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CEM40 iPolar alignment - what can make it wrong? Is 5-10' normal?

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#1 Linwood

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Posted 23 September 2020 - 03:42 PM

I have a CEM40 with iPolar. 

 

According to PHD2 guiding assistant, I am off on polar alignment, the last three times it was 5, 20 and 8 arc minutes. 

 

I have not done other alignments like drift -- I know they are available, this hasn't reached the top of the stack until now.  But last time out I took a LOT of time with iPolar to get that plus really centered in the circle, and I left it running until I started imaging and it stayed centered in the circle. Then GA said it was 8' off.

 

I have (though it was 2 times ago) recalibrated the center of the iPolar camera per the manual.  I do not do that each time.  It's supposed to be once and done unless you put in new firmware.

 

I've read a few older threads suggesting it should be accurate to 1' or less.  I also found a brief comment about comparing to PHD2 and not getting good comparison but no real info.

 

Should a three minute guiding assistant run be accurate?   Should I trust it more?

 

Next time out I'll do a drift align for comparison, but in the meantime so I have things to check -- what can throw off iPolar? 

 

In particular, does the alignment of the guide scope matter (i.e. if it's not perfectly parallel to the RA axis, I'm not talking more than a few degrees, I just have not tried to make them perfect as my guide scope has plenty of FOV). 

 

A slight twist on that question is does it matter for PHD2's determination during Guiding assistant of the polar alignment accuracy? 

 

Other than doing a careful drift align to see which is right, anything else I should plan to check the next time out -- if the clouds ever part again? 

 

Linwood


Edited by Linwood, 23 September 2020 - 03:51 PM.


#2 rgsalinger

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Posted 23 September 2020 - 03:51 PM

I imaged for years with long exposures and polar alignment from 2 to 3 arc minutes off the pole with no ill effects. With shorter exposures and moderate focal lengths you can certainly be 5 arc minutes off the pole and get good results.

 

So, that would be my first port of call. do the alignment and then take you usual length guided exposure. See if you can detect any field rotation. If you can't then just ignore the numbers. Better results beat better numbers every time.

 

This may or may not be helpful to you. 

 

There is no need for the guide scope to be perfectly aligned. You are aligning the mount and not the scope to the pole. 

 

I can also tell you that seeing plays a big role in all these calculations. I've done iterative alignments and somehow unless the seeing is really good I'm always off by a couple of arc minutes!

 

Rgrds-Ross


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#3 Dynan

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Posted 23 September 2020 - 04:18 PM

I use SharpCap's Polar Align utility exclusively. I do both sides of the pier I am mounted on. I rotate the scope 90° (6 Hrs in R.A.) to the East (or West) side of pier. Then start SC Polar Align. When asked to rotate 90°, I just "Goto Zero Position".

 

I alternate sides until they agree. (This counteracts any deviation in mount level.) I only do this if something has radically affected the PA. If a first SC PA check (E or W) is in the excellent range, I move on.

 

The reason I say all this is to agree that when I run PHD2's Guiding assistant, I typically get a significant difference between the PA reported by the GA and what SC gave me to start. Sometimes the difference is significant, in the minutes' range. I assume this is due to the slight cone difference between the imaging camera that does the SC PA, and the OAG guide camera. I might be wrong about that since all cameras are attached tightly with adapters, no thumb screw mis-alignments. And I agree with Ross above about seeing being a big factor in these precise adjustments.

 

The instructions for the Guiding Asst say to "Wait until the PA has stabilized"...not 'reached zero error'. Even with the slight differences between cameras, the guiding should be good with a good Imaging Camera PA.

 

The bottom line is, of course: Do you like your images? Are your stars round? Gratz!


Edited by Dynan, 23 September 2020 - 04:21 PM.


#4 bobzeq25

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Posted 23 September 2020 - 04:30 PM

I have a CEM40 with iPolar. 

 

According to PHD2 guiding assistant, I am off on polar alignment, the last three times it was 5, 20 and 8 arc minutes. 

 

I have not done other alignments like drift -- I know they are available, this hasn't reached the top of the stack until now.  But last time out I took a LOT of time with iPolar to get that plus really centered in the circle, and I left it running until I started imaging and it stayed centered in the circle. Then GA said it was 8' off.

 

I have (though it was 2 times ago) recalibrated the center of the iPolar camera per the manual.  I do not do that each time.  It's supposed to be once and done unless you put in new firmware.

