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shyam

Tracking POlaris

25 posts in this topic

hi

will i be able to track polaris star in my region ie 10.2244° N, 76.1978° E ..

 

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at 10N it will be 10 degrees above the horizon, so as long as you have a clear northern view, yes, but it will be low.

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And my region is 25.4198° N, 55.5092° E, can i or will i be able to?

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40 minutes ago, TareqPhoto said:

And my region is 25.4198° N, 55.5092° E, can i or will i be able to?

Yes, for you it will be higher at 25.4 degrees.....

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9 hours ago, Stu said:

Yes, for you it will be higher at 25.4 degrees.....

Ah ok, i hope i can see it at least, the only way i can find it is following the Big Dipper, funny how that Arcturus is high in the sky above my head while the Polaris is there to the North and not that much visible or not bright.

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Following the two pointer stars of the big dipper is the best way to find it!  Polaris can be difficult to spot if you have some light pollution. 

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26 minutes ago, TareqPhoto said:

Ah ok, i hope i can see it at least, the only way i can find it is following the Big Dipper, funny how that Arcturus is high in the sky above my head while the Polaris is there to the North and not that much visible or not bright.

This is from your latitude. As you say, Arcturus is basically at the zenith and shining brightly at mag 0.1. Polaris is much dimmer at mag 2 and also dimmed by the atmosphere because it is much lower. 

IMG_2026.PNG

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At 57° for me, its almost straight up, I just lay on the ground and scan the skies with binoculars, the telescope is  harder to use as such elevations.

Polaris is  almost the centre of my viewing area, literally, so take  Ursa Major and Cassiopeia as my boundaries, as they rotate about, scribing  the limits for local observations. 
For anything else, I need to be away from the house. 

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I love the idea of tracking polaris. It doesn't seem to wander out of my eyepiece at all and I have an AZ mount. Would it be true to say that the further from polaris an object is, the faster it moves across your eyepiece? 

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2 hours ago, domstar said:

I love the idea of tracking polaris. It doesn't seem to wander out of my eyepiece at all and I have an AZ mount. Would it be true to say that the further from polaris an object is, the faster it moves across your eyepiece? 

Have you ever seen a phonograph-turntable spin a record-album? It gives the impression that the closer you get to the center where the spindle goes through the record-album, the faster it seems to be spinning. And the opposite is true for the edges of the record - they seem to be revolving around the spindle much more slowly than the middle of the same record.

While you know that, logically speaking, they are moving at the same speed - it certainly doesn't look that way. Now apply this to your question.

Have fun -

Dave

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Posted (edited)

........I sense the opposite, as the outer edge is travelling further/faster, with its larger circumference, the speck of dust on the outer edge is hanging on for dear life? 

Edited by Charic
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A larger circumference travelled in the same time means a faster speed. Is there a reason why this doesn't work with stars? I know they aren't really rotating around polaris. Just out of interest, does anyone notice stars far away from polaris moving across the eyepiece at a faster rate? Am I going mad?

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Posted (edited)

29 minutes ago, domstar said:

Am I going mad?

No!
With just enough field of view, looking/locked onto Polaris, most if not all the targets remain within my field of view, they just rotate slowly, but further from, they do race across the field of view, at an alarming rate depending on the field of view and magnification. Also dependent from where you view from does it not? This far North I'm on a sedately trot, whereby those folk on the equator are shifting along at just over 1000mph, Earths rotational speed measured from the equator, the sole reason for images drifting through the viewfinder, eyepiece, on non-tracking telescopes.

Edited by Charic
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So does Sirius move faster across the eyepiece than say Mizar? Logic says it must do but would anyone notice? Would Mizar move across the eyepiece at the same speed at the equator? my brain says yes but I learnt to stop trusting it a while ago.

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Posted (edited)

Put the record-album down on the phonograph's turntable and let it begin spinning at, say, 33 1/3rd RPM.

First off -you've drawn a straight-line, with a black-coloured Sharpie® marker, from the center-hole to the edge. Now watch the black-line go around on the turntable.....

Do you see how they really are moving at the same rate? The black-line shows they're both revolving at an equal rate-of-speed. But you still see the center moving faster than the edges.

This is a matter of 'optical-illusions.' Or, perhaps, A. Einstein's "Special Theory of Relativity." When Einstein was asked where he'd like to sit on a train in order to see the best view from the train, he stated: "I'd wish to sit on the roof of the train!"

<Suffer! :p  :eek:>

Weaving Baskets is Fun!

Dave

 

einstein-bicycle.jpg.e13b0123441d67869bab618b8c20c99a.jpg

 

 

Edited by Dave In Vermont

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37 minutes ago, Dave In Vermont said:

First off -you've drawn a straight-line, with a black-coloured Sharpie® marker, from the center-hole to the edge. Now watch the black-line go around on the turntable.....

my brain is saying that if you place a dot, on the line near the centre and one near the edge, and measure them, they will pass the measuring device at the same time, but the one on the edge has had much further to travel than the one near the centre, this it must be moving faster... ?

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Posted (edited)

1 hour ago, Dave In Vermont said:

Do you see how they really are moving at the same rate? The black-line shows they're both revolving at an equal rate-of-speed. But you still see the center moving faster than the edges.

No, I 'sense' the outer moving faster due to the greater linear speed, even though their moving at the same constant speed.

