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Mount performance testing; what parts of the sky tax the mounts tracking most?

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#1 SimonIRE

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Posted 26 October 2020 - 01:35 PM

Hi Folks,

 

I am doing some mount performance tests

 

I should know the answer to this but I can't seem to find agreement. 

 

What parts of the sky tax the mounts capabilities the most?

 

Is it the Zenith?

 

The Celestial Equator?

 

Neither?

 

Opinions needed!

 

Simon



#2 Jim Waters

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Posted 26 October 2020 - 01:48 PM

For me its polar.


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#3 crgood2

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Posted 26 October 2020 - 01:51 PM

Agreed, near the pole is the most taxing for the mounts I've worked with.


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#4 orlyandico

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Posted 26 October 2020 - 01:54 PM

+1 polar


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#5 descott12

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Posted 26 October 2020 - 02:21 PM

Is it truly more taxing or simply just entirely inaccurate since any change in RA is so compressed?


Edited by descott12, 26 October 2020 - 03:37 PM.


#6 555aaa

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Posted 26 October 2020 - 03:07 PM

Zenith from a practical standpoint.

 

Performance around the poles depends mostly on polar alignment - the pole is basically the 'inaccessible' point in an equatorial mount because if anything is off, the correction rates go to infinity as you get closer and closer to the pole. Math to do pointing corrections has divide by zero type terms at the pole so correction SW usually has some turn-off or clamping of correction terms within a degree or so of the pole.  Field rotation also becomes extreme at the pole if there is any polar misalignment. But that bad patch of sky isn't that big in terms of square degrees on the celestial sphere, and it's not very interesting, unlike zenith.

 

The celestial equator is the orientation of maximum rotational inertia which is a bigger load versus the pole which is the minimal inertial load, but this probably isn't a big effect on smaller mounts.

 

When you say test, what do you mean testing for? Slewing performance? Tracking accuracy? Ability to handle imbalanced loads? Wind gust resistance? Pointing accuracy? stiffness?


Edited by 555aaa, 26 October 2020 - 03:18 PM.

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#7 SimonIRE

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Posted 26 October 2020 - 03:24 PM

Zenith from a practical standpoint.

Performance around the poles depends mostly on polar alignment - the pole is basically the 'inaccessible' point in an equatorial mount because if anything is off, the correction rates go to infinity as you get closer and closer to the pole. Math to do pointing corrections has divide by zero type terms at the pole so correction SW usually has some turn-off or clamping of correction terms within a degree or so of the pole. Field rotation also becomes extreme at the pole if there is any polar misalignment. But that bad patch of sky isn't that big in terms of square degrees on the celestial sphere, and it's not very interesting, unlike zenith.

The celestial equator is the orientation of maximum rotational inertia which is a bigger load versus the pole which is the minimal inertial load, but this probably isn't a big effect on smaller mounts.

When you say test, what do you mean testing for? Slewing performance? Tracking accuracy? Ability to handle imbalanced loads? Wind gust resistance? Pointing accuracy? stiffness?


Hi and thanks,

I’m looking at both the unguided and guided performance of my 10 Microns. By chance, I’ve been imaging the Sadr region. I wondering how these coordinates impact results.

#8 rmollise

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Posted 26 October 2020 - 03:27 PM

Hi Folks,

 

I am doing some mount performance tests

 

I should know the answer to this but I can't seem to find agreement. 

 

What parts of the sky tax the mounts capabilities the most?

 

Is it the Zenith?

 

The Celestial Equator?

 

Neither?

 

Opinions needed!

 

Simon

For a GEM it doesn't matter...it's tracking at the sidereal rate whether the scope is pointed near the pole or the zenith or wherever. But...

 

When you move to a radically different part of the sky expect to rebalance if you want the performance to stay the same. wink.gif


Edited by rmollise, 26 October 2020 - 03:27 PM.

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#9 John Carlini

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Posted 26 October 2020 - 04:07 PM

Interesting question... Atmospheric refraction can contribute to pointing errors, both on targets near the horizon, as well as, the location of Polaris. Here is an engineering article that talks about these pointing issues:

 

https://www.dfmengin..._article_4.html


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#10 WadeH237

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Posted 26 October 2020 - 06:07 PM

The zenith will be most sensitive to backlash errors.  Polar will be most challenging to guide in RA.  Near the horizon will generally have the greatest drift.



#11 spkerer

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Posted 26 October 2020 - 07:25 PM

I would expect the answer to be different for equatorial mounts vs alt-az mounts.


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#12 luxo II

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Posted 26 October 2020 - 07:53 PM

For an altaz mount its the zenith, and for equatorial mounts the celestial pole.

 

The main difficulty in both cases is the mechanical errors resulting in the optical axis and dec (altitude) axis not being exactly perpendicular, and the error between that axis and the RA (azimuth) axis, with the consequence being the scope may have a significant error when attempting to point at the pole (or zenith) and quite possibly can't get it in the file of view if the error is gross.

 

In EQ mode my mount doesn't have any issues with backlash as I set it up with a slight imbalance. In altaz mode, there is a slight backlash in azimuth since it is balanced and the tripod head levelled, but I live with that; maybe one day I'll adjust the worm.


Edited by luxo II, 26 October 2020 - 07:57 PM.

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