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Equipment Discussions >> Mounts

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Cliff Hipsher
Pooh-Bah
***

Reged: 12/31/08

Loc: North Chesterfield, VA
Re: Polar alignment 3 arc minutes acceptable? new [Re: Alph]
      #5936106 - 06/23/13 10:07 AM

Quote:

Quote:

Don't know how things are on Atlas mount, but on CGEM, ASPA is much more precise than polar scope.



That's a myth that I used to believe in for a long time. A well calibrated polar scope will get you closer to the NCP than Celestron ASPA if you can align the polar scope on two stars. I have been using T-Point for over 6 months and ASPA is not as accurate as people claim.
As a side note. The author of T-Point recommends to use a polar scope. He also implied that there is no such thing like all-star polar alignment when you use mount modelling.




I agree 100%. By definition, a Polar Alignment is a physical alignment of the mount RA axis to the NCP. In other words, to do a true polar alignment, you must physically adjust the mount in ALT and AZ to get the RA axis aligned to the NCP.

Any thing else is using some fancy math (probably a variation of plate solving) to tweak the computer pointing model.

Most people fail at, and then subsequently give up on using the PAS because they don't have a grasp of what needs to be done, how to do it, and what can cause errors, and speaking from experience, I understand completely.


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telfish
sage
*****

Reged: 11/17/10

Loc: Adirondack Mountains NY
Re: Polar alignment 3 arc minutes acceptable? new [Re: Cliff Hipsher]
      #5936171 - 06/23/13 10:50 AM

Quote:

Quote:

Quote:

Don't know how things are on Atlas mount, but on CGEM, ASPA is much more precise than polar scope.



That's a myth that I used to believe in for a long time. A well calibrated polar scope will get you closer to the NCP than Celestron ASPA if you can align the polar scope on two stars. I have been using T-Point for over 6 months and ASPA is not as accurate as people claim.
As a side note. The author of T-Point recommends to use a polar scope. He also implied that there is no such thing like all-star polar alignment when you use mount modelling.




I agree 100%. By definition, a Polar Alignment is a physical alignment of the mount RA axis to the NCP. In other words, to do a true polar alignment, you must physically adjust the mount in ALT and AZ to get the RA axis aligned to the NCP.

Any thing else is using some fancy math (probably a variation of plate solving) to tweak the computer pointing model.

Most people fail at, and then subsequently give up on using the PAS because they don't have a grasp of what needs to be done, how to do it, and what can cause errors, and speaking from experience, I understand completely.




The Atlas routine does exactly that. When you have done a slew to the star you are asked to center it using the hand controller. Then the mount points to it's position with the offset for polar error. You are then asked to center the star as close as possible using the alt screws. Then the mount adjust to where the star should be again and you adjust the AZ screws to center the star as close to the middle of your eyepiece as possible.

So you are mechanically adjusting the mount. All the software is doing is telling you how much.

I am going to try again with a longer focal length scope to see if I can get down to one arc minute. It's certainly possible because in ALT I managed 15 arc seconds.


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Alph
Carpal Tunnel


Reged: 11/23/06

Loc: Melmac
Re: Polar alignment 3 arc minutes acceptable? new [Re: telfish]
      #5936210 - 06/23/13 11:29 AM

Quote:

It's certainly possible because in ALT I managed 15 arc seconds.



A typical polar alignment anecdote.


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orlyandico
Post Laureate
*****

Reged: 08/10/09

Loc: Singapore
Re: Polar alignment 3 arc minutes acceptable? new [Re: Alph]
      #5936277 - 06/23/13 12:06 PM

I think the point here is, if the mount tells you that your alignment is within 15 arc seconds, that is not believable.

Most mounts cannot point with that level of accuracy. So how can they claim to measure a polar alignment of that level?

For example, the CGE Pro claims 5 arc-minute pointing, with 1 arc-minute using "precise GoTo." What that tells me is that any polar alignment accuracy reported that is less than 5 arc-minutes is unreliable. At best you're getting +/- 5 arc-minutes. So you could be 5' off the pole and the ASPA routine will still think you're dead-on.


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freestar8n
Post Laureate
*****

Reged: 10/12/07

Re: Polar alignment 3 arc minutes acceptable? new [Re: orlyandico]
      #5936313 - 06/23/13 12:29 PM

Yes - there is no doubt that the displayed precision in the polar alignment is much better than the actual accuracy - but I hope that is somewhat obvious and doesn't detract from the actual performance of ASPA. I think it's more of a consistency thing to show the error including arc-seconds.

If you do a good calibration of the mount you may find your goto accuracy is about 3'. If you then do a careful ASPA, then there is no reason your resulting polar alignment won't be similarly accurate - to within a few arc minutes.