Edit, I started to type, then had to answer the door bell,  hence the  similar answer as rockystar

Edited by Charic
typo

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14 hours ago, domstar said:

I love the idea of tracking polaris. It doesn't seem to wander out of my eyepiece at all and I have an AZ mount. Would it be true to say that the further from polaris an object is, the faster it moves across your eyepiece? 

I think this is essentially correct. You can point an un-driven telescope with a 3 degree field and Polaris will stay in it all night. Try the same with a star on the celestial equator and it will soon move out of the field. However, as you approach the south celestial pole the drift decreases again.

This is why when testing the seeing at sites Polaris is chosen as the target, at least in the northern hemisphere.

I think some of the discussion above does not make the difference between angular velocity (which is constant for a record) and the linear velocity (or speed) at points along the radius of the record which increases from the center to the periphery.

Some confusion may be due that a tracking mount finds it difficult (or impossible) to track at it's main axis (pole for an equatorial mount and zenith foe an Alt Az) as small errors require large movements in the mount.

Regards Andrew

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Posted (edited)

56 minutes ago, Dave In Vermont said:

Put the record-album down on the phonograph's turntable and let it begin spinning at, say, 33 1/3rd RPM.

First off -you've drawn a straight-line, with a black-coloured Sharpie® marker, from the center-hole to the edge. Now watch the black-line go around on the turntable.....

Do you see how they really are moving at the same rate? The black-line shows they're both revolving at an equal rate-of-speed. But you still see the center moving faster than the edges.

 

You're confusing angular velocity with linear velocity. A point near the centre moves at the same angular velocity (radians/second) as a point on the edge but the point on the edge has to move at a higher linear velocity (m/s) as it's travelling a greater distance in the same time. It's the linear velocity that determines how fast the star 'moves' across the eyepiece.

Edited by symmetal
Added extra line
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4 minutes ago, symmetal said:

You're confusing angular velocity with linear velocity. A point near the centre moves at the same angular velocity (radians/second) as a point on the edge but the point on the edge has to move at a higher linear velocity as it's travelling a greater distance (m/s) in the same time. It's the linear velocity that determines how fast the star 'moves' across the eyepiece.

Hit the nail on the head there Re: angular vs linear velocity.

But... Knowing that the star is moving across the sky at a higher linear velocity at the celestial equator (compared to near the pole), how is it that an RA driven mount can track a star at any point in the sky with only one gear speed?

Hurts my head thinking about this! 

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Posted (edited)

19 minutes ago, CraigT82 said:

Hit the nail on the head there Re: angular vs linear velocity.

But... Knowing that the star is moving across the sky at a higher linear velocity at the celestial equator (compared to near the pole), how is it that an RA driven mount can track a star at any point in the sky with only one gear speed?

Hurts my head thinking about this! 

The RA driven mount when polar aligned rotates at a constant angular velocity and all the stars is the sky also rotate at the same angular velocity so tracking occurs no matter where the scope is pointing. However pointing near the celestial equator, the front of your telescope is moving at a higher linear velocity than it would be if it were pointing near to polaris, as the front of the telescope is further away from the axis of rotation. Tracking is only concerned with angular velocity for an eq mount.

Edited by symmetal
Added a couple of words
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Posted (edited)

<Pop-Corn!> <Peanuts!> <C'mon an' get your Tuttie-Frooty!> <Pop-Cor....................

(Glad I brought my beach-chair & some pop-corn!)

Einstein also was asked to explain his "Relativity Theory" in simple terms. Said he:

"Imagine you are sitting for 1 minute on a red-hot stove, and a comfortable sofa next to a beautiful girl. Which 1 minute seems to be longer in time?"

 

<Pop-Corn!> <Peanuts!> <C'mon an' get your Tuttie-Frooty!> <Pop-Cor....................

Evil Dave Mode,

Dave

Edited by Dave In Vermont
sp.

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7 minutes ago, Dave In Vermont said:

<Pop-Corn!> <Peanuts!> <C'mon an' get your Tuttie-Frooty!> <Pop-Cor....................

(Glad I brought my beach-chair & some pop-corn!)

Einstein also was asked to explain his "Relativity Theory" in simple terms. Said he:

"Image you are sitting for 1 minute on a red-hot stove, and a comfortable sofa next to a beautiful girl. Which 1 minute seems to be longer in time?"

 

<Pop-Corn!> <Peanuts!> <C'mon an' get your Tuttie-Frooty!> <Pop-Cor....................

Evil Dave Mode,

Dave

I think you need a lie down and a short sleep.

Regards Andrew

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Nah - just dealing with 2 forums at once DOES take a toll. But thanks for the idea!

By the way, I stole the "pop-corn" routine from a Marx Brothers movie from the 1930's. I knew it would come in handy one day! :p

'Ta,

Dave

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Posted (edited)

You can replicate the real-life situation on a PC planetarium such as the free Stellarium. Set the celestial north pole as the centre of the view and then advance the time in jumps of an hour or so. You'll see that Polaris moves very little, describing a tiny circle, while stars further south move a greater distance around your screen.

In fact a star trail image will show you in a simple static picture what is happening over time. Here's a particularly nice one taken by Maurice Toet at my place. (Who put that light on??? :icon_mrgreen:) Maurice had the nice idea of locating Polaris so that it was close to the apex of the roof.

http://dutchdeepsky.com/index.html

Note that the stars all make arcs describing the same number of degrees but that their trails made during the exposure get longer at greater distances from the pole.

Olly

Edit, I can't get the link to go straight to the image so follow Digital Photography - Nightscapes - and it's the image with house and star trails.

 

Edited by ollypenrice
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