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Equipment Discussions >> Mounts

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telfish
sage
*****

Reged: 11/17/10

Loc: Adirondack Mountains NY
Polar alignment 3 arc minutes acceptable?
      #5933899 - 06/21/13 11:13 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


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wolfman_4_ever
Pooh-Bah


Reged: 07/15/11

Loc: El Segundo, Ca, So. Cal
Re: Polar alignment 3 arc minutes acceptable? new [Re: telfish]
      #5933905 - 06/21/13 11:18 PM

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

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telfish
sage
*****

Reged: 11/17/10

Loc: Adirondack Mountains NY
Re: Polar alignment 3 arc minutes acceptable? new [Re: wolfman_4_ever]
      #5933929 - 06/21/13 11:42 PM

Thanks, so what should I be aiming for?

Terry


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wolfman_4_ever
Pooh-Bah


Reged: 07/15/11

Loc: El Segundo, Ca, So. Cal
Re: Polar alignment 3 arc minutes acceptable? new [Re: telfish]
      #5933934 - 06/21/13 11:46 PM

Are you guiding?

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


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GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Polar alignment 3 arc minutes acceptable? new [Re: wolfman_4_ever]
      #5933936 - 06/21/13 11:46 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.


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telfish
sage
*****

Reged: 11/17/10

Loc: Adirondack Mountains NY
Re: Polar alignment 3 arc minutes acceptable? new [Re: wolfman_4_ever]
      #5933944 - 06/21/13 11:56 PM

Quote:

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


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telfish
sage
*****

Reged: 11/17/10

Loc: Adirondack Mountains NY
Re: Polar alignment 3 arc minutes acceptable? new [Re: GlennLeDrew]
      #5933951 - 06/22/13 12:01 AM

Quote:

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!


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wolfman_4_ever
Pooh-Bah


Reged: 07/15/11

Loc: El Segundo, Ca, So. Cal
Re: Polar alignment 3 arc minutes acceptable? new [Re: telfish]
      #5933987 - 06/22/13 12:36 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.

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orlyandico
Postmaster
*****

Reged: 08/10/09

Loc: Singapore
Re: Polar alignment 3 arc minutes acceptable? new [Re: wolfman_4_ever]
      #5934014 - 06/22/13 01:10 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...

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frito
Pooh-Bah


Reged: 10/05/12

Loc: Fremont, CA
Re: Polar alignment 3 arc minutes acceptable? new [Re: orlyandico]
      #5934050 - 06/22/13 02:33 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.


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orion69
scholastic sledgehammer


Reged: 05/09/10

Loc: Croatia
Re: Polar alignment 3 arc minutes acceptable? new [Re: wolfman_4_ever]
      #5934124 - 06/22/13 05:25 AM

Quote:

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.


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orion69
scholastic sledgehammer


Reged: 05/09/10

Loc: Croatia
Re: Polar alignment 3 arc minutes acceptable? new [Re: GlennLeDrew]
      #5934126 - 06/22/13 05:29 AM

Quote:


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.


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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: telfish]
      #5934183 - 06/22/13 07:54 AM

Quote:



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!




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


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freestar8n
Post Laureate
*****

Reged: 10/12/07

Re: Polar alignment 3 arc minutes acceptable? new [Re: Cliff Hipsher]
      #5934227 - 06/22/13 08:42 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


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tjugo
scholastic sledgehammer


Reged: 11/06/07

Re: Polar alignment 3 arc minutes acceptable? new [Re: freestar8n]
      #5934341 - 06/22/13 10:19 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


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GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Polar alignment 3 arc minutes acceptable? new [Re: tjugo]
      #5934507 - 06/22/13 12:07 PM

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.


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freestar8n
Post Laureate
*****

Reged: 10/12/07

Re: Polar alignment 3 arc minutes acceptable? new [Re: GlennLeDrew]
      #5934517 - 06/22/13 12:15 PM

Quote:

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


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end
sage


Reged: 08/31/11

Loc: Houston, Texas, USA
Re: Polar alignment 3 arc minutes acceptable? new [Re: frito]
      #5934645 - 06/22/13 01:42 PM

Quote:

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.


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Alph
Carpal Tunnel


Reged: 11/23/06

Loc: Melmac
Re: Polar alignment 3 arc minutes acceptable? new [Re: orion69]
      #5934906 - 06/22/13 04:18 PM

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.


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orion69
scholastic sledgehammer


Reged: 05/09/10

Loc: Croatia
Re: Polar alignment 3 arc minutes acceptable? new [Re: Alph]
      #5935028 - 06/22/13 05:21 PM

Quote:


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?


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