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Equipment Discussions >> Mounts

Pages: 1
mpgxsvcd
Carpal Tunnel


Reged: 12/21/11

Loc: Raleigh, North Carolina
How close is good Polar alignment? new
      #6136658 - 10/14/13 11:01 AM

I have a CG-5 and I do as precise a visual Polar Alignment as I can with the Polar Finder scope first. I typically shows within 4-6 minutes when I do the display align after that.

Then I do the all-star align and it always shows zeros. However, that is a misleading spec because the mount is just assuming that it is now perfectly aligned.

Finally I go back and do a new 2 star alignment with 4 additional stars and check the display Align again. Then it almost always shows less than 1 minute error for AZM and about 3-4 minutes for ALT. I can easily do 2 minute exposures with it like that. In most cases the Periodic error becomes the limiting factor instead of the Polar alignment.

So are these decent numbers or should I be in the single digits of seconds for the error? I really don’t want to take the time to do a drift align. With my shorter focal length scope it would take too long to see significant movement for a drift alignment.

Is there any reason that the ALT error is always greater than the AZM error?


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Calypte
Pooh-Bah


Reged: 03/20/07

Loc: Anza, California
Re: How close is good Polar alignment? new [Re: mpgxsvcd]
      #6136713 - 10/14/13 11:30 AM

Keep in mind that even with a perfectly aligned mount, atmospheric refraction has an effect on go-to accuracy. My mount (A-P Mach1) is in an observatory now, but during its portable days I found that using a polar scope was easily good enough for 10-12 min subs. Same for my earlier G11. With Roland's "quick drift alignment" routine or conventional drift alignment, I could do 36 min narrowband subs without field rotation.

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mpgxsvcd
Carpal Tunnel


Reged: 12/21/11

Loc: Raleigh, North Carolina
Re: How close is good Polar alignment? new [Re: Calypte]
      #6136731 - 10/14/13 11:38 AM

Quote:

Keep in mind that even with a perfectly aligned mount, atmospheric refraction has an effect on go-to accuracy. My mount (A-P Mach1) is in an observatory now, but during its portable days I found that using a polar scope was easily good enough for 10-12 min subs. Same for my earlier G11. With Roland's "quick drift alignment" routine or conventional drift alignment, I could do 36 min narrowband subs without field rotation.




How close is "Good Alignment"? Is that less than 1 minute of error? Doesn't your periodic error come into play at 36 minutes?

Edited by mpgxsvcd (10/14/13 11:39 AM)


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WillCarney
Pooh-Bah
*****

Reged: 10/08/09

Loc: Bloomington, ILL
Re: How close is good Polar alignment? new [Re: mpgxsvcd]
      #6137361 - 10/14/13 04:08 PM

The ALT error difference could simply be not perfect level on tripod. Just a little difference in level can show up.

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mpgxsvcd
Carpal Tunnel


Reged: 12/21/11

Loc: Raleigh, North Carolina
Re: How close is good Polar alignment? new [Re: WillCarney]
      #6137424 - 10/14/13 04:38 PM

Thanks for the heads up about the level. I need to make sure I check that every time.

Here is a thread that answers my original question. It looks like anything less than 10 minutes of error will work and anything around a few minutes is considered to be good.

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/1,2,3,4,5,8/Number/5325139/Main/5315934

Edited by mpgxsvcd (10/14/13 04:38 PM)


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AlBoning
professor emeritus


Reged: 03/06/11

Re: How close is good Polar alignment? new [Re: mpgxsvcd]
      #6137688 - 10/14/13 07:16 PM

Altitude of a celestial body would figure in. When I was learning how to use a sextant one of the corrections entered into the sight reduction was for atmospheric refraction. The amount varies, decreasing with increasing altitude.

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Calypte
Pooh-Bah


Reged: 03/20/07

Loc: Anza, California
Re: How close is good Polar alignment? [Re: mpgxsvcd]
      #6138173 - 10/15/13 12:24 AM

Quote:

Quote:

Keep in mind that even with a perfectly aligned mount, atmospheric refraction has an effect on go-to accuracy. My mount (A-P Mach1) is in an observatory now, but during its portable days I found that using a polar scope was easily good enough for 10-12 min subs. Same for my earlier G11. With Roland's "quick drift alignment" routine or conventional drift alignment, I could do 36 min narrowband subs without field rotation.




How close is "Good Alignment"? Is that less than 1 minute of error? Doesn't your periodic error come into play at 36 minutes?



I don't know. I've never quantified it. I'm guiding and doing corrections during 36 min subs (well, really, all of them), but field rotation is what you have to worry about if your polar alignment is off.


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