Author Topic: mount accuracy  (Read 91 times)

calfkommomu

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mount accuracy
« on: December 27, 2017, 03:27:32 PM »
My understanding is if Polaris is centered in the scope you should be able to rotate RA and it should stay in the center. Is that correct?

If it is how much deviation is allowed?

I am still trying to get my GEM to track correctly. Just discovered Mr Cone Error and Mr Periodic Error. Not a friendly pair. I think I have Mr Cone Error in check but not sure.

Thanks in advance



retpoiwerround

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Re: mount accuracy
« Reply #1 on: December 31, 2017, 03:35:37 PM »
Polaris will not move much if centered in the POLAR SCOPE. The polar scope is on the axis of the mount. If you are trying to do the polaris trick on the telescope, you will be off.

bumabbefat

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Re: mount accuracy
« Reply #2 on: January 01, 2018, 05:58:14 AM »
Ideally, Polaris will make a small circle in the polar scope. by doing this, you will be able to figure out "true" north by finding the middle of the circle Polaris made.

Junee Hunt

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Re: mount accuracy
« Reply #3 on: January 01, 2018, 09:32:36 AM »
If you put Polaris in the center (not in the spot reserved for Polaris) of the reticle of a polar scope, rotate the RA axis 180 degrees, and observe that the star moves off center, it means that the optical axis of the polar scope is not aligned with the mechanical RA axis.

This can usually be corrected by using the polar scope alignment screws, and it is not an indication of the accuracy of the mount.

breadexgera

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Re: mount accuracy
« Reply #4 on: January 10, 2018, 02:31:19 AM »
Quote
My understanding is if Polaris is centered in the scope you should be able to rotate RA and it should stay in the center. Is that correct?
If it is how much deviation is allowed?
I am still trying to get my GEM to track correctly. Just discovered Mr Cone Error and Mr Periodic Error. Not a friendly pair. I think I have Mr Cone Error in check but not sure.

The RA axis needs to be aligned with the NCP (North Celestial Pole) rather than Polaris. Polaris is approximately 2/3° from the NCP. Close, but really not close enough for completely accurate tracking for astro-photography, but close enough for visual. It's not the "scope" that needs to point at the NCP, but rather the RA (Polar) axis.

vidysriret

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Re: mount accuracy
« Reply #5 on: January 11, 2018, 03:30:51 AM »
Quote
My understanding is if Polaris is centered in the scope you should be able to rotate RA and it should stay in the center. Is that correct?
No, that is not true.
The North Star is not at the NCP = North Celestial Pole.
It is the NGP point that will not move as your rotate the RA axle.
The North Star is offset by about 1 Degree from NGP.
Therefore, the North Star must make a circle, just like all other stars.

Michael Ritchie

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Re: mount accuracy
« Reply #6 on: January 11, 2018, 03:00:56 PM »
IMHO the main reason for the bad rep of polar alignment scopes lies in the fact that too many people don't bother checking the alignment between the mechanical RA axis and the optical axis of the scope. Once aligned, and if the polar reticle is well made, a polar scope can help achieve a very good alignment, within 1 arc minute, perfectly good for imaging.

So, it's good if you can check the alignment of the two axes by rotating the RA axis 180 degrees (be careful: you want the actual mechanical axis to rotate, not just the declination assembly – this is important in some mounts like the iEQ 45 Pro where it's possible to rotate the dec assembly while keeping the RA axis stationary). Once you are sure of that, you can use the polar reticle to put Polaris in the correctly-offset position so that the mechanical axis points to the NCP.

tidutamar

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Re: mount accuracy
« Reply #7 on: January 12, 2018, 04:27:32 AM »
OK I probably didn't make the first post clear enough. Sorry for that.

I have the main scope centered on Polaris. I know that isn't true north but I am using Polaris as a reference.

I am using the main scope instead of a polar alignment scope. My thought was that since triangulation cannot apply to something light years away with a radius of only a foot or so, moving the scope around RA shouldn't change the position of Polaris in the main scope. If I am wrong about this please explain what I am missing.

Thanks again

Tim Jauregui

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Re: mount accuracy
« Reply #8 on: January 13, 2018, 01:53:02 PM »
Quote
OK I probably didn't make the first post clear enough. Sorry for that.

I have the main scope centered on Polaris. I know that isn't true north but I am using Polaris as a reference.