 

I've read a few older threads suggesting it should be accurate to 1' or less.  I also found a brief comment about comparing to PHD2 and not getting good comparison but no real info.

 

Should a three minute guiding assistant run be accurate?   Should I trust it more?

 

Next time out I'll do a drift align for comparison, but in the meantime so I have things to check -- what can throw off iPolar? 

 

In particular, does the alignment of the guide scope matter (i.e. if it's not perfectly parallel to the RA axis, I'm not talking more than a few degrees, I just have not tried to make them perfect as my guide scope has plenty of FOV). 

 

A slight twist on that question is does it matter for PHD2's determination during Guiding assistant of the polar alignment accuracy? 

 

Other than doing a careful drift align to see which is right, anything else I should plan to check the next time out -- if the clouds ever part again? 

 

Linwood

You're right to be concerned.

 

Did the PhD2 number for polar alignment stabilize?  Watch it, real time.  In my experience, it can easily change from the 3 minute value.

 

For either method, alignment of the polar scope should be pretty irrelevant.  For iPolar, internal platesolving tells the iPolar where the guidescope is pointed.  For PhD2, you're basically doing a drift alignment.

 

But that's what the centering process is for.


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#5 rgsalinger

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Posted 23 September 2020 - 05:30 PM

You are not right to be concerned until you can identify a problem associated with field rotation and not tilt, bad focus, chip too large for scope, large RMS guiding errors, etc. All of these lead people to waste enormous amounts of time trying to perfect something that doesn't need to be perfected. How do your exposures look is the only thing that matters. The rest is just a lot of people worrying over details that only need to be sweated when you actually affect the data you are collecting. 


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#6 Michael Covington

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Posted 23 September 2020 - 05:56 PM

I'm concerned that they differ so much.  Here are two things to consider.

(1) Has correct setup of the iPolar been done (Confirm Position 1, Confirm Position 2, and also take a dark frame)?  I'm assuming so.  You might repeat this and see if it comes up with different numbers -- the iPolar might have shifted in its mounting somehow.

(2) It can take quite a while for a drift alignment measurement to settle down (either PHD2 Guiding Assistant or any drift align procedure).  In rough air, you  might have to wait more than 3 minutes.

 

And a third:

(3) If your mount and telescope have any flexure, drift alignment and iPolar will differ.  In that case, even though iPolar tells you where the polar axis is more accurately, drift alignment is probably what you should go with, to compensate for the flexure.

Pro-tip: All our mounts and telescopes have flexure.

But if you can get within 5', everything is going to work fine.  It's not critical to solve the mystery.



#7 Linwood

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Posted 23 September 2020 - 05:56 PM

You're right to be concerned.

 

Did the PhD2 number for polar alignment stabilize?  Watch it, real time.  In my experience, it can easily change from the 3 minute value.

 

For either method, alignment of the polar scope should be pretty irrelevant.  For iPolar, internal platesolving tells the iPolar where the guidescope is pointed.  For PhD2, you're basically doing a drift alignment.

 

But that's what the centering process is for.

Well, I don't know.  I just did about 3 minutes and quit, as mostly I was interested in the guiding recommendations.  So thanks, now I have another thing to check. 

 

Lots of good intentions during daylight vanish sitting in the dark in the back yard swatting at mosquitoes.  I probably read to wait for it to stabilize, but I also read something about 2 minutes so gave it three.   Now I will (Hopefully) remember.  Thanks.

 

You are not right to be concerned until you can identify a problem associated with field rotation and not tilt, bad focus, chip too large for scope, large RMS guiding errors, etc. All of these lead people to waste enormous amounts of time trying to perfect something that doesn't need to be perfected. How do your exposures look is the only thing that matters. The rest is just a lot of people worrying over details that only need to be sweated when you actually affect the data you are collecting. 

Well, yes and no and maybe.  I've come a long way in that regard in the last two months, but there's round and then there really round.  And eccentricity by subframe selector is a brutal critic.

 

I grabbed a random 30s sub from last night (which I haven't post processed) and ran it -- I got 3.699 FWHM and 0.489 eccentricity.  So not too bad, not great.  I've seen a lot pushing 0.7. Certainly something I need to continue working on.

 

Here's my philosophy: There's so many things that might be impacting me, from optics, to focus (manual, no mask, one on order for this lens; I have a good one for the C11), to guiding, to post processing, to an astounding level of ignorance barely being chipped away, to really awful light pollution.  So I'm trying to find and knock out any weak points I can find.



