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Polar aligning a side by side

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#1 Doug6952

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Posted 25 April 2009 - 06:42 AM

The last 2 nights I've been blessed with clear skies. It was my first time out with my new CGEM and also my first time trying to polar align with a side by side setup. I did the all star align routine and there was no/very little DEC drift, but it slowly drifts west. Every 8 min exposure, it was a bit further west. It didn't cause trailing or odd shaped stars. I have the mount balanced east heavy and Dec axis is balanced.

The 2 scopes that I have set up are a C6 SCT and an EON 72mm. The C6 is closest to center and is the scope I used for the alignment. I wish I could do a drift alignment, but the East and West horizons are blocked by trees. Since neither scope is centered, is it even possible to get a perfect polar alignment?

#2 HunterofPhotons

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Posted 25 April 2009 - 10:28 AM

...... I wish I could do a drift alignment, but the East and West horizons are blocked by trees. Since neither scope is centered, is it even possible to get a perfect polar alignment?

Yes.
You're aligning the mount, not the scopes.
As long as you do your alignment with one scope, there should be no problem.
Why not drift align? You only need access to one horizon and you usually avoid the bottom 20-30 degrees anyway. How much of your eastern horizon is blocked?

dan

#3 DrBuck

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Posted 25 April 2009 - 11:16 AM

...... I wish I could do a drift alignment, but the East and West horizons are blocked by trees. Since neither scope is centered, is it even possible to get a perfect polar alignment?

The distance between the two scopes is absolutely unimportant and miniscule. The important thing is to have the main scope square and aligned with the mount as closely as possible, then align the secondary telescope with that. If you absolutely cannot do a drift method alignment, there is another method. Mount and align a green laser pointer so that it is square and pointing exactly at the same distant spot that the telescope is. (Either land object or star) Then do your best polar alignment of the mount. Then Loosen the locks on the mount so that it will swing freely around the polar axis. Then swing the scopes from side to side with the green laser on. carefully observe the far point of the laser and discern whether it is motionless in space or is it moving.?? If it moves, the polar alignment is definitely off. If it appears motionless then the alignment is quite good. Be very specific when aligning the laser with the RA axis of the Mount!!

#4 Doug6952

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Posted 25 April 2009 - 01:36 PM


How much of your eastern horizon is blocked?

dan


East is completely blocked and west I might be able to hit 50° or maybe only 60°. I might try setting up a single scope tonight and see if it's a problem with the mount.

#5 Doug6952

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Posted 26 April 2009 - 04:39 AM

I managed to do a drift alignment. I set up the mount in a different location so I could get as low in the west as I could. The lowest I could get is somewhere around 40-45°. I've always read that the star in the east or west should be around 20°, but it seems to have worked.

I could do any real imaging due to light clouds, but I did a test shot for 15 min and didn't get any trailing.

Thanks for your help.

#6 Strgazr27

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Posted 26 April 2009 - 06:38 AM

Doc,

That method only proves orthogonality of the OTA and not Polar alignment. I could point the scope at any object and do the above but it would not be polar aligned. To be correctly polar aligned you need to have either polaris visible, a good polar scope, software based polar alignment or iterative drift alignment. It does help to have the OTA orthogonal to the mount though.

#7 HunterofPhotons

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Posted 26 April 2009 - 11:09 AM

I managed to do a drift alignment. I set up the mount in a different location so I could get as low in the west as I could. The lowest I could get is somewhere around 40-45°. I've always read that the star in the east or west should be around 20°, but it seems to have worked.....

That's fine, Doug.
They say that it's possible to achieve an accurate alignment using a star up to 50 degrees above the horizon. The problem is that the higher in the sky the star is, the altitude adjustments that you are making are also affecting the previous azimuth adjustment that you made. That means that you need to go back to a meridian star and repeat the azimuth procedure and then a touch-up to the altitude again.

dan

#8 DrBuck

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Posted 26 April 2009 - 12:29 PM

Bobby, I see what you mean, I think!! In other words, if I pointed the polar axis of the mount anywhere in the sky and then performed that procedure, it would appear motionless?---that could be possible, I have never done that. The only time I have used that method was with a wedge mounted LX200 12 inch with the laser mounted to the OTA. I have a clear shot at Polaris and other points north. Everytime I performed that procedure, it would only appear motionless when pointed at the NCP, not Polaris. moving the wedge controls one direction or the other would cause the end of the laser to move in an arc. My new cge is correctly polar aligned using celestrons software, but I could test the theory with a smaller GEM that I have. :)






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