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What is considered an excellent polar alignment?

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#1 Whichwayisnorth

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Posted 24 September 2013 - 03:50 PM

After doing a polar alignment I've been checking it using PEMPRO. You can set the duration and it will find a star and image it and plot the drift for that duration. Initially it displays drift in arc minutes but you can check the box to show arc seconds.

So after 5 minutes what would be considered excellent? Where do you personally draw the line? If you were setting up a permanent observatory system I would assume you would fuss with it until you can get it as close to 0 as humanly possible however I am not sure if any mounts are capable of such fine adjustment to get it that perfect. At least not on my budget

If someone were to ask me the question I would say anything under 1 arc/min would be excellent.

I look forward to your comments. :kitty:

#2 A. Viegas

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Posted 24 September 2013 - 04:03 PM

I suppose it's a function of how fast you want to get set up and what you are trying to do, visual, video or imaging...

I have my CPC hibernated. When I wake it up it and I do a quick slew to a star I always reset the alignment as its usually off by a little and I am typically 4-8 arc mins off polar alignment. A quick ASPA and I get to 1-4 which for my purposes is just fine.

Visually when I do a quick set up from scratch I get to around 10 arc mins with 2+4 on my CgEm and i find that just fine an this is without using the polar scope. If I try hard with the CgeM and polar scope I can get down to less than an arc minute error with just a few ASPA iterations. Arguably I find the ASPA more accurate an it holds better on the CGEm than on the wedge mounted CPC

Al

#3 Whichwayisnorth

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Posted 24 September 2013 - 08:46 PM

Using Celestron ASPA routine on my AVX mount the typical results are under 2 arc minutes. So my question is, what are the general feelings about that result? Where would it fall on the scale between excellent(don't touch it) to really bad.

Do people keep tinkering with their polar alignment if they get it under 1 arch minute? 30 arch seconds? At what point would someone going for an excellent PA stop fussing and start imaging? If set up in an observatory where you would only do this once, how close to zero drift do people get?

#4 orlyandico

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Posted 24 September 2013 - 08:51 PM

Wow you guys must be getting really good alignments. So how do you measure that 2' ? (in a previous thread it was pointed out that you can't use ASPA to measure the residual polar alignment error, because ASPA uses the pointing model which itself is only accurate to 5' or so).

Me if I get under 5' I'm happy. I can do 20 minute guided subs with round stars with that. But then I can't see Polaris and my mount doesn't have ASPA, so I have to use PEMPro every time..

#5 gdd

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Posted 24 September 2013 - 09:35 PM

Instead of measuring the alignment error in arcmin, why not just measure the drift in arcsec over the longest length of subexposures you plan on taking? When you get to the point where you either reach 0 arcsec or diminishing returns you are done.

Gale

#6 gmartin02

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Posted 25 September 2013 - 01:59 AM

I generally get somewhere between 1.5' to 5' error as calculated by the polar alignment estimator in PHDLab. Of course this may not be accurate, as pointed out by the disclaimer in the PHDLab documentation. Anyway, I always get round stars on 5 to 8 minute subs, and it doesn't appear that the Dec corrections are very frequent most of the time (lots of 30 second spans between Dec corrections), so it probably isn't too far off.

I use ASPA on my CGEM, and always do a 2+1 star alignment (yes, that's right: 2+1, not 2+4). When doing the star alignment I use LiveView in BackyardEOS zoomed with the cross hairs. I may not be doing things the "right" 2+4 way that everyone else does, but I guess whatever works :)

#7 tjugo

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Posted 25 September 2013 - 02:57 AM

Hi,

I've found that the error reported by ASPA or Alignmaster is not very accurate, I mean it gives you a good idea how good/bad the PA is, but pretty much that's it.

For instance many times after performing ASPA, the HC was reporting a PA close to 0 (or even 0) and after checking it with drift alignment it was clear that the error was greater than what the HC reported. Similar situation with Alignmaster.

