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Telescope alignment with polar scope

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#1 Opa Jim

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Posted 22 July 2021 - 09:36 AM

I started my A-P journey last winter. I have a W-O Z61, Canon T3i, recently bought a Sky Watcher HEQ5 (I had been using a star tracker), and use Backyard EOS to run my sessions. Here's my question. I get done with my polar alignment, start up BYEOS, and Polaris is not centered in the Live View screen (or the preview pictures); it's low in the screen but maybe that's correct since Polaris was at about 1AM in the polar scope. Last night was the first good viewing night since I bought the HEQ5, thanks to our wonderful Upstate NY weather. After doing a 1-star alignment, I punched in M51 and the mount slewed to the proper location with M51 located at 1800hrs in the camera view, but again maybe that's where it's supposed to be at this point in the alignment.  Any suggestions or comments?PREVIEW_M51 20210721-23h26m42s.JPG



#2 Opa Jim

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Posted 22 July 2021 - 09:37 AM

Note: 15 sec at ISO1600



#3 Medic002

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Posted 22 July 2021 - 09:54 AM

if your goto's are not getting close to center as u like u do the 2 star alignment and add extra calibration stars and more of your goto's will get closer to center. the more calibration stars u do the closer it should be. when u change sky views from say north, south, east, and west each time u go to a different part of the sky view do at least 1 star calibration and it will help get your goto's closer to center in that part of the sky. another option u can try is using a program to plate solve and the program will take a picture of where ur pointing your telescope and figure out exactly what part of the sky ur looking at and make your goto's closer to center. i use an asiair for plate solving but i think u could use phd2 free program for your setup.



#4 ChiTownXring

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Posted 22 July 2021 - 09:58 AM

Remember your mount and your telescope could be slightly off from each other and that is okay. The main point here is that your mount is polar aligned. Then you use your star alignment via GOTO will get you close but typically not centered.. You should always have the mount GOTO to a bright star and then use the hand controller to center the star in your T3i screen and then sync. You should do this at least on a couple of stars on the opposite side of the night sky. Then GOTO your target and it should be really close to the center of your T3i screen.

Also I started with the Canon T3i and I believe the sweet spot for the ISO is 800..


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#5 barbarosa

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Posted 22 July 2021 - 10:32 AM

You probably already know this but I mention it just in case someone who is not familiar with polar alignment reads this post. Other than that, the previous posts cover the issue of ways to improve go to accuracy

 

Your post  implies that at the end of the PA process you expected Polaris to be centered in the image. This should not be the case unless Polaris was the final alignment star or after PA you initiated a slew to Polaris.  Polar alignment points the RA axis of the mount at the Celestial Pole and not Polaris. .


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#6 terrypaula

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Posted 22 July 2021 - 11:01 AM

Even after polar aligning my mount I can't see Polaris through an EP.  Polaris isn't ever supposed to be centered in your polar scope.  After an alignment slew to Polaris and see what position Polaris takes the scope.  You align to the Celestial North Pole, Polaris is about 1 - 2 degrees away from the CNP.  An astronomer uses Polaris to align the telescope because it is close to the CNP and is easy to see visually.

 

I've been using KStars/Ekos installed on a Fit2 PC and the alignment module contained within.  It works very well and is totally free.  I still haven't had to touch the mount during the alignment procedure.  It put the object right in the center if you let it.



#7 rgsalinger

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Posted 22 July 2021 - 11:17 AM

Polaris is 40 arc minutes from the Celestial North Pole, not 1-2 degrees. You'd be hard pressed to see it in most eyepieces but it generally would appear in the finder  scope. What you need to to have more calibration stars and for those stars to be on both sides of the meridian to minimize mount errors (cone error can be the big one). To get the cal stars just right when using a camera, make sure to turn on your cross hairs. It's really hard to get a star centered (IMHO and IME) without them. Always finish the alignment using the up and right arrow keys to remove any residual backlash from the calibration.

 

Rgrds-Ross


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#8 acrh2

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Posted 22 July 2021 - 11:20 AM

Remember your mount and your telescope could be slightly off from each other and that is okay. The main point here is that your mount is polar aligned. Then you use your star alignment via GOTO will get you close but typically not centered.. You should always have the mount GOTO to a bright star and then use the hand controller to center the star in your T3i screen and then sync. You should do this at least on a couple of stars on the opposite side of the night sky. Then GOTO your target and it should be really close to the center of your T3i screen.