#8 Linwood

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Posted 23 September 2020 - 06:16 PM

I'm concerned that they differ so much.  Here are two things to consider.

(1) Has correct setup of the iPolar been done (Confirm Position 1, Confirm Position 2, and also take a dark frame)?  I'm assuming so.  You might repeat this and see if it comes up with different numbers -- the iPolar might have shifted in its mounting somehow.

(2) It can take quite a while for a drift alignment measurement to settle down (either PHD2 Guiding Assistant or any drift align procedure).  In rough air, you  might have to wait more than 3 minutes.

 

And a third:

(3) If your mount and telescope have any flexure, drift alignment and iPolar will differ.  In that case, even though iPolar tells you where the polar axis is more accurately, drift alignment is probably what you should go with, to compensate for the flexure.

Pro-tip: All our mounts and telescopes have flexure.

But if you can get within 5', everything is going to work fine.  It's not critical to solve the mystery.

Re (1) i'll try a before/after next time, that should be easy. 

 

(2): noted. 

 

(3) I get it, but don't know how to account for it.  I mean that quite literally.  Consider a scenario -- in zero position I get everything aligned to the CNP and it's perfect and the scope and guide camera happen to be exactly parallel (they aren't, but let's pretend).  I slew to something more near straight up.  Things flex, the guide scope is now a degree one way, the camera a degree the other from where it was.

 

OK... but if I guide and move over only a few minutes, not hours -- I would expect the flex to stay the same.  In other words, they aren't changing as it tracks.  Now it might change over long periods, but it seems doubtful it does over the short period of running guide assistant. 

 

So again, pretending -- if it doesn't CHANGE in flex during the GA run, does GA see that it flexed having slewed over to this direction? 

Isn't flexure only a thing seem over time and movement where weight shifts and bends something (I'm ignoring wind for the moment). 



#9 bobzeq25

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Posted 23 September 2020 - 07:08 PM

Flexure is a nasty problem.  Just a little bit, and your stars are toast.  The usual "this is unintuitive" deal.

 

Two points. 

 

Mount the guidescope very rigidly, using two rings to triangulate the load.  Using a finder shoe is very risky. 

 

Know the result.  Your guiding graph is fine, because the guidescope is tracking well, but your stars aren't, because the main scope isn't following perfectly.



#10 PaulE54

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Posted 24 September 2020 - 06:16 AM

I wouldn't place too much reliance on the PA reported by a 3 minute GA run.

For reference here are 4 sets of polar alignment numbers from my CEM40. The first number of each pair is the PA error reported by the PHD2 guiding assistant at the start of a nights observing. The second number of each pair is the PA error reported during 2 or more hours of guiding during that same night by PHD2 Log Viewer. I'd place considerably more reliance on the second number in each pair than the first!

0.9 / 1.1

0.3 / 3.0 

4.4 / 3.9

4.1 / 1.4

 

So in all of these cases I had the dot mostly over, but not completely aligned with, the cross in iPolar. This seems to work for me (focal length around 910mm, 2 minute subs) and I don't sweat trying to get closer. 

 

Clear skies.

 

Paul


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#11 dapalmer

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Posted 24 September 2020 - 08:30 AM

Ipolar should be aligning you mount as close as it can.  PHD2 is trying to alight the telescope. I am suspicious that you may be removing the telescope from the mount each time and when you re-install it you may have some variance in how the telescope sits in the mounting shoe. This might account for why sometimes the discrepancy is higher and sometime it is lower. If this is the case, you may need develop a more consistent method for mounting the OTA into the shoe.  Of course I may also be completely wrong as I am quite the novice myself.



#12 Michael Covington

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Posted 24 September 2020 - 09:05 AM

Moving the OTA in the saddle, provided you lock it firmly into the new position, does not affect polar alignment or star tracking.  It does affect go-to, of course, because the telescope will be aimed in a slightly different direction for the same position of the mount.

To track the stars perfectly, all you need is a shaft parallel to the earth's axis and rotating very smoothly in the opposite direction at the right speed.  That is your polar axis.  Anything attached to that shaft in any manner and aimed anywhere in the sky will track perfectly.

 

This follows from the fact that the only thing we're tracking is the earth's rotation.  The stars aren't swirling about on their own.

This is the single most important fact to understand about equatorial mounts.



