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Asi Air Pro Polar Alignment how can it be accurate?

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#1 lsintampa

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Posted 24 February 2021 - 08:33 PM

This is a bit of a rant and also a question.

 

To do a PA using AAP, you need to set your mount to index points, and be somewhat pointed north.  OK say you do that.

 

Now after plate solve one and two you painstakingly adjust alt az until you get to perfect or good enough (depending on your patience).

 

Then off to guiding, hoping you have a good alignment.

 

During the final adjustments of your PA you will note that just tightening an adjustment on one that is already tight can improve your alignment or throw it off.  I'm talking about torquing say your az adjustment with all the might you can muster.  So minor.

 

So my ponder is this.. Going back to your scope being at your index marks seems to me to have much more room to be "off" than the small torque adjustment at the end of the process.

 

That said, how accurate are any of our polar alignments really?

 


Edited by lsintampa, 24 February 2021 - 08:34 PM.


#2 DennisOrion

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Posted 24 February 2021 - 10:47 PM

As I'm zeroing in my RA and DEC I will start to tighten up the locking mechanisms for each. When I'm making my final adjustments for the PA I'm working against something that is most of the way locked in position. This is for a Celestron CGX mount. Hope this helps.

 

Dennis



#3 jonnybravo0311

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Posted 24 February 2021 - 11:21 PM

I don't use an AAP, but I do polar align :). I follow a very similar process to what Dennis described. As I'm closing in on that "Excellent" alignment, my az knobs are already pretty tightened down and adjustments I make just serve to tighten things up even further. Altitude is adjusted via a wheel that is already pretty tight. Tiny adjustments.



#4 lsintampa

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Posted 25 February 2021 - 07:51 AM

This is my point.  The adjustment knobs are tight as we zero in..  we agree.

 

My point is that those final adjustments are almost impossible to measure they are so small.  That being the case, setting the scope to its index marks has to be off more than those final adjustments.  It's almost certain that each time I line up to index marks I'm never in the same spot.

 

Doesn't the alignment final adjustments assume you are dead spot on your index starting point?



#5 alphatripleplus

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Posted 25 February 2021 - 08:58 AM

What mount are you using? The mechanism for fine azimuth adjustment is going to vary depending on what mount you have. (For example, on my Mach1 there is no need to torque my azimuth adjustment with all the force I can muster to get the final adjustment)



#6 DennisOrion

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Posted 25 February 2021 - 09:12 AM

I'm not certain by what you mean when you say "setting the scope to it's index marks has to be off more than those final adjustments". The only index mark I have on my mount is for the latitude and it only gets you in the ballpark. I would never use this again unless I was taking my scope to a different location. I setup my mount on a patio slab, so my declination is only off by small amounts due to low/high spots on the slab. Certainly not enough for needing to reset the DEC to the proper index mark (latitude). Now it's just a matter of having the mount point north. I have a few subtle marks on the slab that allows me to get the north star in the guidescope. Now I loosen very slightly the lock down for the latitude adjustment and other one that allows the mount to swivel L/R. Now small adjustments are made per the PA process. As the 'scale' on the ASIAir Pro PA screen changes (getting closer to on the money), I'll tighten the lock downs just a bit, but not so tight that further adjustments can't be made. Repeat until satisfied with PA, then lock down and wait to see if that changed PA. I also find that the lock downs are already so tight at this point they don't even need to be tighten further. It can take some practice to get the tightening process down such that it doesn't mess up your PA.

 

Good luck!

 

Dennis



#7 lsintampa

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Posted 25 February 2021 - 09:37 AM

I'm using an AVX mount.

 

I was overthinking this.  Keep thinking starting at the index marks is important and it isn't. 

 

Even if the starting point was off index marks each time, you align the mount not the scope.  Right?



#8 robbieg147

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Posted 25 February 2021 - 10:16 AM

I have the AVX also, the bolts do not need to be that tight, use both hands together tighten one bolt as you slightly loosen the other. You get a feel for it after a while will only take you a couple of minutes.

 

I use the Polemaster once I started using this unguided 3min subs was possible.

 

It's the mount RA axis that is being aligned when you Polar align, not the scope.


Edited by robbieg147, 25 February 2021 - 10:23 AM.


#9 DennisOrion

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Posted 25 February 2021 - 10:26 AM

Even if the starting point was off index marks each time, you align the mount not the scope.  Right?

You are aligning your mount, but it's based on the guidescope the Pro is using to do PA, so it's important that your guidescope and main scope point at the same thing, within reason of course. Your mount should also be in it's 'home' position. Let us know how your next PA goes.

 

Dennis



#10 lsintampa

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Posted 25 February 2021 - 10:29 AM

Thought AAP uses the main scope to do PA?



#11 lsintampa

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Posted 25 February 2021 - 11:16 AM

Another observation... Near the fine adjustments, I can just loosen one side and the alignment moves even without touching the other side.  So that's too tight?  How tight is tight.