Also I started with the Canon T3i and I believe the sweet spot for the ISO is 800..

Correct me if I am wrong, but doesn't the Synscan manual imply that the sync command doesn't alter the pointing model?

In other words, only the last sync command gives extra pointing accuracy within a 5x5 degree area of the sync, and sync commands before that are ignored.

In order to achieve a better pointing accuracy, you need to do an actual star alignment. Synscan pointing model accepts up to 3 alignment points. And the best practices for choice of those stars are described in the manual.

 

There's a separate document that just describes tips for better pointing accuracy - 

https://inter-static...nt_accuracy.pdf

 

From my personal experience, the biggest factor in the pointing accuracy is the actual polar alignment.

 

I usually polar align, Reset alignment, do a goto to somewhere near equator, enable Align with sync, and do plate solve and sync. This creates a single alignment point, just like during a star alignment routine. You can repeat this 2 more times in different parts of the sky to add 2 more alignment points. I don't find more than one alignment point necessary though:

I recently purchased an iPolar camera system for polar alignment. Aligned with this system, my gotos are now to within 1-3 arcmin of the target without a prior sync. And of course, when you plate solve and sync, the next goto in the same area will put you within 5 arcsec of the target.



#9 Opa Jim

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Posted 22 July 2021 - 01:22 PM

After thinking about my initial question a bit more......

For starters, since at the time I was polar aligning Polaris was at the 1o'clock position (slightly above and to the right of NCP) and the polar scope is inverted, it would make sense that Polaris would appear below NCP in the camera image.

Then consider that the polar alignment is independent of the 1-, 2- or 3-star alignment process and the 1-star alignment I did was the least accurate, it shouldn't surprise that M51 wasn't in the exact center.  A 2-star alignment would no doubt have given better results.  

 

Thanks for all your comments and suggestions.



#10 kathyastro

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Posted 22 July 2021 - 01:33 PM

One thing missing from the discussion so far is the accuracy (or lack thereof) of your home position.

 

After completing the polar alignment, the position of Polaris in the imaging field of view is only predictable if you have a perfect home position and if you have no cone error.  Let's forget about cone error for now: you still need a perfect home position in order for the position of Polaris in the scope to have any meaning.

 

Furthermore, the accuracy of the first slew of your goto alignment depends on the accuracy of the home position.  A one-degree error in the home position means a one-degree error in that first slew.  Centering that first star partially corrects for that error, but you really need a three-star alignment to fully correct for it.

 

So, how good is your home position?  How are you ensuring that it is accurate?



#11 Pluggednickels

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Posted 22 July 2021 - 03:59 PM

Make sure your polar scope is in alignment within the mount as well.  if it’s off you won’t get a good polar alignment.     You can test this without the scope on the mount by pointing the mount/polar scope at something on land and fairly low. So you can spin the RA axis 360   Align something in the center of the polar scope.  Then spin the ra axis  if the thing moves from the center your Polar scope is out of alignment with the mount and will need to be adjusted accordingly 

 

This can also be verified after any type of electronic polar alignment 

once polar aligned electronically if Polaris is not where it should be in you visual  polar scope   Then the polar scope is out of alignment from the mount.  
 

this is a good thing to check once and awhile on portable mounts so to keep your polarscope aligned.     
 

 

you don’t want to get to a dark site with a badly aligned polarscope if your not polar aligning electronically somehow

 

i haven’t used any of my visual polar scopes since getting the polemaster after and then later the asiair 


Edited by Pluggednickels, 22 July 2021 - 04:08 PM.


#12 barbarosa

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Posted 22 July 2021 - 04:09 PM

If you have a camera on the guide scope or the main scope, why not use it and software rather than a polar scope to perfect your PA. I highly recommend the PA tool in SharpCap, it alone is worth the modest license fee regardless of whether you use any other feature or tool. 

 

SharpCap also has a cone error tool (ConeSharp) which is both free and easy to use.



