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Polar alignment 3 arc minutes acceptable?

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#1 telfish

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Posted 21 June 2013 - 10:13 PM

I have been playing with the newish all star polar alignment routine in the syscan software on my Atlas mount.

Doing 3 iterations I can get to an average of 3 arc minutes. Is that an acceptable number for AP use or should I just keep going to try and get a lower number?

On one iteration I got the Alt down to 15 arc seconds but each time I do a routine tends to throw up different numbers.


Thanks

Terry

#2 wolfman_4_ever

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Posted 21 June 2013 - 10:18 PM

That's 180 arcsec.. to me.. not close enough.. to others.. and if your guiding.. should be fine.

#3 telfish

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Posted 21 June 2013 - 10:42 PM

Thanks, so what should I be aiming for?

Terry

#4 wolfman_4_ever

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Posted 21 June 2013 - 10:46 PM

Are you guiding?

I believe most (Bisque, Stark, Metaguide, Maxim, etc) recommend 3 arcmin or less in each axis.

#5 GlennLeDrew

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Posted 21 June 2013 - 10:46 PM

If actively guiding, three arcminutes' error poses no problem.

With an axially aligned polar bore scope, you can get to within 1-2 arcminutes. And much more quickly.

I would use the mount's computer alignment only for GoTo pointing, not to set the polar axis.

#6 telfish

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Posted 21 June 2013 - 10:56 PM

Are you guiding?

I believe most (Bisque, Stark, Metaguide, Maxim, etc) recommend 3 arcmin or less in each axis.


Not yet! I have a guider I am going to add to the setup soon. It's a new pier and observatory I am now at the stage of aligning the mount before I start imaging.

Ok so I need to get the numbers a little better in az.

Thanks.

Terry

#7 telfish

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Posted 21 June 2013 - 11:01 PM

If actively guiding, three arcminutes' error poses no problem.

With an axially aligned polar bore scope, you can get to within 1-2 arcminutes. And much more quickly.

I would use the mount's computer alignment only for GoTo pointing, not to set the polar axis.


Thanks Glen. The reason I tried the all star routine was because the polar scope in my Atlas was way off and despite a lot of time adjusting the thing I just could not get a good alignment. I eventually removed it and fitted an adjustable laser in it's place just to get in the ballpark!

I found that with my particular mount it was impossible to get the thing central when I rotated the axis whatever I did with the adjustments. I finally gave up!

#8 wolfman_4_ever

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Posted 21 June 2013 - 11:36 PM

If your not going to use a guider, then you will need to get it closer. The closer you get it, the longer your exposures can be without trailing or field rotation.

#9 orlyandico

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Posted 22 June 2013 - 12:10 AM

that is interesting.. i only get about 7-8 arc-min (using PEMPro) and this is good enough for 10 minutes unguided at a pixel scale of about 2" - 3" / pixel...

#10 frito

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Posted 22 June 2013 - 01:33 AM

i'm constantly confused by this, people often report ASPA's within arc min's but if you are going by the hand control's readout and i must assume you are it will read
xx'xx"
xx' is arc min
xx" is arc seconds

someone correct me if i am wrong here.

i regularly get my CG-5 ASPA'd to within 00'04" or less on both axis. to me this readout says i'm within of course only according to my mount a hand full of arc seconds from the pole. i have successfully done 2 min unguided exposures with this alignment with a scope/camera combo that results in 3.6 arc seconds per pixel resolution.

#11 orion69

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Posted 22 June 2013 - 04:25 AM

If your not going to use a guider, then you will need to get it closer. The closer you get it, the longer your exposures can be without trailing or field rotation.


Not really relevant for Atlas mount.
As I say to all owners of low end mounts, forget unguided imaging.

#12 orion69

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Posted 22 June 2013 - 04:29 AM

With an axially aligned polar bore scope, you can get to within 1-2 arcminutes. And much more quickly.

I would use the mount's computer alignment only for GoTo pointing, not to set the polar axis.


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

#13 Cliff Hipsher

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Posted 22 June 2013 - 06:54 AM

[quote name="telfish"]

I would use the mount's computer alignment only for GoTo pointing, not to set the polar axis. [/quote]

Thanks Glen. The reason I tried the all star routine was because the polar scope in my Atlas was way off and despite a lot of time adjusting the thing I just could not get a good alignment. I eventually removed it and fitted an adjustable laser in it's place just to get in the ballpark!

I found that with my particular mount it was impossible to get the thing central when I rotated the axis whatever I did with the adjustments. I finally gave up! [/quote]

BTDT with an LXD75. If you want to get dead on, drift align.... Other wise, guiding is your best friend.

