Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
· Get a Cloudy Nights T-Shirt · Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User

Equipment Discussions >> Mounts

Pages: 1 | 2 | 3 | (show all)
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


Post Extras: Print Post   Remind Me!   Notify Moderator  
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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
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


Post Extras: Print Post   Remind Me!   Notify Moderator  
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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
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


Post Extras: Print Post   Remind Me!   Notify Moderator  
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!


Post Extras: Print Post   Remind Me!   Notify Moderator  
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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
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...

Post Extras: Print Post   Remind Me!   Notify Moderator  
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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
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


Post Extras: Print Post   Remind Me!   Notify Moderator  
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


Post Extras: Print Post   Remind Me!   Notify Moderator  
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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
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


Post Extras: Print Post   Remind Me!   Notify Moderator  
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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
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?


Post Extras: Print Post   Remind Me!   Notify Moderator  
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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
orlyandico
Postmaster
*****

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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
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


Post Extras: Print Post   Remind Me!   Notify Moderator  
orlyandico
Postmaster
*****

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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
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


Post Extras: Print Post   Remind Me!   Notify Moderator  
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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
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


Post Extras: Print Post   Remind Me!   Notify Moderator  
orlyandico
Postmaster
*****

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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
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


Post Extras: Print Post   Remind Me!   Notify Moderator  
gdd
Pooh-Bah


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


Post Extras: Print Post   Remind Me!   Notify Moderator  
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)


Post Extras: Print Post   Remind Me!   Notify Moderator  
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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
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


Post Extras: Print Post   Remind Me!   Notify Moderator  
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


Post Extras: Print Post   Remind Me!   Notify Moderator  
shawnhar
Postmaster
*****

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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
shawnhar
Postmaster
*****

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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
freestar8n
Post Laureate
*****

Reged: 10/12/07

Re: Polar alignment 3 arc minutes acceptable? new [Re: shawnhar]
      #5949726 - 07/01/13 01:50 PM

Hi-

Your result may indeed be good - but it would help to know the actual fwhm in arc-seconds you obtained. Round is good - round and small, in arc-seconds, is even better.

I don't know what object that is, so I can't tell from context either. Is that a linear stretch?

Thanks,
Frank


Post Extras: Print Post   Remind Me!   Notify Moderator  
shawnhar
Postmaster
*****

Reged: 06/25/10

Loc: Knoxville, TN
Re: Polar alignment 3 arc minutes acceptable? new [Re: freestar8n]
      #5949944 - 07/01/13 03:50 PM

Sorry, that is M16, the Eagle. Used a Canon XS dslr.
One mild curve in PS and a little noise reduction.
Astrometry.net wouldn't plate solve it.


Post Extras: Print Post   Remind Me!   Notify Moderator  
orion69
scholastic sledgehammer


Reged: 05/09/10

Loc: Croatia
Re: Polar alignment 3 arc minutes acceptable? new [Re: shawnhar]
      #5949962 - 07/01/13 04:00 PM

Quote:

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.






Sorry I can't do that, same problem over here...
But it's easier for me since my new scope didn't arrive yet.
This year is really bad.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Steve Mckean
member


Reged: 03/22/10

Loc: Bay Of Plenty, NZ
Re: Polar alignment 3 arc minutes acceptable? new [Re: Cliff Hipsher]
      #6087074 - 09/17/13 11:28 PM

Hi Guys, sorry if this has been covered elsewhere but I was wondering why manually adjusting the mount directly to the NCP or SCP using plate solving and direct camera imaging would not work, the plate solving would let you know exactly which star you looking at through the main OTA so as long as you can keep the cross hairs in the center using manual adjustments would that not work ? (For polar alignment I mean).

Cheers

Steve


Post Extras: Print Post   Remind Me!   Notify Moderator  
frolinmod
Carpal Tunnel


Reged: 08/06/10

Loc: Southern California
Re: Polar alignment 3 arc minutes acceptable? new [Re: orlyandico]
      #6087288 - 09/18/13 03:07 AM

Quote:

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.



That sounds like raw uncorrected pointing or a very poor pointing model. Maybe they weren't even enabling Tpoint pointing corrections at all. They're certainly not needed if the workflow includes plate solving and re-slewing as most do these days as a matter of course.

