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Polar Alignment scope vs. Drift Align?

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#1 Footbag

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Posted 18 August 2012 - 11:58 AM

I used my PASILL4 for the first time the other night. I aligned the three stars and they were as close as I could get them. Then I began my PHD drift align routine. I finished the drift alignment, but must have done something wrong. The PHD graph just didn't look right and I was certain I messed up the drift alignment.

So I used the polar scope again and lined up the stars, but then I decided to just snap an image with my WO66 to see if there was visible field rotation. There was none. Then the clouds rolled in so I had to break it down.

I am wondering, though. After you finish a drift alignment, how far does Polaris typically move from where it should be in the polar scope? Is it just slightly off from where it should be or is it possible that it will be as much as half the FOV away in the PA scope?

#2 jchoy46

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Posted 18 August 2012 - 02:04 PM

with a correct polar alignment, polaris should be off 3 degrees since youd be looking at true north instead of magnetic north.
and since youre using a pasill I assume youre talking about your mach1.
but with any scope that is motorized, it would take a very long time to see polaris move in your finderscope. since polaris doesnt move (much). i am going to guess that youre asking how far from dead center on your polar scope with polaris move once you have a perfect polar alignment? 3 degrees. if its not what youre asking i appologize for jumping the gun. if polaris if moving drastically then your finder scope isnt aligned with your scope. hope this helps.

#3 Footbag

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Posted 18 August 2012 - 03:14 PM

Thanks for the reply, but it isn't quite what I was asking. Let me ask a different question because it is a bit confusing.

After aligning using the polar scope, during the drift alignment would you expect any adjustments to the alt and az adjusters to be a full turn or more or would it just be a fraction of a turn?

#4 frolinmod

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Posted 18 August 2012 - 03:15 PM

@jchoy46, I don't follow your reference to magnetic North. What does magnetic North have to do with anything?

#5 Peter in Reno

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Posted 18 August 2012 - 03:30 PM

Hi Adam,

I think what you are experiencing is similar to mine. When I first received my Mach1, the PAS was not yet available so I used Rolando's Quick Drift Alignment (QDA) and got very good results. Then I finally received my PAS, I re-adjusted the mount according to Polaris and second star positions (can't see the third star, too dim for my LP area), the GoTo's were pretty bad. So, I re-adjusted polar alignment using Rolando's QDA and GoTo's and tracking were much better.

Polar scope is never really intended for super accurate polar alignment. Any drift alignment methods will always be better. PAS is still an excellent tool for going to a dark site and you can polar align quickly with PAS before it gets dark. PAS will get you close and help to drift align quicker.

I don't think the rotatable collar of A-P PAS is round enough. I can see a slight wobble as I rotate the PAS's collar. I believe this may be the source of the inaccuracy of this PAS. Since PAS does not rotate while Mach1 RA axis rotates, A-P had to customize the PAS' collar to make it rotatable and I don't think A-P did a good job.

You must live at a dark site. I cannot see the third star through PAS. I can just barely see the second star but I have to wait till it gets dark which is a little too late. I like to polar align using bright stars before it gets dark so I can start imaging early.

I hope A-P's new right angle polar scope will be as accurate as drift alignment. If it behaves like Takahashi polar scope, I have read great things about it and supposed to make extremely accurate polar alignment quickly. It looks like the new polar scope is fixed to the mount and does not rotate, it may be more accurate this way. You just use any planetarium software to find out Polaris' current hour angle and position Polaris in A-P's new polar scope. I hope A-P's new polar scope won't be expensive.

Peter

#6 DonR

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Posted 18 August 2012 - 03:30 PM

Magnetic north doesn't have anything to do with it. Both drift alignment and alignment using the polar alignment scope will align to true north, if done correctly.

When using the polar alignment scope, it is necessary to assure that the polar alignment reticle is centered on the mount's polar axis. When using drift alignment, it is necessary to make sure that cone error is eliminated, i.e., that the OTA's optical axis is parallel to the mount's polar axis. If both of these conditions are true, the polar alignment scope method and the drift alignment method will agree perfectly, assuming both methods are executed perfectly.

#7 Peter in Reno

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Posted 18 August 2012 - 03:45 PM

When using the polar alignment scope, it is necessary to assure that the polar alignment reticle is centered on the mount's polar axis.


