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Stars for iterative polar alignment?

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#1 UniversalMaster

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Posted 29 March 2009 - 11:35 AM

Hi!

What stars do you use for iterative polar alignment?
I saw somewhere that it should be more than 3h away in RA, but what about dec? Could, say, any of the other stars in Ursa Minor be use (e.g. Kochab)?

Søren

#2 HaleBopper

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Posted 29 March 2009 - 12:16 PM

Here are a couple of articles that you may find informative:

http://www.covington...o/iterating.pdf

http://www.allabouta...tivemethod.html

#3 UniversalMaster

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Posted 29 March 2009 - 01:09 PM

Thanks!

The 2nd says 90 deg. from polaris in Dec, so this method is a no-go for me :(

Have anyone tried it with a star like 45 deg. from polaris?

Søren

#4 Kolenka

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Posted 29 March 2009 - 05:05 PM

It will work, but you will remove less of the actual alignment error.

The reason for using stars near the equator is to make the alignment process more sensitive to error. This is true for both drift and iterative alignment.

#5 UniversalMaster

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Posted 30 March 2009 - 01:35 AM

Thanks. Unfortunately, many times I can not see so much of the sky.

Søren

#6 Skylook123

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Posted 30 March 2009 - 01:11 PM

For iterative polar alignment, I've found that I can use almost any star. The difficulty is how to balance "precision" with "accuracy". Iterative polar alignment requires an alignment on a star, then GOTO Polaris and adjust out the offset with the mount mechanicals. Using a star more than 3 hours RA and around 90 degrees in DEC will spread the offset error over a larger range and make it easier to crank out the error on fewer cycles. If you use close-by stars like Dubhe, Merak, or Kochab, then your adjustment back to Polaris just needs to stop with a bigger remainder. Otherwise, you'll rattle around the solution (e.g. overcorrect) due to the lack of optimum star choice, so if you take out too much offset you may have crossed to the opposite side of the error. Then the next cycle looks worse. However, I have used stars occasionally that would seem to not be ideal; in that case, I just take out only half of the offset, and redo again. It may take me three or four cycles, but if I don't take out too much offset I converge nicely. If I don't, the next 1-star alignment is worse than the previous, so it's a clue that I chose a less than perfect star and I took out too much. So I only take out half of what I took out on the last cycle, and it seems to do OK, just takes longer.

If need be, pick what star you can, just don't take out too much offset ( I know how tempting it is to try to take out most of a large error; don't do it!) and you should converge. Maybe take out only half the error, not 3/4 or more. It might take several cycles though; be patient.

#7 UniversalMaster

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Posted 30 March 2009 - 03:22 PM

I usually use my polar scope with "Kochab's clock", but I find that sometimes Im a bit too much off, so if I could just do a few iterations to improve this, it would be great. It does not have to be perfect, the autoguider can handle the rest.
I'll try it the next time out. Thank you for your help :D

Søren

#8 Peter in Reno

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Posted 30 March 2009 - 03:42 PM

I use polar scope with Atlas EQ-G. I rotate the RA axis and make sure that Polaris stays on circle line in polar scope. That's good enough for me and my auto guider (SSAG). I find iterative and drift alignment too time consuming for portable use.

Peter

#9 UniversalMaster

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Posted 31 March 2009 - 02:15 AM

Hi Peter!

I do the same, but sometimes the pol-scope is out of alignment since I move my mount a lot. It would be nice to easily be able to spend like 5 min to correct for this possibility.

Søren






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