Can you polar align using your guide scope?

[5haan_A]

Hi,

It's a silly question in many ways, but I wanted to know why it works. Why can you polar align with your guide scope attaced to your OTA even if its not pointing at the exact same thing as your main OTA? I would have thought that you would only be able to PA with your main OTA on your mount. 

Thanks,

[Davey-T]

Needs to be aligned with the RA axis which is why the polar scope is usually through it, it should give you a good enough result for visual use though.

Some software can work out the orthogonal error between the mount axis and the scope axis which can then be adjusted to match but probably not worth the time spent.

Dave

[jjosefsen]

It works fine.

It's the RA axis that needs to be aligned, not the scope it self. I use sharpcap to do my polar alignment and phd2 confirms it is very accurate.

It uses platesolving to determine the starfield around the North Pole, then it compares that star field to a 90 degree turned platesolve of the same region. The difference can be used to work out the offset of the RA axis to the NCP, giving you a solution in ra and Dec coordinates that you need to move the RA axis.

 

[JamesF]

2 hours ago, jjosefsen said:

It works fine.

It does indeed.

Some software works by using the telescope and camera (whichever one you choose) to measure the drift due to misalignment of the RA axis.  Some works by taking multiple images at various RA values and plate-solving to work out what axis of rotation would give those images and thereby determines how far the RA axis is from the Earth's axis.  It doesn't really matter that much what the OTA is pointing at, especially in the second case, though it can be helpful to know that you're somewhere close to an intended target (because drift alignment is a bit easier if you pick alignment stars in specific positions in the sky, for instance).

The OP's original question might be reversed to instead ask: "Why should you not be able to polar align with your guidescope?"  Short of a valid answer it's hard not to conclude that you can

James

[Ouroboros]

Is this one way the appreciate why it works?  Suppose the guide scope is not perfectly parallel to the RA axis. It will sweep out a wider field of view as it is rotated about the RA axis - wider that is when compared with an equivalent polar scope which is perfectly aligned with the RA axis. That's why it works. It's equivalent to using a polar scope with a wider field of view. 

Edited October 10, 2019 by Ouroboros

[ollypenrice]

I think what we are asking is, 'Can a scope with cone error be used for polar aligning?' Since none of us is likely to own a scope mounted without cone error I think the answer has to be 'yes.'

Olly

[GraemeH]

The reason it works is based on 3D geometry.  If you platesolve images which are separated by a move of the mount's RA axis only (i.e. no move in Dec between images), you can work out exactly where on the celestial sphere your RA axis is pointing, because it is the centre of rotation between the platesolved images.  It works best if you take 3 images because this gives three points to work with and any 3 non-colinear points in space define the circumference of a circle.  The centre of that circle gives the pointing coordinates of the mount's RA axis.  It can also work with 2 images, because it's a reasonable approximation to use the midpoint of the 2 images as the third point for calculating the circle.

I have used this method myself using an Excel spreadsheet to calculate the PAE, and then using APT's GOTO++ function to point to a position offset from a suitable alignment star by the amount of Polar Alignment Error.  It is then a matter of adjusting Alt-Az bolts to centre the alignment star.  On my last test, this gave me a calculated PAE (using my spreadsheet) of <2 arcminutes.  This was confirmed by running the Guiding Assistant in PHD2, which also reported <2 arcminutes error.

My reason for developing the spreadsheet for this approach is that I use an OAG on my Edge HD8, and I image with a DSLR.  Sharpcap's polar alignment tool needs a larger FOV than I get on my guide camera, and it doesn't work with DSLR live view.  I'd be happy to share my spreadsheet if it's of interest to anyone.

Graeme

[PaulR1]

+1 for sharpcap polar align , works a treat

[PaulB]

I use mine with PHD 2 "Drift Align" No problem.

If you wanted to be more precise you could centre a bright star in your eyepiece and align your guide scope. But it doesn't really matter.

[Owmuchonomy]

Of course.  Thats what the Synscan polar alignment routine does after accounting for cone error.

[5haan_A]

Thanks for the replies. I felt there was a little bit of the answer in everyone's replies.

 

The analogy that makes sense to me is that the rotation of the mount in RA is akin to a compass that you used to use in maths class. Once the software figures out, platesolves, where the pole star is it can figure out how far away your mount is from the that centre and then you adjust accordingly. The fact that your guide scope is pointing in a slightly different direction to your mount is not relevant to the software calculation. 

 

To some extent that makes sense to me.

[Ouroboros]

32 minutes ago, 5haan_A said:

Thanks for the replies. I felt there was a little bit of the answer in everyone's replies.

 

The analogy that makes sense to me is that the rotation of the mount in RA is akin to a compass that you used to use in maths class. Once the software figures out, platesolves, where the pole star is it can figure out how far away your mount is from the that centre and then you adjust accordingly. The fact that your guide scope is pointing in a slightly different direction to your mount is not relevant to the software calculation. 

 

To some extent that makes sense to me.

