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PA with one scope, use with another scope?

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#1 twidget

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Posted 19 October 2018 - 05:46 PM

This might be a dumb question, and this might not be the correct forum to post it, but here goes...

 

Is it possible to do a PA with one scope on your mount, then change out the scope/rings combo for another scope/rings combo, and continue using the same PA?

 

Like I said, maybe it's a dumb question....



#2 ImNewHere

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Posted 19 October 2018 - 05:48 PM

Since your polar aligning the mount as long as changing the scope doesn't move the mount there is no problem. I'd still give ti a quick check anyway unless it is a permanent pier mounted one.


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#3 freestar8n

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Posted 19 October 2018 - 05:55 PM

The only thing that matters for polar alignment is how the polar axis is aimed at the sky.  As long as that doesn't change, the polar alignment is good.

 

Cone and other aspects of how the scope itself is attached to the mount will not matter.

 

You could point the other 'scope 20 degrees off in any direction and the system would still be polar aligned - but it might be hard to get it aligned to the sky so you have goto accuracy.  Goto accuracy will need awareness of cone and other aspects of how the 'scope is attached to the mount - but none of that affects polar alignment.

 

Frank


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#4 twidget

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Posted 19 October 2018 - 06:03 PM

Scotty and Frank,

 

Thanks you, sirs!!  Your replies pretty much confirmed my thoughts, even down to the goto accuracy if the scopes don't mount up the same. Guess it is worth a try.

 

(The situation which caused this question is that of a FOV with one setup that is marginal at best with the new Sharpcap PA routine....another setup has a good FOV for Sharpcap)

 

Thanks again !!!



#5 J A VOLK

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Posted 19 October 2018 - 06:11 PM

For a GOTO scope there may be difference in pointing accuracy. If you have a 3-star alignment capability that usually means your pointing software compensates for 'cone error', which is the misalignment between the scope optical axis and the RA axis. If you change scopes the cone error will be different, thus pointing accuracy will be affected. As far as basic tracking, there should be no effect unless the weight distribution causes some flexure in the mount.

#6 twidget

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Posted 19 October 2018 - 06:16 PM

For a GOTO scope there may be difference in pointing accuracy. If you have a 3-star alignment capability that usually means your pointing software compensates for 'cone error', which is the misalignment between the scope optical axis and the RA axis. If you change scopes the cone error will be different, thus pointing accuracy will be affected. As far as basic tracking, there should be no effect unless the weight distribution causes some flexure in the mount.

Would one be able to do another 3-star alignment with the 2nd scope on the mount after the PA?



#7 ImNewHere

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Posted 19 October 2018 - 06:33 PM

Would one be able to do another 3-star alignment with the 2nd scope on the mount after the PA?

Yes, but clear any old alignment data first if you can.


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#8 Stelios

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Posted 19 October 2018 - 06:35 PM

I am not sure where you are confused with Sharpcap. 

 

You can use a guidescope to polar align rather than the main scope for a larger FOV. But there's no "mounting/dismounting" of scopes necessary. 

 

The same guidescope used for PA can be used for star alignment (although frequently the alignment stars will be in the FOV of the main camera), and later on for guiding.

 

Actually I *do* dismount the guidescope, but that's because I have an OAG. I balance the scope without the guidescope, then put it on to use Sharpcap. Once I'm polar aligned, I take off the guidescope and go about my business (my mount doesn't require star alignment). 



#9 twidget

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Posted 19 October 2018 - 07:44 PM

I am not sure where you are confused with Sharpcap. 

 

You can use a guidescope to polar align rather than the main scope for a larger FOV. But there's no "mounting/dismounting" of scopes necessary. 

 

The same guidescope used for PA can be used for star alignment (although frequently the alignment stars will be in the FOV of the main camera), and later on for guiding.

 

Actually I *do* dismount the guidescope, but that's because I have an OAG. I balance the scope without the guidescope, then put it on to use Sharpcap. Once I'm polar aligned, I take off the guidescope and go about my business (my mount doesn't require star alignment). 

No guidescope on the rig in question, use OAG instead. But, that is another way to "skin the cat."  Thanks!!



#10 J A VOLK

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Posted 19 October 2018 - 08:37 PM

Yes, but clear any old alignment data first if you can.

Cone error can be shimmed out for each scope/ring/dovetail system, in which case one would not need to redo the 3 star alignment when changing scopes. There are some Youtube videos available on this.
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#11 twidget

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Posted 19 October 2018 - 11:08 PM

Cone error can be shimmed out for each scope/ring/dovetail system, in which case one would not need to redo the 3 star alignment when changing scopes. There are some Youtube videos available on this.

I'll have to give those vids a look-see in the morning. Shimming sounds like the way to go, when I decide to try this PA "approach." Thanks very much for the tip!


Edited by twidget, 19 October 2018 - 11:08 PM.



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