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What is considered excellent/good/fair PA error?

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#1 robininni

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Posted 22 May 2013 - 10:57 PM

Would you tell me if this is good polar alignment? I assume this means 1 minute on Az and 4 minutes on Alt. Is this okay, good, excellent?

Thanks,

Rob

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#2 robininni

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Posted 23 May 2013 - 10:51 AM

Maybe I should rephrase my question, and that is, what is considered excellent vs. good vs. okay polar alignment?

Thanks,

Rob

#3 Mike Wiles

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Posted 23 May 2013 - 11:01 AM

I don't know how to quantify excellent vs good vs okay. Experience taught me a couple of years ago that I could consistently and easily get within 5 arc minutes of the pole with my Celestron mounts and that was sufficient to expose up to 15 minutes. I don't actually know how long I could have exposed before polar alignment became an issue - I never tried.

When I got my AP900 and began using PemPro's polar alignment routine when I setup, I could get within 2 arc minutes of the pole without much effort. I shot subs as long as 45 minutes with that setup in a portable configuration and never had an issue. Now that I have the mount placed permanently, the polar alignment is 35 arc seconds from the pole. During moonlit nights, I shoot 60 minute narrowband subs without issue.

So I'd quantify:

> 10 arc minutes = not ideal
< 10 arc minutes = okay
< 5 arc minutes = good
< 2 arc minutes = excellent

Mike

#4 raf1

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Posted 23 May 2013 - 11:28 PM

Maybe I should rephrase my question, and that is, what is considered excellent vs. good vs. okay polar alignment?


Rob - I have an AP1200 polar aligned <30 a/s in ALT and AZ with PEMPro2. Guiding 30-60 minute subs with an OAG is no problem. My Peak to Peak corrected PE is .86 a/s. I'm wondering if another indicator of good polar alignment would be the length of an unguided image without star trails. Anyone measured this just for the halibut?

CS, Ron

#5 David Pavlich

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Posted 24 May 2013 - 12:57 PM

There's PE then there's PE. If you have a jagged graph, in other words, jumpy, the PE will be more difficult to guide out. But, if the PE graph is smooth, the guider will have little trouble getting you round stars as long as you've done your part and have a good polar alignment.

David

#6 Jared

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Posted 24 May 2013 - 06:57 PM

Ron, wouldn't that be determined more by periodic error and focal length rather than polar alignment error? PA is just going to determine how long you can go without field rotation.

#7 alpal

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Posted 24 May 2013 - 07:32 PM

Hi Rob,
I find that the results of Deep Sky Stacker let
me see how well I have polar aligned.
Note the PHD guiding cannot stop polar alignment error -
the field will still rotate slightly from mal adjustment.

See the image below.
That was the results for my latest Centaurus A image.

For polar alignment you can see that I obtained worst +.22°
from the reference frame & -.13°.
That's a total error of 0.35° over 2 hours & 11 minutes -
or an error of 0.16° per hour.

Your measurement seems bad at 4 minutes 28 seconds.

May I suggest that you look at what you obtain in DSS
in case your information above is misleading?

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#8 pfile

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Posted 24 May 2013 - 09:18 PM

alpal, that kind of drift while guiding is usually caused by differential flexure rather than polar alignment error. if you have perfect guiding then as you point out you will have field rotation but there should be no drift.

if you turn off guiding and measure the drift then you can draw some conclusions about polar alignment.

#9 raf1

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Posted 24 May 2013 - 09:29 PM

Ron, wouldn't that be determined more by periodic error and focal length rather than polar alignment error? PA is just going to determine how long you can go without field rotation.


Good question Jared and I don't know the answer - never had cause to consider it before. I thought that how long one could go unguided without star trailing at longer focal lengths might be an indicator of polar alignment accuracy.