Now what I do is just align the mount using the polar scopes, if properly calibrated they can be very accurate, usually that's enough for 20m subs @ my image scale (1.8 arcsec per pixel).

I have checked the accuracy of my polar scopes with PEMpro and they give me around 3' polar error, more than enough for my needs.

Cheers,

Jose

#8 Cyclop_si

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Posted 25 September 2013 - 03:02 AM

...
I've found that the error reported by ASPA or Alignmaster is not very accurate, ...


Problem might be in star position errors in their database.

#9 gmartin02

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Posted 25 September 2013 - 10:17 AM

Jose, the Nexstar controller always shows 0 or very close to 0 after you finish ASPA. Only after resetting the alignment/calibration stars after ASPA will the the Nextar controller show what it thinks the alignment error is. This is necessary anyway to correct the pointing model after ASPA (unless you don't care about the pointing model after ASPA - the mount will still track the same, just be less accurate in pointing).

#10 mich_al

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Posted 25 September 2013 - 02:36 PM

Instead of measuring the alignment error in arcmin, why not just measure the drift in arcsec over the longest length of subexposures you plan on taking? When you get to the point where you either reach 0 arcsec or diminishing returns you are done.

Gale



Yep, fuction over form. Work till ya get the result that you need and you're done. On the down side that may not provide suitable metrics to brag to your buddies about.

#11 Pak

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Posted 25 September 2013 - 02:40 PM

I think you guys misunderstood his question. He isn't asking if ASPA report is accurate. He is testing the result of ASPA against a 3rd party program that shows in arcminutes or arcseconds what the drift is. After doing an ASPA it appears that pempros polar alignment helper is telling him that it is under 2 arcminutes. If that is true, that is really good I think. Also to answer the OP's second question, if I was setting up a mount in an observatory I would want to get it under an arcminute and then I would try to see if I could get t lower just because I won't ever touch it again but I think anything under 1 arcminute is excellent.

#12 Whichwayisnorth

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Posted 25 September 2013 - 03:42 PM

I think you guys misunderstood his question. He isn't asking if ASPA report is accurate. He is testing the result of ASPA against a 3rd party program that shows in arcminutes or arcseconds what the drift is. After doing an ASPA it appears that pempros polar alignment helper is telling him that it is under 2 arcminutes. If that is true, that is really good I think. Also to answer the OP's second question, if I was setting up a mount in an observatory I would want to get it under an arcminute and then I would try to see if I could get t lower just because I won't ever touch it again but I think anything under 1 arcminute is excellent.



Correct. Ultimately my question is "What is excellent". I have two methods I currently use to polar align. On my Celestron AVX I use the ASPA and on my LX850 I use the Starlock assisted Drift Alignment. On the AVX I get under 2 arc/minutes typically. If I am careful during alignment. On the LX850 it is under one arc/minute. I can't stop tinkering though so even then I want to spend way too much time trying to get it lower. Based on some of the responses here I would say that both results are excellent. If anyone disagrees please respond here.

#13 jbalsam

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Posted 27 September 2013 - 01:11 PM

Excellent polar alignment = you don't notice field rotation in your images.

For the focal lengths and exposure times I've imaged with (usually ~800 or 1000mm and 10 to 30 min exposures, using an 8300 chip) I've found that if I get my polar alignment (measured in PemPro) to be better than about 2 arc mins, my images look good and I don't notice any field rotation. Finer adjustments than that get very tedious with my CGE. On my neighbors AP1200 and AP1600, it's a lot easier to make those fine tweaks (though probably not necessary).

#14 Tonk

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Posted 29 September 2013 - 01:28 PM

fuss with it until you can get it as close to 0 as humanly possible however I am not sure if any mounts are capable of such fine adjustment to get it that perfect.


My 10Micron GM1000HPS is currently measuring 15 arc seconds error from polar alignment. That was achieved with 3 iterations of its built in Polar Alignment procedure using 15 alignment stars. I tried the procedure out 6 times now with results ranging from 25 arc seconds error to the best around 12 arc seconds error.