I am using the main scope instead of a polar alignment scope. My thought was that since triangulation cannot apply to something light years away with a radius of only a foot or so, moving the scope around RA shouldn't change the position of Polaris in the main scope. If I am wrong about this please explain what I am missing.

Thanks again

This is equivalent to evaluating the parallelism between the telescope optical axis and the mechanical axis. Once you have minimized the "wandering" by using the declination control, all that's left is cone error and the error in perpendicularity between the mechanical RA and dec axes.

Gary Eldridge

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Re: mount accuracy
« Reply #9 on: January 16, 2018, 11:16:41 PM »
Quote
Quote

OK I probably didn't make the first post clear enough. Sorry for that.

I have the main scope centered on Polaris. I know that isn't true north but I am using Polaris as a reference.

I am using the main scope instead of a polar alignment scope. My thought was that since triangulation cannot apply to something light years away with a radius of only a foot or so, moving the scope around RA shouldn't change the position of Polaris in the main scope. If I am wrong about this please explain what I am missing.

Thanks again

This is equivalent to evaluating the parallelism between the telescope optical axis and the mechanical axis. Once you have minimized the "wandering" by using the declination control, all that's left is cone error and the error in perpendicularity between the mechanical RA and dec axes.
That's it.

I was wondering what precision I should hope to get. Should Polaris move at all?

Tom Durham

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Re: mount accuracy
« Reply #10 on: January 18, 2018, 03:34:01 AM »
Quote
Quote

Quote

OK I probably didn't make the first post clear enough. Sorry for that.

I have the main scope centered on Polaris. I know that isn't true north but I am using Polaris as a reference.

I am using the main scope instead of a polar alignment scope. My thought was that since triangulation cannot apply to something light years away with a radius of only a foot or so, moving the scope around RA shouldn't change the position of Polaris in the main scope. If I am wrong about this please explain what I am missing.

Thanks again

This is equivalent to evaluating the parallelism between the telescope optical axis and the mechanical axis. Once you have minimized the "wandering" by using the declination control, all that's left is cone error and the error in perpendicularity between the mechanical RA and dec axes.
That's it.

I was wondering what precision I should hope to get. Should Polaris move at all?
What do you need to minimize cone error for? That determines the precision you need. If you plan to use the main telescope as a polar scope, that's really hard as you would need less than 1 arc minute axis alignment error to get an AP-quality polar alignment. But for a visual low-power GoTo you can be happy with maybe 10-20 arc minutes of cone error...

Robert Estrada

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Re: mount accuracy
« Reply #11 on: January 20, 2018, 02:46:22 PM »
Quote
What do you need to minimize cone error for? That determines the precision you need. If you plan to use the main telescope as a polar scope, that's really hard as you would need less than 1 arc minute axis alignment error to get an AP-quality polar alignment. But for a visual low-power GoTo you can be happy with maybe 10-20 arc minutes of cone error...


It is a manual GEM. I wanted to eliminate cone error for the mount to track as best as possible and eventually use the main scope for polar alignment.

Can you please explain what you mean by "need less than 1 arc minute axis alignment error".

I hope I'm not being a pest. Just trying to understand all this.

Joe Maillet

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Re: mount accuracy
« Reply #12 on: January 20, 2018, 10:08:21 PM »
hi,ill put my 2 cents in,.....Polaris is I think about 45 minutes from the ngp. so if you center Polaris depending on field of view of eyepiece unless your using an eyepiece with a 1 degree field of view an you were centered on ngp,you would I imagine see Polaris make a circle around the edge of eyepiece ,if your mount mechanical axis was in sync with your optical axis.but if you center on Polaris you will just see an arc as you rotate.

poithegepur

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Re: mount accuracy
« Reply #13 on: January 23, 2018, 03:59:59 PM »
Quote
Quote

What do you need to minimize cone error for? That determines the precision you need. If you plan to use the main telescope as a polar scope, that's really hard as you would need less than 1 arc minute axis alignment error to get an AP-quality polar alignment. But for a visual low-power GoTo you can be happy with maybe 10-20 arc minutes of cone error...


It is a manual GEM. I wanted to eliminate cone error for the mount to track as best as possible and eventually use the main scope for polar alignment.

Can you please explain what you mean by "need less than 1 arc minute axis alignment error".

I hope I'm not being a pest. Just trying to understand all this.
Hmmm... cone error of the main telescope has no bearing at all on tracking. It only affects the precision of GoTos.

What I meant with the 1 arc minute thing is that 1 arc min is a level of precision I'd shoot for polar alignment for astrophotography.