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Parallax


Guest dawson

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Guest dawson

Does the goto system on my mount take into account any parallax effect, when the relative positions of the stars change by very small amounts as the earth circles the sun?

James

Edited by dawson
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Guest Tibbz2

From what I have found the parallax effect is 1 arcsecond on an object 1 parsec away (3.26 ly), and seeing as the nearest star is more than that, I highly doubt that it is something goto systems need to take into account because its so small! (this is annual parallax, which i'm guessing is the most the star/object moves over a full orbit or something)


 


You can also find out the parallax of a star if that's your kind of thing with:  distance (parsecs) = 1 / parallax (arcsecs)


 


And a copy and paste for you:


 



The angles involved in these calculations are very small and thus difficult to measure. The nearest star to the Sun (and thus the star with the largest parallax), Proxima Centauri, has a parallax of 0.7687 ± 0.0003 arcsec. This angle is approximately that subtended by an object 2 centimeters in diameter located 5.3 kilometers away.



Edited by Tibbz2
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Guest Tibbz2

0.14 arcsecs is pretty accurate, more accurate than I imagined, however if the annual parallax of the closest star is 0.7 arcsecs (and that's down south anyway) then it will probably only be noticeable on very very few stars...


 


I don't know anything about mounts but I doubted such a tiny tiny matter would be implemented in 'amateur' kits...


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Guest Tibbz2

Oh right lol!


 


I can't find anything on it, but I guess it's the smallest rotations the mount can make or be measured or something?


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Guest dawson

That would make sense, the small movement it can make.


 


I would be interested to know what the accuracy of the GOTO computer is. Does it GOTO an object with arc second accuracy, or just arc minute accuracy?


 


JD

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Guest Tibbz2

I would think it is possible some go to arcsecond accuracy, seeing as the resolution is smaller than an arcsecond by quite a bit


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Guest peepshow

I always find it very difficult to get my head around these tiny angles and vast distances.


So I scale them down to size !


It's about 186 million miles across on our journey around the Sun.  That is about


15 light minutes. Take Proxima, as an example,  which  is 4.25 light years away


 which is approx 2 million light minutes.


 


So the relationship between Proxima distance and our orbit size is a ratio of 144,000 to 1


 


Scaling that  right down that is an angle of 1 inch base and 2 .5 miles long. 


Quite a tiny angle but I find it now easier to visualize.


 


I hope my very rough calcs are right. :)


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Guest dawson

No that is useful, thank you.

So if i was looking at something 2.5 miles away theough my scope, the position of the star at the two extremes would be one inch?

I doubt i could differentiate a one inch gap at 2.5 miles by eye or if imaging. But i'd be interested to know what spatial differentiation (in inches) i could discern at 2.5 miles. A foot? A metre?

That still doesn't answer the question though whether the GOTO takes parallax into account or not, which if it did would make the equations much more complicated presumabely.

James

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Guest peepshow

No, the naked eye can only make out an inch resolution at roughly 100 odd feet......well my ol' eyes


can just  about do that, and that's with glasses on :)


 


It's amazing, isn't it, that the central part of the eye which resolves all fine detail is such


a tiny, tine area on the retina.  No more than a few thou. 


 


Now I'm getting off topic.


 


Sorry,can't help on the GOTO.  


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Guest dawson

No, i meant looking through the telescope, not with naked eye, sorry. I can barely make put what is tge other side of the room qothout my glasses on!!!!! :)

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Guest Tibbz2

I can't read 'normal' sized text like in books from more than a foot away!


Edited by Tibbz2
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Guest peepshow

So if i was looking at something 2.5 miles away theough my scope, the position of the star at the two extremes would be one inch?

 

 

No I don't think so. Not 1 inch in a scope. 

 

 It was the angular change looking at Proxima over a six month period (when we are on the other

side of the Sun) , that is the same as the angle subtended by either side of a 1 inch object at 2.5 miles away.

 

It was just a scaled down example to try and get one's head around these tiny angular changes.

The only way I can get my head around the Universe is to bring it down to size so that I can visualize it.

 So we had two similar triangles but vastly different in size ! 

 

Try it out on other distances etc.  Make the Earth the size of a pin head, for example, and co relate that reduction to the planets and stars etc. 

I would give some more examples but it's off topic and it's too hot to work :)  :)  

Edited by peepshow
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Guest dawson

Sorry, again i think my poor typing has led to confusion.

If two pencils were stood upright, one inch apart, and then viewed theough a telescope based 2.5 miles away; is the movement of the star, due to parallax, effectively jumping from the tip of pencil one to the tip of pencil two, as biewed in said telescope 2.5 miles away, which i suspect would be indisguishly different in the scope. I don't think i'd even be able to see a pencil in my scope at 2.5 miles, let alone differenate one pencil from another when they were just one inch away from each other.

James

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If two pencils were stood upright, one inch apart, and then viewed theough a telescope based 2.5 miles away; is the movement of the star, due to parallax, effectively jumping from the tip of pencil one to the tip of pencil two, as biewed in said telescope 2.5 miles away, which i suspect would be indisguishly different in the scope.

James

 

Hi James,

Your above statement is correct.

I just ran through Richards calculations with my rum and coke (hic!), and can confirm his numbers are as near as makes no difference.

Most larger amateur scopes will nearly, but not quite resolve two stars at this tiny separation, approx. 0.7 arcsecs. This parallax angle is for the nearest star, and other stars are much further away, so if you want to examine the parallax on Deneb for instance, over 2000 light years away, you would need to resolve down to about 0.002 arcsecs, and be able to set up and maintain your equipment with such accuracy for a period of 6 months! So the parallax is going to make no difference to the positioning of a goto mount. Any polar alignment will not give anything like a 1 arcsec accuracy.

 

I also use Richards methods to scale down and understand the sizes of things in the universe, it works really well if you enjoy doing the calcs.

 

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Guest dawson

I see.

I clearly need to take up rum, whiskey a d other shorts; will it make me good at these sums like you two :)

Thanks.

James

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It might, provided you can maintain a mental calculation of exactly how many units you have consumed, and can accurately establish the point at which your blood alcohol level will cause a calculation overflow error in your brain making further calculations totally invalid :wacko: .


 


Practice, practice, practice :) .


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