 

Hate to be so anal about this, but seems to me these finer adjustments seem to be very sensitive to the point just loosening one side moves it quit a bit from where it was.



#12 rgsalinger

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Posted 25 February 2021 - 11:25 AM

Unfortunately, with less expensive mounts you are going to find every time that the mount is going to move when you tighten up the ALT adjuster. I guess that could even extend to the AZ adjuster in some mounts. The reason that this is happening is that your mount is balanced in RA and DEC but not in ALT and (maybe) AZ.

 

However, the important thing to understand is that it doesn't matter. I image all the time when my mounts are as much as 3 arc minutes off the pole and never see any field rotation in long exposures. So, it just doesn't matter if you are off a bit at all. The reason that this is happening is that your mount is balanced in RA and DEC but not in ALT and (maybe) AZ.  

 

It's a mistake to worry too much about perfecting PA or leveling your mount in order to achieve PA closer than a couple of arc minutes. Use this calculator to convince yourselves that this is correct. 

 

Rgrds-Ross



#13 DennisOrion

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Posted 25 February 2021 - 11:28 AM

I think you are correct, for some reason I just had a thought it was pulling a picture from the guide camera.

 

As for adjusting the AVX mount, I'm not sure how it works, maybe robbieg can help with that. For the CGX, the L/R adjustment has two opposing bolts, so for fine adjustments you can loosen one a 1/4 turn, then tighten the other a 1/4 turn. Then go 1/8 loose, then 1/8 tight. Repeat in whatever small increments you need.

 

Dennis



#14 lsintampa

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Posted 05 March 2021 - 07:37 PM

Well using the AAP polar alignment routine, it made a big difference in my tracking if I have a better PA.  IE great vs good.. or under 1 total error PA vs 2 or 3 total PA error.

 

My point is the last tiny adjustments seem nearly impossible to get to near perfect and being off from your best effort seems to have a huge impact on your guiding.   At least that seems to be so in my situation.



#15 Cresfactor

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Posted 05 March 2021 - 07:42 PM

Wait, are you guys saying the PA on AsiAirPro uses the main cam and scope, and not the guide scope? 

So then does it use that for autoguiding too? 

 

I think you are correct, for some reason I just had a thought it was pulling a picture from the guide camera.

 

As for adjusting the AVX mount, I'm not sure how it works, maybe robbieg can help with that. For the CGX, the L/R adjustment has two opposing bolts, so for fine adjustments you can loosen one a 1/4 turn, then tighten the other a 1/4 turn. Then go 1/8 loose, then 1/8 tight. Repeat in whatever small increments you need.

 

Dennis



#16 lsintampa

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Posted 05 March 2021 - 07:46 PM

AAP uses the main scope for PA (as far as I know).  It uses your guide scope for guiding.



#17 Kring

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Posted 06 March 2021 - 12:24 AM

AAP uses the main scope for PA (as far as I know).  It uses your guide scope for guiding.

This is correct



#18 Alex McConahay

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Posted 06 March 2021 - 08:50 AM

There are two questions in this thread.

 

One is about how to get the last tightening of the knobs such that it reflects the best alignment. That is, since the final torque on the set screws, etc. affects the position slightly, it is bound to be a bit off. This question  has been discussed rather thoroughly in this thread. It is a point of "easing" into that final position and tightening. And, yes, you are right. There probably always will be some final effect!. But not enough to worry about. The differences in the torquing of the mount adjustments are probably dwarfed by the potential errors induced by flexing, flopping, refraction, tripod settling, weight shifts, and all that. And at any rate are less than will show up as rotation in the longest of exposures. 

 

The second question, and it was actually more prominent in the original post, has not been addressed well.

 

It was the original posts "ponder" that:

 

>>>>>>Going back to your scope being at your index marks seems to me to have much more room to be "off" than the small torque adjustment at the end of the process. 

 

Yeah, you can be a lot further off on the "index marks" than any torque on the mount adjusting bolts. Your perception of that is correct.

 

But the point is, that does not at all affect the alignment of your mount. A polar alignment affects the alignment of the MOUNT. It is not really about the pointing of the tube. Setting your tube to the index points just affects where the tube is pointed, not where the mount is aligned. To make sure you have a good alignment, you have to slew to a target, center it, and then synch the mount. And, considering the vagaries of the mounts, you probably need to do that to several stars so the mount software can calculate a "pointing model."

 

Even if you are fifteen degrees or more off in setting the tube back to the indexes, the mount still has the same polar alignment. 

 

Alex


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#19 lsintampa

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Posted 06 March 2021 - 09:22 AM

There are two questions in this thread.