That's true but if the PAS rotatable collar wobbles like mine does, then the center of reticule will change which is not good.

Using Rolando's QDA, you are adjusting the finderscope to be orthogonal with the mount which is why QDA works so well and quickly. The only drawback of QDA is you have to be somewhat lucky to find a named star within one hour of the Meridian AND near Zenith for latitude adjustment.

Peter

#8 Footbag

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Posted 18 August 2012 - 04:19 PM

Yes Peter. That is exactly what I experienced. I had polar aligned a few times in the evening sky using the hand control routines and PHD drift align method before even using the polar scope.

I figured that Polaris would be very close to where it should be in the polar scope, but it wasn't. It was off by more then I was expecting.

Every time I've taken the mount out, I got clouded out before doing any imaging, so I never really got to confirm what I suspected. Once I get my tripod, I'll have a lot of experimenting to do. The more time I spend not using my mount the more questions I have.

#9 Starhawk

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Posted 18 August 2012 - 05:18 PM

In my experience, polar scope alignments are very approximate. I don't know if I just have the knack, but I have consistently been able to get about as accurate a polar alignment eyeballing the mount as I can with the polar alignment scope- about within a degree.

So, drift alignment it is- the only really accurate polar alignment there is. I turned my polar scope opening over to cables, and that's working for me.

-Rich

#10 orion69

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Posted 18 August 2012 - 07:01 PM

When using drift alignment, it is necessary to make sure that cone error is eliminated, i.e., that the OTA's optical axis is parallel to the mount's polar axis. If both of these conditions are true, the polar alignment scope method and the drift alignment method will agree perfectly, assuming both methods are executed perfectly.


OTA's optical axis doesn't have to be parallel to the mount's polar axis or to be precise doesn't have to be dead on (but usually it's close enough), it's maybe easier to do drift alignment if optical and mount polar axis are parallel.
Also, you'll match precision of good drift alignment with polar scope only if you're very,very lucky, in fact never.

Knez

#11 dawziecat

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Posted 19 August 2012 - 09:58 AM

I am wondering, though. After you finish a drift alignment, how far does Polaris typically move from where it should be in the polar scope? Is it just slightly off from where it should be or is it possible that it will be as much as half the FOV away in the PA scope?



This PAS has been around a long time. Many report a degree of accuracy for it that I find hard to accept based on my personal experience with my own.

Point is, unless you test it, both to ensure the reticle is centered and that the optical axis is well aligned with the polar axis, you just can not be sure how accurate yours may be. I struggled with mine. I still find it useful and my tests indicate mine can be anything from dead on to about 20 arc minutes off. It depends on the rotation angle of the PAS within the polar axis. This is AFTER you ensure your reticle is centered!

Harold Leinbach wrote a detailed analysis of this whole matter years ago. I have not heard of a soul who found the accuracy of their PAS was as high as they hoped/expected AFTER they actually tested theirs! Others are happy not knowing.

There is a long, detailed discussion in a CN thread from about a year ago. The conclusion was that you should NOT rotate the PAS within the polar axis housing after centering the reticle. Instead you adjust for HA by rotating the entire PA, NOT just the PAS.

So, if you really want an answer to your question as to how far Polaris may be off in your PAS after a drift alignment, you will have to do some work. No one can tell you the answer as it depends on YOUR PAS. One revealing test: Align as best you can with the PAS, getting the 3 stars in their proper locations on the reticle. At least 6 hours later, check to see if the PAS still shows you are well aligned. Obviously it should. Likely it will not. If it does not, you have a problem and should not be overly relying on the PAS to deliver all the accuracy promised in that lovely reticle.

Be aware that, as H. Leinbach states, adjusting this alignment scope is not an easy matter!

I post the link to Leinbach's paper here but you will have to register with the Losmandy Yahoo group before you can access it.

This is the link.

All this said, I use my PAS, AFTER adjusting it, to do AP with success. Up to 30 minute exposures but at short FLs using camera lenses up to 600mm. But I don't bother using the three stars anymore as I appreciate my particular PAS just can not deliver the accuracy promised by doing so.

To recapitulate, no one can answer your question. You have to test your particular PAS to determine its accuracy. Leinbach's paper will tell you how.