I have heard some strange interpretations of how best to do the ASPA, and I hope it is also somewhat obvious that you should use a star low down, perhaps 20 degrees above the horizon, regardless of your latitude, and near the meridian. If someone is using a star high up, or not first calibrating the mount, then accuracy will suffer.

Frank


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orlyandico
Post Laureate
*****

Reged: 08/10/09

Loc: Singapore
Re: Polar alignment 3 arc minutes acceptable? new [Re: freestar8n]
      #5936324 - 06/23/13 12:36 PM

I remember when I was renting on GRAS (now iTelescopes). The typical pointing accuracy, even with the T-Point modeling, the fixed observatory, etc. etc. etc. was about 1' to 2'. The reason I know this is because as part of the workflow, after a slew, the GRAS software would plate-solve and display how far off the pointing was. And these were Paramount ME's.

So I think Celestron saying 5' for the CGE Pro is probably the absolute best case, which would not be typically achievable by someone who just plunked it down in a field.


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orion69
scholastic sledgehammer


Reged: 05/09/10

Loc: Croatia
Re: Polar alignment 3 arc minutes acceptable? new [Re: orlyandico]
      #5936332 - 06/23/13 12:41 PM

Well, if you are satisfied with polar scope, that is perfectly OK.

Just to be clear, we are talking about Atlas or CGEM polar scope, not TAK.
Problem is not just polar scope alignment, also you can't be sure if drawings on polar scope reticle are correct... Also there is not enough magnification for polar scope to be precise enough.
You can achieve polar alignment with polar scope similar to ASPA but that involves certain amount of luck, ASPA in my experience is much more reliable.
Since nearly all my images are 30 min subs @ 765mm or 900mm difference is easily noticable.
I would really like to see 30 min subs image(s) that were done by mount (Atlas or CGEM) polar aligned with just polar scope.


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freestar8n
Post Laureate
*****

Reged: 10/12/07

Re: Polar alignment 3 arc minutes acceptable? new [Re: orlyandico]
      #5936335 - 06/23/13 12:42 PM

I thought modern mounts with good modeling were in the sub 1' range - perhaps 15" rmsd. For cge or cge-pro, a lot depends on how loaded it is. For an actual all-sky rmsd above perhaps 20 degrees altitude, I think 5' is not hard to do - and that's with 6 stars.

The main skill involved is in accurately centering the alignment and calibration stars, and finishing with up/right. And using two widely separate on one side for alignment - and four widely separate on the other side for calibration.

Frank


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orion69
scholastic sledgehammer


Reged: 05/09/10

Loc: Croatia
Re: Polar alignment 3 arc minutes acceptable? new [Re: freestar8n]
      #5936347 - 06/23/13 12:55 PM

Quote:


The main skill involved is in accurately centering the alignment and calibration stars, and finishing with up/right. And using two widely separate on one side for alignment - and four widely separate on the other side for calibration.

Frank




Exactly, I think those are the main reasons why many people have problems with ASPA (and using wrong star for final polar alignment).
I use permanently mounted camera carefully perpendicular to scope and crosshairs in Maxim DL.


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neilson
professor emeritus


Reged: 08/22/10

Re: Polar alignment 3 arc minutes acceptable? new [Re: orlyandico]
      #5936349 - 06/23/13 12:58 PM

Hi,
I think the 5 arc minute pointing accuracy on the CGE Pro has probably improved since then. In recent firmware updates the pointing accuracy has been improved quit a bit. On my CGE, I usually check my ASPA with a drift alignment and rarely need to make any adjustments. Then I redo my 2+4 alignment and my goto's are dead center. But then my CGE wasn't made in China like the CGE Pro was. lol The Nexstar compensates for cone error automatically so I'm guessing its accuracy is pretty good on all their mounts. At least if you do the 2+4 alignment. And I never use the last star for my ASPA, it's rarely in the right place.

neilson


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orlyandico
Post Laureate
*****

Reged: 08/10/09

Loc: Singapore
Re: Polar alignment 3 arc minutes acceptable? new [Re: neilson]
      #5936367 - 06/23/13 01:08 PM

The 5' for the CGE Pro is the current figure Celestron publishes on their web site - where they also claim +/- 3" PE.

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neilson
professor emeritus


Reged: 08/22/10

Re: Polar alignment 3 arc minutes acceptable? new [Re: orlyandico]
      #5936470 - 06/23/13 02:04 PM

Hi,
Yes you are correct. If it's still just 5 arc minutes then that could effect accuracy of the ASPA. Is it possible that after doing the 2+4 alignment the computer is able to take that in effect though.

neilson


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gdd
scholastic sledgehammer


Reged: 11/23/05

Loc: N Seattle suburb, WA
Re: Polar alignment 3 arc minutes acceptable? new [Re: neilson]
      #5936719 - 06/23/13 04:39 PM

Can we work the problem from the other direction, measure the polar alignment error from the tracking error?