#13 acrh2

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Posted 22 July 2021 - 04:54 PM

If you have a camera on the guide scope or the main scope, why not use it and software rather than a polar scope to perfect your PA. I highly recommend the PA tool in SharpCap, it alone is worth the modest license fee regardless of whether you use any other feature or tool. 

 

SharpCap also has a cone error tool (ConeSharp) which is both free and easy to use.

Sharpcap can be very deceiving for polar alignment. If you get different polar alignment errors depending on the direction and amount of rotation of the RA axis, you have a flexure in your setup. Sharpcap will report an error and direct your to correct it, but in fact, you can be getting an even worse polar alignment.

 

I wonder how many people use Sharpcap without realizing this.

Until I got an iPolar camera, I was one of those people - getting a worse polar alignment with Sharpcap after doing a rough polar alignment with the built in polar scope. 



#14 Alex McConahay

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Posted 22 July 2021 - 10:15 PM

>>>>>and Polaris is not centered in the Live View screen (or the preview pictures);

 

There is generally no reason for Polaris to be centered in your Field of View unless you have in fact centered that star on purpose. 

 

In the traditional "Home" position (counterweight down, at 90d declination, you are not pointing at the coordinates for Polaris. For me to get Polaris centered in my telescope, I must slew significantly to one side, and move the declination a bit. 

 

Do not confuse Celestial alignment with Polar alignment. A scope is not actually "polar aligned." The MOUNT is. The scope is then loaded on top of that. Hopefully it is pointed parallel to the axis of the RA rotation of the mount, and the axis of the declination of the mount. But it is not necessarily so. Once you are polar aligned, you may choose to celestially align the mount and scope. You do this by pointing to a known bright star, centering it, and telling the mount the star is centered. Do that a few times (two and three or more star alignment) and the mount knows where it is, and probably a lot about what errors are there in polar alignment, RA movement, mirror flop, and other things. 

 

But, at any rate, there is no reason for Polaris to be centered in a Field of View unless you have specifically told the mount and scope to go there. 

Alex


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#15 barbarosa

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Posted 23 July 2021 - 03:56 AM

Sharpcap can be very deceiving for polar alignment. If you get different polar alignment errors depending on the direction and amount of rotation of the RA axis, you have a flexure in your setup. Sharpcap will report an error and direct your to correct it, but in fact, you can be getting an even worse polar alignment.

 

I wonder how many people use Sharpcap without realizing this.

Until I got an iPolar camera, I was one of those people - getting a worse polar alignment with Sharpcap after doing a rough polar alignment with the built in polar scope. 

Not a big problem my experience and I have used three other PA tools or methods.

 

The objective is to point the mount's RA axis at the celestial pole. Not the finder, not the main scope but the mount. If you guide with guide scope there is as you say the problem of flexure, that is the guide scope can move relative to the main scope. Your setup has some flexure and so must mine. My preference is to use the main scope rather than the guide scope for polar alignment but not everyone is setup for a wide field. 

 

I have done the PA using a guide scope in fact three different scopes from 50mm to 85mm, all of them mounted with D style dovetails and 3 point rings. Pretty stiff but still they would have some flexure. More or less than the Orion Mini on a finder scope stalk and shoe I can't say but I would guess less.

 

Here is what the manual says-

 

The most common cause of poor results is that something is shifting as the mount is being rotated around the RA axis. If you are using a guide scope/camera it could be that the scope is not mounted firmly or that a cable is pulling (or just hanging loose) which can shift the camera slightly. This problem has also been experienced by people who have a problem with their RA axis bearings!

 

The first thing to do if you suspect this problem is to confirm you have an issue. Run a SharpCap polar align normally (starting in the home position and rotating 90 degrees), and adjust as usual. Once you are finished, leave the scope at the 90 degree position and re-run polar alignment in SharpCap (this time rotating back to the home position when prompted to rotate). If the measurement from the second PA run matches the first then you probably don’t have this issue. If it doesn’t match by a big margin then flexure/movement as you rotate is the likely cause.

 

You can test for this problem quite easily by rotating the mount in stages of about 15 degrees. SharpCap plots a dark red cross at the point it calculates to be the centre of rotation (where the RA axis is pointing) for each stage of the rotation – these should form a tight group on screen if there is no flexure. In the screenshots below I deliberately let the cable to the guide camera hang loose for the polar alignment run shown on the left. Here you can see that the measured RA axis positions have drifted as I have rotated further due to the weight of the cable pulling on the camera. Once the cable was properly secured the RA axis positions form a much tighter group.