#13 bobzeq25

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Posted 24 September 2020 - 10:32 AM

Moving the OTA in the saddle, provided you lock it firmly into the new position, does not affect polar alignment or star tracking.  It does affect go-to, of course, because the telescope will be aimed in a slightly different direction for the same position of the mount.

To track the stars perfectly, all you need is a shaft parallel to the earth's axis and rotating very smoothly in the opposite direction at the right speed.  That is your polar axis.  Anything attached to that shaft in any manner and aimed anywhere in the sky will track perfectly.

 

This follows from the fact that the only thing we're tracking is the earth's rotation.  The stars aren't swirling about on their own.

This is the single most important fact to understand about equatorial mounts.

Pretty close, close enough to have value.

 

But we _are_ tracking other things like atmospheric dispersion changes.  The beauty of the guide star approach is that all gets folded in.

 

My opinions often contain some waffle words.  This business is _really_ complex.



#14 Linwood

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Posted 24 September 2020 - 10:34 AM

The second number of each pair is the PA error reported during 2 or more hours of guiding during that same night by PHD2 Log 

THANK YOU.  I never hit that tab on the log viewer before, and did not know I could get PA error from a session, only from the guide assistant. 

 

So I went to the last night out.  My guide assistant said 8.5 arc minutes in 193s.  My first guiding session before the clouds rolled in and I stopped was 37m and said PA Error of 1.3 arc min.  I Think but cannot see any way to confirm I was pointing in the same direction for guide assistant and guiding.

 

So indeed, there's a lot of difference there.

 

But what bothers me is the next session, 48m, was 7.5 arc min in the log, and the final one of 32m guiding was 2.6'.  No change in equipment position other than the slew.

 

The middle of those three was toward an area near the pole (M31 specifically), the other two much further to the celestial equator (or whatever you call it)  at M8, trying to find holes in the clouds.  So I wonder if its ability to detect error varies by declination? 


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#15 Linwood

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Posted 24 September 2020 - 10:36 AM

Ipolar should be aligning you mount as close as it can.  PHD2 is trying to alight the telescope. I am suspicious that you may be removing the telescope from the mount each time and when you re-install it you may have some variance in how the telescope sits in the mounting shoe. This might account for why sometimes the discrepancy is higher and sometime it is lower. If this is the case, you may need develop a more consistent method for mounting the OTA into the shoe.  Of course I may also be completely wrong as I am quite the novice myself.

Each night I have to start fresh and assemble everything, so I redo all calibrations each night - polar align, star align (if I bother), guide calibration, guide assistant. In theory guide assistant should not be needed, but I routinely run it at the beginning of a session and may do it again later just for comparison.

 

I envy you guys that have permanent fixed setup and a lot less to do to get started!


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#16 Michael Covington

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Posted 24 September 2020 - 11:38 AM

Yes, a portable mount is a different piece of machinery every time you use it!


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#17 DuncanM

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Posted 24 September 2020 - 12:02 PM

I have a CEM40 with iPolar. 

 

According to PHD2 guiding assistant, I am off on polar alignment, the last three times it was 5, 20 and 8 arc minutes. 

 

I have not done other alignments like drift -- I know they are available, this hasn't reached the top of the stack until now.  But last time out I took a LOT of time with iPolar to get that plus really centered in the circle, and I left it running until I started imaging and it stayed centered in the circle. Then GA said it was 8' off.

 

I have (though it was 2 times ago) recalibrated the center of the iPolar camera per the manual.  I do not do that each time.  It's supposed to be once and done unless you put in new firmware.

 

I've read a few older threads suggesting it should be accurate to 1' or less.  I also found a brief comment about comparing to PHD2 and not getting good comparison but no real info.

 

Should a three minute guiding assistant run be accurate?   Should I trust it more?

 

Next time out I'll do a drift align for comparison, but in the meantime so I have things to check -- what can throw off iPolar? 

 

In particular, does the alignment of the guide scope matter (i.e. if it's not perfectly parallel to the RA axis, I'm not talking more than a few degrees, I just have not tried to make them perfect as my guide scope has plenty of FOV). 

 

A slight twist on that question is does it matter for PHD2's determination during Guiding assistant of the polar alignment accuracy? 

 

Other than doing a careful drift align to see which is right, anything else I should plan to check the next time out -- if the clouds ever part again? 