#12 DonR

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Posted 19 August 2012 - 10:16 AM

Hi Knez,

OTA's optical axis doesn't have to be parallel to the mount's polar axis or to be precise doesn't have to be dead on (but usually it's close enough), it's maybe easier to do drift alignment if optical and mount polar axis are parallel.


I stand corrected. It is not necessary to eliminate cone error in order to perform an accurate drift alignment.

Also, you'll match precision of good drift alignment with polar scope only if you're very,very lucky, in fact never.


Correct. I did not mean to imply that polar scope alignment is as precise as drift alignment. I was just answering the OP's question - polar scope alignment and drift alignment should agree when carefully executed, within the precision limits of each technique. If each technique is executed correctly, you won't be able to tell the difference when looking through the polar scope.

#13 Footbag

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Posted 19 August 2012 - 10:36 AM

Thanks for the info. I guess my biggest problem is that I haven't been able to do enough testing. I really only got a couple hours in with the polar alignment scope and even then I was dodging the clouds.

I will do some testing when my tripod shows up and hopefully have some conclusions to update this thread.

#14 Peter in Reno

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Posted 19 August 2012 - 11:48 AM

The conclusion was that you should NOT rotate the PAS within the polar axis housing after centering the reticle. Instead you adjust for HA by rotating the entire PA, NOT just the PAS.


A-P Mach1 mount is different than others. Rotating the RA axis does NOT rotate the PAS. That's how they designed it. So A-P had to make the PAS rotatable and they did not do a good job because it does not rotate in perfect circle. No matter how well the reticule is centered, rotating PAS to match the Polaris' Hour Angle is necessary but also causes the reticule to be off center.

Peter

#15 Coastal

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Posted 19 August 2012 - 09:35 PM

My project for the week is to adapt a T-thread to the polarscope of my EQ6 (removal of the eyepiece is obviously necessary). Hopefully the PS has enough aperture for my lodestar to pick up several stars. If it can I plan on using astrometry.net to solve the image and overlay the image with a crosshair where NCP is. In theory all I'd need to do is place the NCP at the image centre. I have no idea how well this will work however...

#16 Tom and Beth

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Posted 19 August 2012 - 11:35 PM

The conclusion was that you should NOT rotate the PAS within the polar axis housing after centering the reticle. Instead you adjust for HA by rotating the entire PA, NOT just the PAS.


A-P Mach1 mount is different than others. Rotating the RA axis does NOT rotate the PAS. That's how they designed it. So A-P had to make the PAS rotatable and they did not do a good job because it does not rotate in perfect circle. No matter how well the reticule is centered, rotating PAS to match the Polaris' Hour Angle is necessary but also causes the reticule to be off center.

Peter


When you bought the PAS, did you align it to the mount by sighting a distant object then rotating the PAS? I could be wrong but from your description of the errors, it's the first thing I would check. Procedure is more throughly discussed in the manual.

#17 Peter in Reno

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Posted 19 August 2012 - 11:59 PM

I cannot align PAS to the mount because rotating Mach1 RA axis does NOT rotate PAS. I would need to get get something like a V-Block and position PAS on the V-Block to see if the reticule is centered but I chose not to do it because I didn't really feel like it and PAS is never intended for accurate polar alignment for imaging. It's a good tool if you go to a dark site and allows you to get pretty close so that Drift Alignment would be faster.

I tried to find a link to A-P PAS for Mach1 about why Mach1 RA axis will NOT rotate PAS but it's not there.

I believe the main reason for this design is to allow "through-the-mount-cabling" otherwise the cables will get all twisted. A-P Mach1 is a fantastic mount and polar alignment is very easy. Just read the manual.

Peter

#18 jmiele

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Posted 20 August 2012 - 07:17 AM

A-P does align them before shipment. So, unless you are having issues with the quality of your alignment it's most likely fine.. That said, I'd normally say there's no substitute for drift alignment....however, there is! PEMPro. :)


Joe

#19 psu_13

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Posted 20 August 2012 - 07:49 AM

The thing I found hard about the AP polar scope was that when you turn the reticle to align the stars it seems to loosen up and become more sloppy depending on where you need it to be. I finally gave up and use the scope just to get a rough sighting of Polaris in the scope. It's then usually close enough that the quick drift align goes faster. I put my name on the list for the new Polar scope just because it's such a clever idea. :)

#20 dawziecat

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Posted 20 August 2012 - 09:36 AM

I am interested in the new AP PAS. Does anyone have any idea if it might be useable with a G11?