Also, they were probably using TheSky6 and Tpoint for Windows. TPoint for Windows was implemented using TPoint version 1 from Patrick Wallace. I'll bet their pointing model was fairly lousy because back then anything beyond a basic model had to be artfully hand crafted one term at a time, which was a PITA.

The TPoint Add On to TheSkyX is implemented using TPoint version 18. The new Tpoint supermodel feature automatically creates the pointing model for you, no artful hand crafting skills required. A new pointing model created by supermodel is likely to be far superior to any previously hand crafted one (especially when given a few hundred well distributed pointing samples to chew on).


Post Extras: Print Post   Remind Me!   Notify Moderator  
Phil Sherman
Pooh-Bah


Reged: 12/07/10

Loc: Cleveland, Ohio
Re: Polar alignment 3 arc minutes acceptable? new [Re: telfish]
      #6089315 - 09/19/13 02:01 AM

Quote:


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.





If you're setting up the mount on a pier then you should be using your polarscope and/or any hand controller polar alignment routine for no more than a rough polar alignment. Once you've done this; you should be refining it using the drift method, preferably using your camera to measure the drift. Drift measurements are made along the celestial equator at the meridian (just W of the meridian) and close to either the E or W horizon. Close means around 30 degrees in altitude above the horizon. The meridian measurement is used to adjust azimuth while the horizon location is for altitude.

Set your tracking rate to sidereal, slew rate to 1x sidereal, exposure time to 70 seconds then start the exposure. Let the mount track for 5 seconds then slew E for 30 seconds, then slew W for 35 seconds. Look at the image. There will be stars (the 5 second tracked portion) with two trails forming a V shape. The size of the opening of the V at the star is an indication of how much drift occurred in 60 seconds. The relationship between the two trails will show you which way to adjust the mount. If the first adjustment makes the V wider, you moved the mount the wrong way. When the mount is polar aligned, the trails collapse into a line. At a minute per measurement, this technique is much much faster than using an eyepiece. It also makes it very easy to compare the current measurement with the last one.

If you're using an Atlas mount on a pier inside an observatory, you should be planning on running the mount with EQMOD. Everything to run the mount, including guiding is available as free downloads; EQMOD, ASCOM platform, CdC, PhD, and even AstroTortilla for plate solving. PhD can guide the Atlas using pulse guide, avoiding the need for an ST4 connection to the mount's guide port. This also allows webcams and any other camera that PhD can control to be used for guiding. You'll also want a motorized focuser that can be controlled from your computer. Orion has a relatively inexpensive geared DC motor unit that can be adapted to almost any focuser. Shoestring Astronomy makes a USB run controller for this motor that works with current Windows systems.

Phil


Post Extras: Print Post   Remind Me!   Notify Moderator  
orlyandico
Postmaster
*****

Reged: 08/10/09

Loc: Singapore
Re: Polar alignment 3 arc minutes acceptable? new [Re: Phil Sherman]
      #6089401 - 09/19/13 05:58 AM

the problem with the "V technique" is that it's not very accurate. the rule of thumb for declination drift is 1 arc-second in 5 minutes for 1 arc-minute of misalignment.

if your camera is 2"/pixel (a common size) in order to get a drift of 1 pixel (2") in 1 minute would require you to be out by 10 arc-minutes, which is a lot.

i have been doing this V-method for a long time and PEMPro polar alignment is bulletproof. Just leave it for 10-15 minutes for each of alt and az, and (assuming you didn't move the mount the wrong direction) you can get < 5' with one iteration of each Alt and Az.

also helps if you know how much (arc-minutes) the mount moves with every rotation of the adjuster bolts.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Steve Mckean
member


Reged: 03/22/10

Loc: Bay Of Plenty, NZ
Re: Polar alignment 3 arc minutes acceptable? new [Re: orlyandico]
      #6090339 - 09/19/13 05:13 PM

Thanks for that, ill work on it.

Steve


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | 3 | (show all)


Extra information
26 registered and 40 anonymous users are browsing this forum.

Moderator:  Dave M, richard7, bilgebay 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 2470

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics