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#26 Tony Flanders

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Posted 06 June 2020 - 01:22 PM

that’s the idea people have, but I’m not sure it’s possible to point again at Vega and NOT have the tube in the same orientation to the sky.  Think about it.  

Of course it's possible to point to the same object before and after moving the scope yet not have the same orientation. Think about it.

 

If the ground is dead level, then it is indeed the case that pointing to the same object before and after the move guarantees that the scope is oriented the same in all three space axes. Moreover, it means that you necessarily did not adjust the length of any of the legs.

 

But if the ground is not level, then you will probably need to adjust the length of the legs after moving the scope. And at that point, the fact that Vega is still in view only proves that the scope is oriented properly in one plane. It could be wildly off if you point it 90 degrees away from Vega.

 

You may have somewhat of a mental block in this regard, living as you do in Florida, which is more or less dead flat from one end to the other. In my backyard in the Apallachian Mountains, level ground is a very rare commodity.

 

In any case, it's fairly clumsy to fine-tune a scope's direction by tweaking the legs. In practice, it would probably be easier to re-align the scope after moving it.



#27 Andrekp

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Posted 06 June 2020 - 02:00 PM

Of course it's possible to point to the same object before and after moving the scope yet not have the same orientation. Think about it.

 

If the ground is dead level, then it is indeed the case that pointing to the same object before and after the move guarantees that the scope is oriented the same in all three space axes. Moreover, it means that you necessarily did not adjust the length of any of the legs.

 

But if the ground is not level, then you will probably need to adjust the length of the legs after moving the scope. And at that point, the fact that Vega is still in view only proves that the scope is oriented properly in one plane. It could be wildly off if you point it 90 degrees away from Vega.

 

You may have somewhat of a mental block in this regard, living as you do in Florida, which is more or less dead flat from one end to the other. In my backyard in the Apallachian Mountains, level ground is a very rare commodity.

 

In any case, it's fairly clumsy to fine-tune a scope's direction by tweaking the legs. In practice, it would probably be easier to re-align the scope after moving it.

 

I know what you are saying - I am just questioning whether or not it might be just an assumption.

 

If I align a GoTo alt-az scope, then I look at a star that is (made up numbers)  Alt: 60.0 degrees, Az: 136.5 degrees.  The star is in the center of my field and is the only star in the field at all.  I can say that from my position on Earth, Alt:60.0, Az:136.5 is that star's coordinates at this time.  BUT I can also say that that particular orientation of the tube and mount is unique to that particular star.  Only one star appears in that field with that orientation.  

 

If I carefully move the scope 1 inch to the left, but keep that star in the field, right in the center, the scope is still aligned.  NOTE: I am NOT changing the orientation of the scope and mount, only the position of the tripod.  The same orientation of the scope and mount, points to the same spot in the sky, uniquely populated by that star. The tripod leveling and orientation, in this example, hasn't really changed, since I'm on the same slab and moved only an inch.   Alignment hasn't changed.  I think we all agree on that.

 

And the star's relative orientation to you, Alt:60.0, Az:136.5, is the same from everywhere in your yard.

 

Why is that true?  Because your latitude and longitude on Earth have not changed by any amount that exceeds the error built into the mechanics of the scope and whatever formula the scope uses to find a star.  Anywhere in the yard, if you align the scope, then goto that star, at the same time, you will get the same numbers, fed to the same motors, and the scope will point to the same place, assuming you level and point the tripod it the same way.  Your yard is simply not big enough to even approach a different angle off from that star.  And if you use that star as a sort of level and orientation guide, then it is leveled and oriented the same way anywhere in your yard.

 

So I say, if you can move the scope 1 inch with no change, so long as you keep the scope orientation, then you can move the scope 50 feet the same way.  You are simply not going to be carrying that scope far enough away from your initial alignment spot to change your Earthly coordinates enough to have a different off angle to that star.

 

As you mention, and as I made clear (I thought), you can only manipulate the tripod legs and orientation - you MUST maintain the relative orientation of the scope and mount - but if you do it successfully, the scope will still be aligned.  Remember: that particular orientation, only points to one spot on the sky when aligned.  So if you move the scope, but reacquire that point in the sky, without changing the orientation, you are still aligned.  If you can drag a scope a few inches across the patio, keeping a star centered, you have kept alignment.  There is no reason why that becomes false with greater distance, until you start talking REAL distance.

 

I agree, that it's probably easiest to just realign in the new spot.  I've said this as well.  But it is incorrect, barring some actual explanation to the contrary, which I have never seen, to say that you cannot IN THEORY move a GoTo scope and maintain alignment.  You CAN, it's just too much trouble to be practical.


Edited by Andrekp, 06 June 2020 - 02:17 PM.


#28 Tony Flanders

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Posted 07 June 2020 - 04:16 AM

As you mention, and as I made clear (I thought), you can only manipulate the tripod legs and orientation - you MUST maintain the relative orientation of the scope and mount - but if you do it successfully, the scope will still be aligned.


Sure. That is also true, by the way, if you fly the scope to Australia. As long as the telescope is oriented the same way in space, it will stay aligned. Latitude and longitude alter what will be above the horizon and what will be below the horizon, but they do not affect the telescope's alignment. Obviously, however, if the scope is level where you started, it will be tilted with respect to the ground and with respect to gravity at its new location -- which may cause some distress to the motors and other mechanical components. In Australia it will be upside down, which will require some creative engineering.

Actually, the statement above isn't 100% true because of parallax. When observing a nearby object -- the Moon in particular -- its celestial coordinates will be different in the U.S. and Australia due to the shift in perspective.
 

If you can drag a scope a few inches across the patio, keeping a star centered, you have kept alignment.

 
No, that is true only if the patio's side-slope remains unchanged. If the new location on the patio has the same slope in the direction pointing to the star, but a different slope in the direction orthogonal to the star, then the telescope will no longer be aligned.

 

Put another way, maintaining the scope's orientation guarantees that the star will remain visible. But maintaining the star's visibility does not guarantee that the orientation will remain unchanged.



#29 Myk Rian

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Posted 07 June 2020 - 01:44 PM

I have 2 places I use my scopes, rear deck and front driveway, and have solar noon set compass points on both.

If I move from one to the other place, make sure the tripod is level, the same object is centered, and the polar scope is adjusted to Polaris, I don't see how it can possibly be thrown out of calibration.


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