#21 jmiele

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Posted 20 August 2012 - 12:10 PM

The thing I found hard about the AP polar scope was that when you turn the reticle to align the stars it seems to loosen up and become more sloppy depending on where you need it to be. I finally gave up and use the scope just to get a rough sighting of Polaris in the scope. It's then usually close enough that the quick drift align goes faster. I put my name on the list for the new Polar scope just because it's such a clever idea. :)


Yes, the entire assembly feels like it's "slightly" spring loaded and can get hung up not returning to center on occasion. At least that's the best a can do to describe it. :)

I ended up using PEMPro or TPoint via the computer for any alignment where I required accuracy for imaging. If not, the polar scope has always been sufficient for visual. Even the daytime polar routine outlined in the manual work very well IME. If I thought my A-P polar scope was out of wack, I think I'd send it to A-P and have them look at it. Otherwise I'd just use it.

Joe

#22 Tom and Beth

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Posted 20 August 2012 - 01:52 PM

I cannot align PAS to the mount because rotating Mach1 RA axis does NOT rotate PAS. I would need to get get something like a V-Block and position PAS on the V-Block to see if the reticule is centered but I chose not to do it because I didn't really feel like it and PAS is never intended for accurate polar alignment for imaging. It's a good tool if you go to a dark site and allows you to get pretty close so that Drift Alignment would be faster.

I tried to find a link to A-P PAS for Mach1 about why Mach1 RA axis will NOT rotate PAS but it's not there.

I believe the main reason for this design is to allow "through-the-mount-cabling" otherwise the cables will get all twisted. A-P Mach1 is a fantastic mount and polar alignment is very easy. Just read the manual.

Peter


Please note I wrote "rotate the PAS", not the mount. I'm well aware of the Mach1 axis.

#23 WadeH237

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Posted 20 August 2012 - 05:57 PM

My project for the week is to adapt a T-thread to the polarscope of my EQ6 (removal of the eyepiece is obviously necessary). Hopefully the PS has enough aperture for my lodestar to pick up several stars. If it can I plan on using astrometry.net to solve the image and overlay the image with a crosshair where NCP is. In theory all I'd need to do is place the NCP at the image centre. I have no idea how well this will work however...


I know of someone whose normal polar alignment routine is similar to this. Instead of a PAS and guide camera, he just uses his regular imaging camera in the main OTA. With the mount aligned near the NCP, he takes an exposure for a few seconds. During the exposure (with the RA clutch released), he rotates the RA axis 180 degrees.

This gives him an image where Polaris transcribes a 180 degree circle in the field. He moves the mount such that the semicircle is centered on the center of the camera's field.

He claims that this works well for him and that he never has a problem with drift or field rotation.

#24 Coastal

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Posted 20 August 2012 - 06:20 PM

My project for the week is to adapt a T-thread to the polarscope of my EQ6 (removal of the eyepiece is obviously necessary). Hopefully the PS has enough aperture for my lodestar to pick up several stars. If it can I plan on using astrometry.net to solve the image and overlay the image with a crosshair where NCP is. In theory all I'd need to do is place the NCP at the image centre. I have no idea how well this will work however...


I know of someone whose normal polar alignment routine is similar to this. Instead of a PAS and guide camera, he just uses his regular imaging camera in the main OTA. With the mount aligned near the NCP, he takes an exposure for a few seconds. During the exposure (with the RA clutch released), he rotates the RA axis 180 degrees.

This gives him an image where Polaris transcribes a 180 degree circle in the field. He moves the mount such that the semicircle is centered on the center of the camera's field.

He claims that this works well for him and that he never has a problem with drift or field rotation.


That sounds a LOT easier than what I'm doing. The polarscope has the benefit of being pretty easy to collimate and therefore has minimal cone error.

I'd be tempted to try that technique with my guidescope (I can eliminate cone error via the adjustable rings)...I just need a way to start with the dec axis at the correct angle.

#25 shams42

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Posted 20 August 2012 - 08:47 PM

Reading this makes me very happy that I have a Takahashi mount. I can align with enough accuracy for perfect 30 minute subs in 60 seconds or less with the polar scope in my NJP. I've never drift aligned it.






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