 

Linwood

In my experience it takes about 20min before PHD2 begins to give accurate PA error values.  IMHO, it's best to ignore PHD2's PA error because it is too influenced by atmospheric conditions and it does not give fast enough feedback to be useful.

 

I have found that Sharpcap is accurate, and gives rapid feedback as the mount is adjusted,  and it's best to simply trust it. I have no experience with iPolar.


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#18 rgsalinger

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Posted 24 September 2020 - 04:07 PM

The guiding assistant is measuring drift. That includes not just polar alignment but ANYTHING that causes the star to move. Anything. That can be periodic error. It could be flexure of some kind. It could be a cable pull. It could be wind. That's why you need a really long run with any drift alignment technique to get the right answer. Just use the iPolar and be happy.  


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#19 Linwood

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Posted 25 September 2020 - 08:31 PM

So trial #1 was calibration of center position.  I polar aligned very carefully, then cleared the center and reset it by moving 90 degrees.  The derived pixel position was off by 2 in both directions, which does not seem like much.  When I swung back to zero and let it plate solve, I was off by about 3/4 of an inch on the screen (i.e. it stopped the big circle/plus and went back to sky mode -- I realize inches is not descriptive but I do not know what the scale is on the screen). 

It is possible that was due to me swinging the RA axis +90 and back (I actually did it a couple times before realizing I needed to lock it, my hand wasn't steady enough when stopping to have it auto-enter). 

 

But I was surprised how far it was off -- whether from the 2 pixels, or from swinging +90 and back, it seemed far.  And I had the tripod really firmly in the ground (spikes) and the mount tight to the tripod.

Rather than doing a conventional drift align I am now going to image for ... well, as long as the clouds let me and see how guiding things it looks, then check what it calculated as drift error.  I gather it doesn't show that as a running figure (while guiding not in Guide Assistant). 


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#20 Linwood

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Posted 25 September 2020 - 08:57 PM

And 42 minutes later the clouds rolled in.  During that 42 minutes PHD2 claimed the alignment error was 0.1', so that's really close.  Or coincidentally two wrong measures matched.  smile.gif

 

By the way, in the above, I did re-align with the new center before imaging.

 

Now let's see if the clouds will stay to the north, since now I'm imaging south.  What are the chances.  frown.gif


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#21 Michael Covington

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Posted 25 September 2020 - 11:21 PM

The guiding assistant is measuring drift. That includes not just polar alignment but ANYTHING that causes the star to move. Anything. That can be periodic error. It could be flexure of some kind. It could be a cable pull. It could be wind. That's why you need a really long run with any drift alignment technique to get the right answer. Just use the iPolar and be happy.  

I agree with all of that except periodic error, which is always east-west, never north-south, which is what drift alignment measures.

 

And in fact the superpower of drift alignment is that, if you do it in the same part of the sky as you're imaging, it compensates for flexure.  That is, it gives you the polar alignment that makes the stars stand still (all things considered), not the alignment that is theoretically correct.

But I also agree with "just use the iPolar and be happy."  If flexure is enough to make a difference, you have other (improbable) problems.  Flexure can well result in a maybe 1' or 2' disagreement in the readings between the two methods, but it's not enough to matter.



#22 Michael Covington

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Posted 25 September 2020 - 11:22 PM

As best I can determine, in the iPolar software, the dot and cross are each 8' wide, and the whole field of view is about 8 x 11 degrees.



#23 Linwood

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Posted 25 September 2020 - 11:31 PM

As best I can determine, in the iPolar software, the dot and cross are each 8' wide, and the whole field of view is about 8 x 11 degrees.

So at 11 degrees and half way is 640 pixels (at least that's my "Y" the widest), a pixel would be half an arc minute?  Am I doing that right?   So if my center was 2 pixels off that's an arc minute.  That's not consistent with falling completely outside the dot/cross, I was at least 2 dot withs off when I re-centered.  So something else must have moved.

 

Thanks. I appreciate the scale. 



#24 rgsalinger

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Posted 25 September 2020 - 11:46 PM

Sorry my bad. I forgot that the only drift term is declination. I hadn't had my afternoon nap and should not have been online. Still, the point is that it takes time for seeing, etc to be cancelled out. I used to use PEMPRO all the time and I just had to wait it out between adjustments to get it right. You don't need to be spot on you just need to be close when it comes to polar alignment. So, 1 arc minute of potential error is meaningless for almost all use cases. 

 

 

Rgrds-Ross




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