 

My experience and that of two people I know with better skills than I have is that flexure is not a problem with a well setup guider. That is not to say that it can never be a problem, but only that it is not a problem for some.

 

What do people know about possible problems? I cannot say but I can hope that they read the instructions.

 

 We will pause now and give time to the advocates of off axis guiding.shocked.gif  



#16 acrh2

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Posted 23 July 2021 - 04:09 AM

Not a big problem my experience and I have used three other PA tools or methods.

 

The objective is to point the mount's RA axis at the celestial pole. Not the finder, not the main scope but the mount. If you guide with guide scope there is as you say the problem of flexure, that is the guide scope can move relative to the main scope. Your setup has some flexure and so must mine. My preference is to use the main scope rather than the guide scope for polar alignment but not everyone is setup for a wide field. 

 

I have done the PA using a guide scope in fact three different scopes from 50mm to 85mm, all of them mounted with D style dovetails and 3 point rings. Pretty stiff but still they would have some flexure. More or less than the Orion Mini on a finder scope stalk and shoe I can't say but I would guess less.

 

Here is what the manual says-

 

The most common cause of poor results is that something is shifting as the mount is being rotated around the RA axis. If you are using a guide scope/camera it could be that the scope is not mounted firmly or that a cable is pulling (or just hanging loose) which can shift the camera slightly. This problem has also been experienced by people who have a problem with their RA axis bearings!

 

The first thing to do if you suspect this problem is to confirm you have an issue. Run a SharpCap polar align normally (starting in the home position and rotating 90 degrees), and adjust as usual. Once you are finished, leave the scope at the 90 degree position and re-run polar alignment in SharpCap (this time rotating back to the home position when prompted to rotate). If the measurement from the second PA run matches the first then you probably don’t have this issue. If it doesn’t match by a big margin then flexure/movement as you rotate is the likely cause.

 

You can test for this problem quite easily by rotating the mount in stages of about 15 degrees. SharpCap plots a dark red cross at the point it calculates to be the centre of rotation (where the RA axis is pointing) for each stage of the rotation – these should form a tight group on screen if there is no flexure. In the screenshots below I deliberately let the cable to the guide camera hang loose for the polar alignment run shown on the left. Here you can see that the measured RA axis positions have drifted as I have rotated further due to the weight of the cable pulling on the camera. Once the cable was properly secured the RA axis positions form a much tighter group.

 

My experience and that of two people I know with better skills than I have is that flexure is not a problem with a well setup guider. That is not to say that it can never be a problem, but only that it is not a problem for some.

 

What do people know about possible problems? I cannot say but I can hope that they read the instructions.

 

 We will pause now and give time to the advocates of off axis guiding.shocked.gif  

I have an Orion mini guide scope, and that goes into a finder scope shoe with a single thumb screw of my Maks. So, that's the source of flexure, I think.

So I bought an iPolar and I couldn't be happier - polar align in less than 2 minutes tonight. No need to rotate anything, no pressing ok buttons. Just align a cross hair with a circle after pressing connect button.

 

And I absolutely agree with you. People who have rings for their guide scopes, or who use wide field main scope for polar alignment would most likely not have the flexure problems.

 

But I think it's still a good idea to check - start polar align, rotate RA one way, restart polar align and rotate the other way. If both rotations give you identical polar alignment errors, then you are good to go.

Mine were different by 10 arcmin.


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#17 BQ Octantis

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Posted 23 July 2021 - 06:39 AM

A lot of opinions here. But no analysis of your pointing. M51 is just 21 arcminutes from the center of your image:

 

(Click for full size.)

gallery_273658_12412_114366.jpg

Animated toggle

 

For open loop pointing (and no PAE), that's not bad at all. And your target has heaps of margin against the edge of your FOV. Two taps on the RA and one tap on the DEC will put M51 at the center with nothing more than a look at the image on the preview screen on the back of the camera.