I image at 2,550 mm and can image unguided for some time without trailing. I suspected that pretty good polar alignment was a major contributor to that unguided success at my focal length. And yes, my PE is smooth so using an autoguider for 30-60 minute sub-exposures is uneventful. I'm really looking forward to APCC being released and what its pointing model will do for improved tracking. Thanks for your reply.

CS, Ron

#10 alpal

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Posted 24 May 2013 - 09:44 PM

alpal, that kind of drift while guiding is usually caused by differential flexure rather than polar alignment error. if you have perfect guiding then as you point out you will have field rotation but there should be no drift.

if you turn off guiding and measure the drift then you can draw some conclusions about polar alignment.



Yes - I don't know why I had drift because I use an OAG.
Those results were typical.
I did not use dithering.

#11 David Ault

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Posted 25 May 2013 - 12:46 AM

I was trying to do the math and determine how long of an exposure you could take at a given image scale without seeing the effects of field rotation. Determining the limits for an unquided exposure based on drift from polar mis-alignment was not difficult, but my attempts to determine the field rotation did not really work out. I found this site, but once they started talking about spherical trigonometry and derivatives my brain just melted. Maybe someone smarter than me can turn that paper into something usable.

Regards,
David

#12 mikeschuster

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Posted 25 May 2013 - 01:06 AM

Here is a field rotation calculator and a link to an article:
http://celestialwond...xErrorCalc.html
Mike

#13 orlyandico

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Posted 25 May 2013 - 04:20 AM

What I found was that I still had field rotation in 20-minute guided subs with about 12 arc-minutes of mis-alignment. So I agree with the statements above that you have to get under 10'

#14 robininni

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Posted 25 May 2013 - 08:36 AM

This is my best polar alignment so far. I tried adjusting it once when it was slightly not as good as this using Maxpoint's Polar Align function where it moves a star to where it *should* be and you adjust your mount to center it. This however, is extremely hard to do with my Atlas (and a full load makes it worse I think).

I finally got it but by the time I did it must have been off due to PE during the several minutes of work and then after running another 100 pt Maxpoint routine, my polar alignment was worse!

This current polar alignment I did using the EQMOD Polar Alignment routine and my polar scope. This was much more accurate then trying to adjust the mount to get a star centered.

I have wondered, do Paramounts and AP mounts adjust smoothly and easily compared to Atlas mounts? Particularly the Altitude is very difficult to adjust with any precision.

I would love to get less than 1 arc sec on each ALT and AZ. If I only adjust one, this theoretically should not change the error in the other, right?

Thanks,

Rob

#15 David Ault

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Posted 25 May 2013 - 12:22 PM

Mike,

That is awesome. Using that calculator you can plug in your image scale information and either determine a maximum exposure based on pixel or sky deviation (i.e. so many arc-seconds). Thanks for finding and posting that.

Regards,
David

#16 raf1

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Posted 25 May 2013 - 02:29 PM

I have wondered, do Paramounts and AP mounts adjust smoothly and easily compared to Atlas mounts?


Hi Rob, the AP1200 has easy to use ALT and AZ adjusters which makes precise adjustments of each axis drama-free.

Hth, Ron

#17 orlyandico

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Posted 26 May 2013 - 03:43 AM

Robin, yes.

Adjusting the ALT and AZ on my Mach1 (with precision azimuth adjuster) is smooth, precise, backlash-free, and stiction-free.

Even my old AP600 is nowhere as smooth.

It's part of the price you pay. May seem extremely minor but it makes a huge difference when doing polar alignment.

#18 vdb

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Posted 26 May 2013 - 04:00 AM

It depends on a lot of factors, but technology can help.
-TheSky X with a PM mout can correct for PA error
-The 10micron
- or Sitech controller ...
All build a model that can correct for errors including PA
Or have a guide star close to the centre, you will still have rotation with a FF camera but smaller chips/smaller FOV will get away with it. Of course you loose OAG and you can't correct for errors like mirror shift ...
Or you use one of these: http://www.innovationsforesight.com






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