Gale


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mikeschuster
super member


Reged: 08/25/11

Loc: SF Bay area
Re: Polar alignment 3 arc minutes acceptable? new [Re: gdd]
      #5936754 - 06/23/13 05:01 PM

Yes.
http://celestialwonders.com/tools/polarErrorCalc.html

Rule of thumb: 1 arcsecond of drift for a declination 0 star near the meridian in 5 minutes equals about 1 arcminute of polar misalignment in azimuth.

Edited by mikeschuster (06/23/13 05:02 PM)


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neptun2
scholastic sledgehammer


Reged: 03/04/07

Loc: Bulgaria
Re: Polar alignment 3 arc minutes acceptable? new [Re: mikeschuster]
      #5948739 - 06/30/13 09:40 PM

Well i use the new synscan polar alignment routine on my heq5 pro mount and am very happy with it. I don't know how correct is the reading of the synscan controller but after the polar align it is usually under 1 minute. I checked the result visually through the polar scope after that and the polar star is where it should be (i use polar finder program to see where it should be in the polar scope).

To sum it up - as other people said if you center the stars carefully and finish the movement with up and right keys the polar realign feature is working very good. I use the live view of my nikon d90 dslr which thanks to the grid allows me to center the stars with good precision.


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Cotts
Just Wondering
*****

Reged: 10/10/05

Loc: Toronto, Ontario
Re: Polar alignment 3 arc minutes acceptable? new [Re: neptun2]
      #5948917 - 06/30/13 11:58 PM

If anyone is trying to achieve 5 arcmin polar accuracy or better and is not discussing the orthagonality of their mount (cone error) and the orthagonality of the telescope's attachment to the mount then they are ignoring significant sources of error.

Your polar scope is not nearly as accurate if the telescope doesn't point exactly the same direction as your mount's saddle. Look at your rings - is the felt the same thickness all around? Does each ring have the same thickness? Are the bolt holes centered to .01 inch tolerances or better? Your polar alignment will suffer also if your scope has cone error - the telescope/mount points to slightly different places in the sky when on exactly opposite sides of the mount.

Dave


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freestar8n
Post Laureate
*****

Reged: 10/12/07

Re: Polar alignment 3 arc minutes acceptable? new [Re: Cotts]
      #5949027 - 07/01/13 02:36 AM

Cone and orthogonality don't affect the polar alignment. All that matters is that the single polar axis of rotation is pointed at the pole. The saddle could be 37 degrees off and the dec. axis bent 23 degrees - and as long as the polar axis is pointed right, the telescope will track well.

For all star polar alignment and other systems where terms like cone are modeled, you can use the goto accuracy as a gauge of how accurate the polar alignment will be - because it indicates how well that term is known and modeled.

So cone and dec. error do affect the way the mount behaves across the sky and they would need to be modeled - but ASPA does that - and an accurate polar scope doesn't need to know them in the first place.

But you would definitely want the polar scope well aligned in the polar axis - and 5' accuracy there isn't trivial to achieve.

Frank


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shawnhar
Post Laureate
*****

Reged: 06/25/10

Loc: Knoxville, TN
Re: Polar alignment 3 arc minutes acceptable? new [Re: orion69]
      #5949417 - 07/01/13 10:39 AM Attachment (14 downloads)

Quote:

Well, if you are satisfied with polar scope, that is perfectly OK.

Just to be clear, we are talking about Atlas or CGEM polar scope, not TAK.
Problem is not just polar scope alignment, also you can't be sure if drawings on polar scope reticle are correct... Also there is not enough magnification for polar scope to be precise enough.
You can achieve polar alignment with polar scope similar to ASPA but that involves certain amount of luck, ASPA in my experience is much more reliable.
Since nearly all my images are 30 min subs @ 765mm or 900mm difference is easily noticable.
I would really like to see 30 min subs image(s) that were done by mount (Atlas or CGEM) polar aligned with just polar scope.



How about a 100% crop of a 10 minute sub at 2500mm using only the polar scope on an EQ6.
I normaly use the CCD Drift method but this session was only polar scope. It can be done.


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orion69
scholastic sledgehammer


Reged: 05/09/10

Loc: Croatia
Re: Polar alignment 3 arc minutes acceptable? new [Re: shawnhar]
      #5949550 - 07/01/13 12:14 PM

Of course it can be done, just not every time...
Since you are using CCD drift method I'm sure you agree with me.


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shawnhar
Post Laureate
*****

Reged: 06/25/10

Loc: Knoxville, TN
Re: Polar alignment 3 arc minutes acceptable? new [Re: orion69]
      #5949657 - 07/01/13 01:24 PM

Challenge accepted!
Now, would you please get rid of all the clouds and rain here so I can polar align each night using only the polar scope and report my findings.


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