or declination could worsen polar alignment.  The other factor is measuring drift with the motor running or off.  How drift alignment works is makes sense, but I am having difficultly knowing how to orient the reticle correctly to measure the drift north and south to make the necessary adjustments in the mount.  I know there are polar scopes and software programs that  make this much easier and more precise, but right now I would like to learn to do this manually.

 

Thanks,

Mike 



#2 Kendahl

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Posted 21 May 2019 - 12:35 AM

Here is a discussion of several polar alignment methods including drift alignment: https://uncle-rods.b...alignment party



#3 Dynan

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Posted 21 May 2019 - 12:57 AM

Can you pop a camera in your scope? If so:

 

https://www.cloudyni...bert-vice-r2760



#4 kathyastro

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Posted 21 May 2019 - 07:12 AM

You want the reticle to be orthogonal to the celestial coordinate system.  So one line is north-south and the other is east-west.  You test the orientation by observing the star for a few minutes with tracking turned off.  The star should drift along the east-west line.  Then turn on tracking and begin the drift alignment.


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#5 dakinemaui

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Posted 21 May 2019 - 09:39 AM

One way is to center a star on the retical then quickly slew in right ascension by a half field and rotate the reticle until that star is back on one of the lines. This is similar to the camera approach linked above where slewing during an exposure will essentially "draw a reticle line" in the image.

 

One point, though: you said "finder" in the OP. If your goal is to image, you'll obviously have a camera so you might as well use that. If your goal is visual, then doing the above in the finder may be sufficient, but better results can be had by using an eyepiece with a reticle.


Edited by dakinemaui, 21 May 2019 - 09:45 AM.

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#6 Kendahl

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Posted 21 May 2019 - 09:41 AM

You want the reticle to be orthogonal to the celestial coordinate system.  So one line is north-south and the other is east-west.  You test the orientation by observing the star for a few minutes with tracking turned off.  The star should drift along the east-west line.  Then turn on tracking and begin the drift alignment.

Orienting the reticle would go faster if he ran the mount up and down in declination. When the orientation is correct, the star will stay on the north-south line.


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#7 MMBARS

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Posted 21 May 2019 - 10:43 AM

Thanks for your comments and links.  I think I now understand how to correctly orient the reticle in the focuser.  If the skies permit, I will give it a try in the next few days.



#8 Alex McConahay

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Posted 21 May 2019 - 11:42 AM

By the way, it does not matter where the reticle is. All that matters is if the star drifts up or down from where it was when the run started. Yes, if the reticle is correct it is easier to see the change, and define the direction of change. But it is not essential that you have it rectilinear. I would follow the excellent advice already given. But realize that as long as you can see the drift, it does not matter whether the reticle lines are precisely NSEW.

 

 

Alex


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#9 Phil Sherman

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Posted 22 May 2019 - 04:07 PM

Can you pop a camera in your scope? If so:

 

https://www.cloudyni...bert-vice-r2760

This procedure works better with the following changes:

1. Slew E then W. An E slew stops the mount's tracking and keeps the gears engaged.

2. Try using a 70 second exposure with 5 seconds tracked, 30 slewing E, then 35 slewing W. When your alignment is very close, the W slew will pass back through the starting point making it easy to judge how close to aligned you are. You can always use a 130 second exposure with doubled slew times (60,65 seconds) for higher resolution of the drift.

 

The big advantage of this technique is that it's mich faster than visual measurement of drift. It's easy to observe a 0.5 pixel drift error in the image, something that is usually less than one arcsec of drift.


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