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5haan_A

Can you polar align using your guide scope?

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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,

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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

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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.

 

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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 :D

James

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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 by Ouroboros
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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

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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

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+1 for sharpcap polar align , works a treat :)

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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.

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

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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.

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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. 

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