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Old 09-05-2011, 06:23 PM
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seeker372011 (Narayan)
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A polar alignment mystery

I need figuring out what went awry

I did a really good polar alignment...I thought. The star stayed in the crosshairs for five minutes when I finished doing both alt and az adjustments
I didn't bother doing a second iteration because I was trying to image at 240mm,while auto guiding, so I thought close enough would be adequate

To my surprise I found huge star trails in my 25 minute first shot;I cut back to 5 minutes, and found I was still getting trails
Cloud came over before I could do any more
But how could this be?
One thing I did different though..I polar aligned using the guide scope, not the imaging scope that I would normally use
Surely this shouldn't matter?
The imaging scope and guide scope were not perfectly aligned..not even close..still surely that shouldn't matter?
The guide scope has a focal length of 900mmm and I was using star targ with a dsi as the camera

Any thoughts?
Would appreciate any comment, especially as it will probably be three months before I get another clear night to actually try it out again

Thanks in advance
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Old 09-05-2011, 08:28 PM
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tlgerdes (Trevor)
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I used to do one iteration, now I do two. When I first started doing 2 iterations, I noticed how far off my alt was, even though it was good first iteration.
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Old 09-05-2011, 09:05 PM
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Octane (Humayun)
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Did something slip or snag?

H
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Old 09-05-2011, 09:27 PM
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Did something slip or snag?

H
Nope, certainly don't think so
Also checked the locks on both axes, they looked ok
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Old 09-05-2011, 10:32 PM
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If your guide scope was not aligned with the main scope and the guide scope was polar aligned, that means the main scope wasn't aligned at all....
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Old 09-05-2011, 10:39 PM
adman (Adam)
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I always align through my imaging scope using the QHY5, then switch it back to the guide scope. You just have to remember to recalibrate afterwards.
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Old 10-05-2011, 08:42 AM
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Rigel003 (Graeme)
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Quote:
Originally Posted by jenchris View Post
If your guide scope was not aligned with the main scope and the guide scope was polar aligned, that means the main scope wasn't aligned at all....
Don't think so. You are polar aligning the mount, not specific scopes on it. When that's done properly any scope on it should follow the stars perfectly, no matter where it's pointed.
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Old 10-05-2011, 09:07 AM
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Quote:
Don't think so. You are polar aligning the mount, not specific scopes on it. When that's done properly any scope on it should follow the stars perfectly, no matter where it's pointed.
That's true. My bad.

I normally find those two little illusive 10.7 and 11.5 stars which are 3 mins from the CP.
Does your guide scope give you that opportunity or do you do it a different way? I'm not that cluey - but my stars seem to be drift free when I set up that way.
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Old 10-05-2011, 09:22 AM
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Yep it is the mount that has to be aligned perfectly and then the scopes will follow.

Leon
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Old 10-05-2011, 09:39 AM
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Quote:
Quote:
If your guide scope was not aligned with the main scope and the guide scope was polar aligned, that means the main scope wasn't aligned at all....
Don't think so. You are polar aligning the mount, not specific scopes on it. When that's done properly any scope on it should follow the stars perfectly, no matter where it's pointed.
Following on from Rigel003's comments, as long as the mount is accurately polar aligned the scope can be pointed anywhere and the guide scope anywhere within about 20 deg, quite possibly more, of the scope and it will still track.

This is useful when imaging close to the CP where the movement is slow and there may not be enough change for the guider to follow, especially when the arcsec/pixel of the guide camera is significantly bigger than that of the main camera.

Andrew
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Old 10-05-2011, 06:54 PM
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Yes it's the mount not the scope that needs to be aligned, and the guide scope was well within say 10 degrees of the imaging scope..probably only 5 degrees or maybe even less..I didn't take particular care to align I just eyeballed it...so why was I so badly out of alignment?...clearly I did something wrong...maybe one of my "stars" was a hot pixel..though I am sure it was not
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Old 10-05-2011, 08:21 PM
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Go back and try 2 iterations and see if it improves, that would eliminate a hot pixel.

Drifting is part art, part science.
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Old 10-05-2011, 09:19 PM
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Moon (James)
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Quote:
huge star trails
How huge is huge?
Why do you suspect it was polar alignment? It could easily be something else. Can you post the images? Comparing the 25 min and 5 min exposures will be interesting.

Quote:
Would appreciate any comment, especially as it will probably be three months before I get another clear night to actually try it out again
So true.

James
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Old 11-05-2011, 06:01 PM
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How huge is huge?
Why do you suspect it was polar alignment? It could easily be something else. Can you post the images? Comparing the 25 min and 5 min exposures will be interesting.


So true.

James
Can't unfortunately..deleted them in disgust..but it's the worst star trailing I have seen in four or five years of imaging auto guided..and that too at less than half the usual focal lengths I image at.
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