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Polar Alignment

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#1 B. Hebert

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Posted 14 February 2010 - 04:29 AM

Last night I performed a careful polar alignment using my polar scope on my SVP mount. Using PHD my guiding graph has a regular deep sawtooth pattern with occasional wild swings that I suspect is due to very poor polar alignment. I got no visible difference using 1 or 3 second sampling or by lowering RA aggressiveness.
After polar alignment I was able to rotate my mount around RA and Polaris neatly traced the inscribed circle on the scope recticle and was well centered in its proper location.
Obviously I need to do a drift alignment (and I eventually will) but how far off can I be if it is carefully aligned in a polar scope?

Is my poor guiding graph likely a result of polar alignment or some other cause?

B. Hebert

#2 guyroch

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Posted 14 February 2010 - 10:58 AM

I did the same last week with my new NEQ6, polar aligned with the polar scope only and I ended up with considerable filed rotation around the edge on 10min subs with PHD. I've always dreaded drift aligmenent because I've never been able to fully grasp the technique but I found the simulator last week and now I can't wait for the clouds to move away so I can try it.

http://www.petesastrophotography.com/

Click on the "Drift Technique" about half way down the left menu and then completly at the bottom of the page you'll have a link "Guiding Simulator".

#3 Eric P

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Posted 14 February 2010 - 11:00 AM

Not familiar with the SVP, but it could also be the motors struggling or clutches slipping on an unbalanced load.

#4 DaemonGPF

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Posted 14 February 2010 - 11:40 AM

With your DSI you could use WCS which will help you drift align perfectly.

http://wcs.ruthner.at/index-en.php

It's a little strange the first run, but once you realize what you're doing with the software it takes no more than 5-10 minutes to get extremely precise polar alignment.

#5 drksky

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Posted 14 February 2010 - 12:07 PM

Hmmmm...I have a NexImage. I may have to try WCS when I actually get a mount...

#6 Starhawk

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Posted 14 February 2010 - 12:21 PM

The polar alignment scope is good enough to START alignment from, and that's it. I took the polar scope out of my Vixen mount so I could use its path for a cableway (yep, it's that useful). Look up www.starizona.com, they have the best instructions on doing a drift alignment. Drift isn't that hard to do, but it's a bit time consuming, but at that point, you can have a really good alignment.

-Rich

#7 David Pavlich

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Posted 14 February 2010 - 04:24 PM

The only company that makes an extremely accurate polar scope that I know of is Takahashi. It's legendary in its accuracy.

David

#8 B. Hebert

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Posted 14 February 2010 - 10:08 PM

I had much better luck with my polar scope out of the box. I suspect that an attempt at "alignment" may have affected it.

Any's Shotglass has a great description of scope alignment, I'll try again.

#9 BlueGrass

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Posted 14 February 2010 - 10:43 PM

and .... using a polar scope to perfrom an initial alignment means that the polar scope reticule itself must be accurately aligned within the polar scope itself. If the reticule is not aligned, you're simply wasting your time using it. The link guyroch posted gives a very good visual representation of how drift alignment works. Thanks.

#10 EricJD

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Posted 15 February 2010 - 09:57 AM

I did the same last week with my new NEQ6, polar aligned with the polar scope only and I ended up with considerable filed rotation around the edge on 10min subs with PHD. I've always dreaded drift aligmenent because I've never been able to fully grasp the technique but I found the simulator last week and now I can't wait for the clouds to move away so I can try it.

http://www.petesastrophotography.com/

Click on the "Drift Technique" about half way down the left menu and then completly at the bottom of the page you'll have a link "Guiding Simulator".


I'm a bit baffled by this. I only use the polar scope on my Atlas to align with, and I don't think it's even properly centered in the mount, and I'm able to get 8 minute guided subs with zero rotation.

#11 guyroch

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Posted 15 February 2010 - 01:08 PM

I did the same last week with my new NEQ6, polar aligned with the polar scope only and I ended up with considerable filed rotation around the edge on 10min subs with PHD. I've always dreaded drift aligmenent because I've never been able to fully grasp the technique but I found the simulator last week and now I can't wait for the clouds to move away so I can try it.

http://www.petesastrophotography.com/

Click on the "Drift Technique" about half way down the left menu and then completly at the bottom of the page you'll have a link "Guiding Simulator".


I'm a bit baffled by this. I only use the polar scope on my Atlas to align with, and I don't think it's even properly centered in the mount, and I'm able to get 8 minute guided subs with zero rotation.



Then take a look at one on my thread a few weeks ago which ended up in a Field rotation conversation.

You'll notice that the wide field does not really show the amount of field rotation so if you crop you're images it's fine. However, a 100% crop of all 4 corners does show significant field rotation.






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