 

One is about how to get the last tightening of the knobs such that it reflects the best alignment. That is, since the final torque on the set screws, etc. affects the position slightly, it is bound to be a bit off. This question  has been discussed rather thoroughly in this thread. It is a point of "easing" into that final position and tightening. And, yes, you are right. There probably always will be some final effect!. But not enough to worry about. The differences in the torquing of the mount adjustments are probably dwarfed by the potential errors induced by flexing, flopping, refraction, tripod settling, weight shifts, and all that. And at any rate are less than will show up as rotation in the longest of exposures. 

 

The second question, and it was actually more prominent in the original post, has not been addressed well.

 

It was the original posts "ponder" that:

 

>>>>>>Going back to your scope being at your index marks seems to me to have much more room to be "off" than the small torque adjustment at the end of the process. 

 

Yeah, you can be a lot further off on the "index marks" than any torque on the mount adjusting bolts. Your perception of that is correct.

 

But the point is, that does not at all affect the alignment of your mount. A polar alignment affects the alignment of the MOUNT. It is not really about the pointing of the tube. Setting your tube to the index points just affects where the tube is pointed, not where the mount is aligned. To make sure you have a good alignment, you have to slew to a target, center it, and then synch the mount. And, considering the vagaries of the mounts, you probably need to do that to several stars so the mount software can calculate a "pointing model."

 

Even if you are fifteen degrees or more off in setting the tube back to the indexes, the mount still has the same polar alignment. 

 

Alex

Thanks for the clarity..  the process for me is painstakingly long to get the PA into the excellent range.  Those last adjustments, no matter how small I try to make them, seem to always overshoot center mark.  I'm hopeful that more experience will be my ticket to success.  

 

Too bad they don't have a "simulator" to hone your alignment skills.  



#20 Alex McConahay

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Posted 06 March 2021 - 09:44 AM

It is easy to obsess about polar alignment. And I in no way wish to encourage sloppiness. It is good to be as close to polar aligned as you can be. But it is not as important as getting usable shots that show no rotation (polar alignment error). At some point, to get some shots, you have to stop the hours of hunting for precision, and start the imaging run. 

 

Just what is that point?

 

Have a look here:

 

http://celestialwond...rErrorCalc.html

 

Play around with that, and you will find that polar alignment errors do not show nearly as quickly as you might think. 

 

If you have done a yeoman's job of aligning--you should be good enough to get round stars. You do not need to be perfect. 

 

Alex



#21 Jeff Morgan

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Posted 06 March 2021 - 11:20 AM

There are two questions in this thread.

 

One is about how to get the last tightening of the knobs such that it reflects the best alignment. That is, since the final torque on the set screws, etc. affects the position slightly, it is bound to be a bit off. 

 

Wouldn't that fall under the category of Mount Issues (since they are all fine-tuned with alt/az threads), rather than the "fault" of alignment device or software?



#22 lsintampa

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Posted 06 March 2021 - 02:26 PM

It is easy to obsess about polar alignment. And I in no way wish to encourage sloppiness. It is good to be as close to polar aligned as you can be. But it is not as important as getting usable shots that show no rotation (polar alignment error). At some point, to get some shots, you have to stop the hours of hunting for precision, and start the imaging run. 

 

Just what is that point?

 

Have a look here:

 

http://celestialwond...rErrorCalc.html

 

Play around with that, and you will find that polar alignment errors do not show nearly as quickly as you might think. 

 

If you have done a yeoman's job of aligning--you should be good enough to get round stars. You do not need to be perfect. 

 

Alex

 

I'm no way an expert, but my limited experience has been that the better my PA is, the better my guiding is.  And not by just a little bit better.  Like small PA improvements make a big guiding difference.

 

Say my PA using AAP is 10" x 53" vs 9" x 11"  my guiding may show total error of over 9 verses under 4 (just examples not real numbers here).  My point being is that getting PA down under 4 seconds each axis is a royal pain (for me) and if I don't take the time to get it there (maybe 20 to 30 minutes or more), I find my guiding suffers a lot.

 

Now maybe the PA is good enough, but the AAP guiding is more of my issue?  Last time out, I had a "good" alignment (riding my bike slow), but my guiding was for the most part under 4 total error at it's worse (dithering aside).  Some times under 2, or 3, but never over 4.

 

BTW - I always recalibrate before guiding.   



#23 KenS

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Posted 06 March 2021 - 03:44 PM

I'm no way an expert, but my limited experience has been that the better my PA is, the better my guiding is.  And not by just a little bit better.  Like small PA improvements make a big guiding difference.

There is no reason why that should be the case. The effect of PA on guiding is simple to calculate. Let's say you have 5 arcmin PA error: that is the total drift you will see over a 6 hour period. So 5 arcmin over 6 hours translates to just 50 arcseconds per hour, 0.8 arcseconds per minute or 0.014 arcseconds per second. The peak drift rate is about 1.5 times that so 0.02 arcseconds per second. If your guiding exposures are 3 seconds long then the drift in that time is just 0.06 arcseconds. Basically imperceptible and easily guided out.




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