 

Obsessing over GoTo pointing accuracy is outside the scope of good AP IMO. But if you want tighter polar alignment, you shouldn't be using Polaris for anything but getting in the ballpark of the NCP through the guide scope. My good mate SAO 3788 is my northern hemisphere counterpart just 15 arcminutes from the NCP. That is your pole star. Polaris is a red herring—unless you're trying to work your way north through the wilderness at night bare-eyed and without a compass.

 

Cheers,

 

BQ of Octans.


Edited by BQ Octantis, 23 July 2021 - 08:38 AM.

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#18 rgsalinger

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Posted 23 July 2021 - 06:58 PM

The OP said that "the mount slewed to the proper location with M51 located at 1800hrs in the camera view" so, I thought that he was after better polar alignment since it was intuitively obvious to any casual observer that the scope was roughly pointed at M51. I suspect that others read the ask the same way.

 

What's much more relevant is that if you want your target dead center, plate solve and precision slew is the way to go. That wasn't the ask, but it's a good thing to understand, just like don't obsess over polar alignment and/or leveling the tripod. 

 

Rgrds-Ross



#19 BQ Octantis

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Posted 23 July 2021 - 07:18 PM

Polar alignment has nothing to do with reducing open loop GoTo pointing errors. That is what 3-star alignment and Pointing Accuracy Enhancement (PAE) is for. I get at least this much offset when I've got my polar alignment to under 2 arcminutes (directly measured). Platesolving with closed loop pointing (like N.I.N.A) is indeed the machine answer. But I just use the eyeball-brain-finger controller to close the loop. And for eyeball centering enhancement, I turn on the grid on the LiveView (on my 600D/T3i, just like the OP's)—this gives me realtime feedback when I'm precision slewing.

 

As to the position of the NCP on the sensor in the home position, the location could be skewed by misalignment between the dovetail plane and the OTA-sensor centerline. Along the DEC axis is easy, but orthogonal offset is cone error. (The only way to know is to measure the location of the pole in the home position with SAO 3788 as the rough NCP target.) Only 3-star align accommodates cone error.


Edited by BQ Octantis, 23 July 2021 - 07:40 PM.


#20 Alex McConahay

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Posted 23 July 2021 - 07:41 PM

Polar alignment has nothing to do with reducing open loop GoTo pointing errors. That is what 3-star alignment and Pointing Accuracy Enhancement (PAE) is for. I get at least this much offset when I've got my polar alignment to under 2 arcminutes (directly measured). Platesolving with closed loop pointing (like N.I.N.A) is indeed the machine answer. But I just use the eyeball-brain-finger controller to close the loop. And for eyeball centering enhancement, I turn on the grid on the LiveView (on my 600D/T3i, just like the OP's)—this gives me realtime feedback when I'm precision slewing.

 

What is "open Loop Pointing?"   (And, it may be obvious after you tell me what that means, but when you say "open loop GOTO pointing errors" you just mean the scope is not pointing to where it belongs?)

 

Alex



#21 kathyastro

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Posted 23 July 2021 - 07:54 PM

What is "open Loop Pointing?"  

Open loop: goto.

Closed loop: plate solving



#22 BQ Octantis

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Posted 23 July 2021 - 08:17 PM

In control theory, you represent a process as a system with a plant and a controller. An open loop control system has no feedback loop—you just dial in a setting and let the plant respond as it may. A closed loop has a feedback loop from the output to generate a measurement error to get the system to converge on the desired output.

 

Just dialing in a GoTo target or position has no feedback loop, so it is an open loop control system. But when you look at the image and you use the RA/DEC hand control to center your target, you become the feedback loop. So you close the loop.

 

Platesolving alone is not closed loop—it is just a process to find your image's position, scale, and orientation. I upload images to astrometry.net to be platesolved all the time; Astrobin does this for all of my images automatically. But that's way after I've torn down my rig and returned to civilization, so those platesolver results are merely academic. For platesolving to be part of the closed loop, the imaging system has to be able to use the position measured by platesolving to generate a pointing error to feed into the mount's RA/DEC. That is what N.I.N.A in Synch mode does.



#23 Alex McConahay

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Posted 23 July 2021 - 08:28 PM

I would have called that "No loop" because an open loop is not a loop. (in my mind)

 

Silly me. 

 

Alex




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