#14 freestar8n

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Posted 22 June 2013 - 07:42 AM

There's nothing wrong with using the AllStar polar alignment on a celestron mount. It should be about as accurate for polar alignment as it is in its GoTo - since both involve modeling errors in the mount and correcting for them. I expect both to be around a few minutes of arc if the alignment is done carefully and with 3-4 calibration stars.

I don't think the stated error in polar alignment with ASPA should be treated too accurately - because it is limited by the same error of a few arc minutes.

Polar alignment is important for guided imaging in two ways: it limits the amount of field rotation, and it gives more control over dec. guiding. The amount of field rotation you can tolerate depends on how long the exposure is - so for short exposures you may not need to be aligned well at all. But dec. guiding is much more subtle and I find that for tight guiding with OAG and mid-range mounts, I want the dec. drift to be small enough that dec. guiding needs to be chased both directions. This is completely opposite those who think dec. drift is always in the same direction - and some even recommend intentionally offsetting it. I can imagine that might help very low end mounts that are almost unguideable - but for mid-range mounts I would polar align well and chase dec. in both directions.

Frank

#15 tjugo

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Posted 22 June 2013 - 09:19 AM

++Frank

Even if you are guiding if you want to go for long subs +20m you need very good alignment. I can only get round stars if the polar alignment error is within 1arcmin. Having said that, for 10m subs 3arcmin produce round stars.

3arcmin error is within the range of a good polar scope, ASPA, alignmaster. OTOH 1arcmin is only achievable with drift alignment.

Cheers,

Jose

#16 GlennLeDrew

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Posted 22 June 2013 - 11:07 AM

I get the impression that many amateurs needlessly fear drift aligning. If nothing else, after one or the other of the other alignment techniques have been employed, spend a little time checking dec drift on a meridian and E/W star to confirm no great drift rate is happening.

Myself, I would have a hard time trusting implicitly a computer readout of polar alignment error to a precision far better than the actual pointing accuracy.

#17 freestar8n

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Posted 22 June 2013 - 11:15 AM

Myself, I would have a hard time trusting implicitly a computer readout of polar alignment error to a precision far better than the actual pointing accuracy.



Yes - you shouldn't and there is no need to. If your pointing with a celestron mount is about 3' accurate, and if you ask the mount what the polar alignment currently is - and it says you are 45' off in az and 30' off in alt - you can be confident you are off by about 45 and 30 - plus or minus a few arc minutes in each.

If you then do the ASPA procedure and you do it carefully, you should be within about 3' of aligned.

When you complete the procedure the mount assumes it is perfectly aligned, and if you ask the mount it will give either 0 or some small number in arc-seconds for the assumed error. This should be interpreted as "I am aligned perfectly to within the GoTo error."

If you then recalibrate the mount after polar aligning - it will give a true, measured estimate of the polar alignment error - and it may be something like 2' in az and 1' in alt. Again - these should be viewed as approximate values within the 3' or so goto accuracy - which means the values are consistent with being polar aligned to within 3'.

Frank

#18 end

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Posted 22 June 2013 - 12:42 PM

i'm constantly confused by this, people often report ASPA's within arc min's but if you are going by the hand control's readout and i must assume you are it will read
xx'xx"
xx' is arc min
xx" is arc seconds

someone correct me if i am wrong here.

i regularly get my CG-5 ASPA'd to within 00'04" or less on both axis. to me this readout says i'm within of course only according to my mount a hand full of arc seconds from the pole. i have successfully done 2 min unguided exposures with this alignment with a scope/camera combo that results in 3.6 arc seconds per pixel resolution.


Keep in mind that after you do an ASPA the hand controller will report an extremely low polar alignment error - usually only a few arc seconds. This number is meaningless. You can only know what your error is after restarting your system and going through a new 2+4 alignment. At that point, check your polar alignment again and you will see an accurate assessment of your polar alignment error.

If I am doing visual observing or planetary videos I am satisfied with a polar error of less than 10 minutes in each axis, but if I'm going to be doing careful work I will usually shoot for less than one arc minute in both Alt and Az. It usually requires two or three rounds of ASPA for me to get this.

#19 Alph

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Posted 22 June 2013 - 03:18 PM

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.

#20 orion69

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Posted 22 June 2013 - 04:21 PM

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.


Well, this has been my experience, nothing to do with what I believe in. I used polar scope (carefully aligned) and results in 30 min subs were always worse than when I use ASPA. I use polar scope to get fairly close and then use ASPA.

Do you use polar scope for polar alignment? If not,why?

#21 Cliff Hipsher

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Posted 23 June 2013 - 09:07 AM

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.

#22 telfish

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Posted 23 June 2013 - 09:50 AM

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.

#23 Alph

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Posted 23 June 2013 - 10:29 AM

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


A typical polar alignment anecdote.

#24 orlyandico

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Posted 23 June 2013 - 11:06 AM

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.

#25 freestar8n

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Posted 23 June 2013 - 11:29 AM

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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