And me. You speculated on this being a silly question, which of course it isn't, and not just because we don't have silly questions on SGL, only silly answers 🙂.   It's a question I had vaguely puzzled about myself and not satisfactorily answered when I recently started using the polar alignment routine in KStars EKOS which very succesfully uses plate solving with either the guide scope or main scope.  But your question prompted my own thinking which, combined with the answers from others, has enabled me to see why it works. So thanks for posting the question. 

[JIm19130]

Hi.  Sounds like one can polar align using a guide scope, but is there a spec for the guidescope or the camera that would be required.  

In my case, I have a Williams Optics 50mm  guide camera and a ZWO ASI120MM MINI mono camera attached to it.  Would this work, or would I be best served with an upgraded camera like the ASI 290Mm Mini?

[Pitch Black Skies]

@JIm19130 I think you meant you have a 50mm guidescope.

Anyway, that set up will work fine.

A lot, if not most people use a 50mm guidescope with a 120mm camera for guiding.

Edited May 24, 2022 by Pitch Black Skies

[michael8554]

Hi Jim

The PA routine in SharpCap is popular and fast, but has some requirements:

"The camera must be supported by SharpCap. The cameras field of view should be 1-2.5 degrees. You need to see at least 15 stars in the FOV. You need to be aligned within 5 degrees of the pole (rough polar alignment)

[AstroMuni]

On 10/10/2019 at 13:52, 5haan_A said:

Hi,

It's a silly question in many ways, but I wanted to know why it works. Why can you polar align with your guide scope attaced to your OTA even if its not pointing at the exact same thing as your main OTA? I would have thought that you would only be able to PA with your main OTA on your mount. 

Thanks,

This diagram that I put together may help clarify the views that have been said above. Most software like Sharpcap, PHD2, Ekos use this method to help do PA. As you can see from the diagram, it doesnt matter if the images are being captured via main or guidescope. Its just the radius of the arc that would differ.

EDIT: Added text to explain ....The red circle is what the mis-aligned mount is rotating about and green circle (rotating around NCP) is where it should be rotating about. Our aim should be to get red circle to line up as closely as possible with green.

Edited May 24, 2022 by AstroMuni

[Ouroboros]

@AstroMuni Nice diagram to illustrate polar alignment. Thanks.

In practice Polaris and the celestial pole are within the field of view of the scope (main or guide).  I have sometimes wondered how polar alignment methods work when Polaris is not in view.  Not that I’ve ever have to use that method. 

[AstroMuni]

3 hours ago, Ouroboros said:

I have sometimes wondered how polar alignment methods work when Polaris is not in view.

As you can see from the diagram, visibility of Polaris is not a must. You could describe an arc that is further and further away from the pole, but the accuracy is going to be slightly less as you go further away.

[Ratlet]

On 17/05/2022 at 13:10, AstroMuni said:

This diagram that I put together may help clarify the views that have been said above. Most software like Sharpcap, PHD2, Ekos use this method to help do PA. As you can see from the diagram, it doesnt matter if the images are being captured via main or guidescope. Its just the radius of the arc that would differ.

This is it.  The software is calculating the polar axis direction, not the guide camera direction.  It takes the info from the guide camera to figure out where the polar axis is pointing based on where the snaps from the guide scope tell the software the axis  of rotation is.  Really it doesn't matter where the scope is pointing since it is just using that information to figure out where the axis of rotation is pointing.  Once it knows where you are pointing it can tell you where to move it to.  I love stuff like this, although I'm rubbish at maths so I don't understand that part.  But the 'crayon level' understanding does the job for me (and honestly, you can get pretty far in life with a big enough box of crayons)

[AstroMuni]

12 minutes ago, Ratlet said:

The software is calculating the polar axis direction, not the guide camera direction. 

You are correct. Guide or main camera direction is not important 🙂

Edited May 24, 2022 by AstroMuni

[StevieDvd]

1 minor correction to the diagram is that the axis of the mount should point at the NCP (not at polaris). If your mount is polar aligned and you view through your polar scope, polaris itself will go around a circle with the NCP in the middle.  Like the PA software the polarscope reticule is to place polaris on the small circle at the right point for the date/time to place your mount axis on the NCP.

[AstroMuni]

39 minutes ago, StevieDvd said:

1 minor correction to the diagram is that the axis of the mount should point at the NCP (not at polaris). If your mount is polar aligned and you view through your polar scope, polaris itself will go around a circle with the NCP in the middle.  Like the PA software the polarscope reticule is to place polaris on the small circle at the right point for the date/time to place your mount axis on the NCP.

It is shown as pointing to NCP (white line). Polaris is shown as a separate marker.

[StevieDvd]

7 minutes ago, AstroMuni said:

It is shown as pointing to NCP (white line). Polaris is shown as a separate marker.

Apologies, on a second look I can see that's what was intended.  Though the line marked as the mount axis rotation point is wrong then.

 

[AstroMuni]

1 hour ago, StevieDvd said:

Apologies, on a second look I can see that's what was intended.  Though the line marked as the mount axis rotation point is wrong then.

That mount axis line is showing where the mount is pointing at present (in blue). The red circle is what the mount is rotating about and green circle is where it should be rotating about. Our aim should be to get red circle to line up as closely as possible with green.

[Ouroboros]

As I understood it, the diagram shows the mount misaligned and pointing at neither Polaris or the NCP.