Within this range I'm getting 10 minute unguided imaging with a very small drop rate (2 images slung due to tracking errors so far) - so I'm incredibly happy. Nice mount!!!

#15 Alph

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Posted 16 October 2013 - 02:31 AM

My 10Micron GM1000HPS is currently measuring 15 arc seconds error from polar alignment. That was achieved with 3 iterations of its built in Polar Alignment procedure using 15 alignment stars. I tried the procedure out 6 times now with results ranging from 25 arc seconds error to the best around 12 arc seconds error.



You should apply for a Guinness World Record. Here is how

#16 frolinmod

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Posted 16 October 2013 - 03:54 AM

Trouble is, there isn't any one specific perfect polar alignment setting for all objects at all declinations, so getting that close to any one specific alignment is entirely pointless unless you're observing only objects at one specific declination rather than all over the sky.

#17 HKang

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Posted 16 October 2013 - 12:28 PM

Approximate max drift with polar inaccuracy ~ 0.26" * error(in degree)* exposure time.

So you can calculate how often you need to correct DEC.

#18 Dwight J

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Posted 16 October 2013 - 12:48 PM

3 to 5 arc minutes is likely good enough and with all the variables involved probably at the limit of practicality. Movements of the mount like play and metal contraction, play, and tripod sinking and variability over the sky of the alignment make getting really close and keeping it there tough. If you are imaging a few dec corrections help reduce elongated stars that you may have if all you are correcting are errors in RA.

#19 mpgxsvcd

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Posted 16 October 2013 - 01:56 PM

I typically get around 4-5 arc minutes by simply doing a very precise visual align with my CG-5 and the polar finder scope.

When I also do an ASPA it typically gets that down to less than 1 arc minute of error.

Honestly, the 4-5 arc minutes of error gets me 1-2 minutes unguided. I barely notice any difference when I get down to less than 1 minute of error. Periodic error is a much greater issue that Polar alignment with my equipment then.

If you are guiding then 4-5 are minutes of error should work. Less than 1 minute is great for bragging rights though. Less than 15 seconds is just obsessive. However, I am still envious of Tonk’s achievements even if it probably isn’t necessary.

If I had an observatory I would try for less than 1 minute and then just forget about it.

#20 Footbag

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Posted 16 October 2013 - 03:51 PM

Right now my mount is out by 2 arc minutes. I can get 30m exposures without any field rotation @2032mm FL. I'm sure I could improve it, but I haven't come up with a good reason why.

#21 Skunky

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Posted 17 October 2013 - 06:18 PM

Epic fail with a side by side and PA with pempro...

I'm sure most of you saw my PA in pempro, if not here it is again:

Posted Image

Posted Image

Well, what I find odd is that pempro said my PA was below 1 arcmin (30 arcsec average) in both axis (5+ repeats to test for repeatability) yet I get nice long star trail shooting M2 at 30min subs. Now we know that a side by side does not affect PA. I can't believe diffraction would cause pempro to be that far off... Especially when my seeing is a 5 (excellent) this week (clean ocean air)

Back to the tried and true method of polar alignment scope (RAPAS) and drift align tonight to see how far off pempro is in a real world test....

ORTHOGONALITY? Maybe, but not that much... more testing Watson!

#22 PGW Steve

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Posted 17 October 2013 - 07:23 PM

Skunky, are you guiding those 30 minute shots? Which OTA are you using?

#23 Skunky

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Posted 17 October 2013 - 07:52 PM

no guiding.. testing my pa..

#24 PGW Steve

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Posted 17 October 2013 - 09:03 PM

Which OTA? If you were using that SCT without a mirror lock in one of your other pics, that is where the trails are coming from. I've got a lock on my 14" and the mirror still has a gradual shift over time, never mind one with no lock.

#25 Skunky

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Posted 18 October 2013 - 01:22 AM

HD11... I figured it out. My secondary is not square to the baffle on the HD11. The other scope on the side by side is a refactor.. and I was able to get the PA squared away..






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