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Why Wedge is set to local latitude

Polar Alignment
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14 replies to this topic

#1 burtba

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Posted 20 July 2021 - 07:13 AM

Hi All. I'd REALLY appreciate some guidance here. I'm really struggling to understand what I think is a very basic concept. Thats is why we set our wedge to the same latitude of our location. In this specific case as per the Celestron picture the fork of the mount is suppose to point to your celestial pole. In my picture, Adelaide is at about 35deg(south), then if I tilt my wedge another 35deg toward the South, this to me seems like it is now pointing 70deg, where is should be 90deg due South. Obviously my thinking is wrong. Can someone please let me know where my thinking is going wrong. Thanks in advance

B0243F79-B03C-42D6-8317-2C79A58A3BDB_1_102_o.jpeg


Edited by burtba, 20 July 2021 - 07:33 AM.


#2 kel123

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Posted 20 July 2021 - 07:21 AM

I think it will help for you to upload the pictures you are referencing, so that we know exactly what is causing the confusion.

Meanwhile, you set the wedge at your latitude, so that it will have the same elevation as the north pole and have your mount at the same axis of rotation of the earth, allowing objects to remain in the view of your telescope.
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#3 Jim Davis

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Posted 20 July 2021 - 07:29 AM

If you are on the equator, the celestial north and south pole would appear to be on the horizon.

 

As you move away from the equator, the celestial pole will appear one degree higher for each degree you move north or south. So, if you are 35 degrees south, it will appear 35 degrees above the horizon.

 

Untitled.jpg


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#4 astrokeith

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Posted 20 July 2021 - 07:35 AM

You are right. You may be having a problem with your starting reference point and the direction of the adjustment scale.

 

So in effect the base of your 'unmounted' scope should be at (90-latitude). So at the pole it is horizontal and the forks are pointing straight up.


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#5 Mike K

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Posted 20 July 2021 - 07:47 AM

In your picture, if you set the wedge angle to 0 degrees the forks would be pointing straight up.  I suspect that in fact setting the wedge to 0 degrees would point the forks horizontally.


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#6 Dynan

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Posted 20 July 2021 - 08:02 AM

Face north behind your scope and point the base of your wedge directly  (perpendicular) at the NCP (or SCP). This will be an angle equal to your latitude.


Edited by Dynan, 20 July 2021 - 08:29 AM.

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#7 gnowellsct

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Posted 20 July 2021 - 08:03 AM

You are trying to make your telescope spin exactly opposite to the way the world spins so that the telescope stays stationary relative to the Stars. Your telescope appears to be moving but that's only because you think you are stationary when in fact you are spinning along with the world.

The axis of your telescope Mount and the axis of the world are parallel when your telescope declination is the same as your latitude.

You're not going to break anything by not being Polar aligned. You can run these experiments. It is a lesson in applied geometry.

The accuracy of your tracking will gradually improve the more accurately you are polar aligned.

It took me some years before I realized that although it seems a universal principle that stars and other things rise in the east and set in the West, that when you get around the North Pole things behave differently. The Stars which are above the North Pole move east to west. The Stars under the North Pole move west to east. They never sent and are called circumpolar. At the North Pole All Stars are circumpolar including the Sun. The only alteration of the light comes from the Earth's movement around the sun and the tilted axis. So you get 6 months of light when you're pointed at the Sun and 6 months of darkness when you're pointed away.

As you move away from the pole the amount of sky which is circumpolar gets smaller. And you get more regular night and day alterations.

There is something to be gained by watching this stuff without a telescope. You can watch Ursa Minor turn completely around the sky (but not in one night). It is rotating around celestial North. It is a giant clock that moves at 15° an hour.

And if you puzzle that out you will begin to realize that if you set your telescope Mount too low it will not exactly counteract the rotation of the Earth and it will compromise your tracking. And that if you set your telescope too far to the right or left the same thing will happen. And if you set your telescope Mount too high you also will not get good tracking.

The place that is just right is when you are polar aligned.

The advantage of polar alignment, or at least one of them, is that it allows completely stupid mechanical devices to provide excellent sidereal tracking. You can set your fork up in alt azimuth configuration where it moves up and down and swivels around exactly like your head on your neck. These days there are computerized controls which can make that work for tracking. The controls work by using two motors on two axes in order to keep an accurate track. But when you pull her a line one motor will do the trick.
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#8 JohnnyBGood

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Posted 20 July 2021 - 08:14 AM

If I'm understanding your question and your drawing correctly, I think the think what is confusing you is where you're measuring the 35 degrees from. In the drawing it looks like you're measuring a 35-degree angle between the base and wedge. That angle will actually be 55 degrees for you.

 

The "35 degrees" on the lattitude scale measures the other direction. Facing south with the scope parallel to the ground (i.e. the wedge flopped all the way over so that the scale reads "0 degrees") you raise the scope 35 degrees to have it pointed at the South Celestial Pole. If you look at the scale on the wedge you'll see that it runs backwards: the more you "open it up" the smaller the angle reading becomes. So when the wedge is set at "35 degrees" and the angle it makes with the base is 55 degrees then the scope is parallel to the earth's N-S axis. It took me a while to wrap my head around that.

 

Jim Davis's modification to your drawing above shows this correctly though he didn't spell it out in detail.


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#9 michael8554

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Posted 20 July 2021 - 08:31 AM

On my wedge, if the angle between the ground and the wedge were 90 degrees, the scale would read 0 degrees.

 

So when your wedge is set to 35 degrees on the scale and the forks are pointing at the SCP, it is actually tilted 55 degrees

 

For your convenience the scale on the wedge is "backwards" so you can set it to your Lat, instead of 90 - Lat

 

Wedge.jpg


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#10 michael8554

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Posted 20 July 2021 - 08:32 AM

On my wedge, if the angle between the ground and the wedge, the Tilt", were 90 degrees, the scale would read 0 degrees.

 

So when your wedge is set to 35 degrees on the scale and the forks are pointing at the SCP, it is actually tilted 55 degrees

 

For your convenience the scale on the wedge is "backwards" so you can set it to your Lat, instead of 90 - Lat

 

Wedge.jpg


Edited by michael8554, 20 July 2021 - 08:32 AM.

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#11 burtba

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Posted 20 July 2021 - 09:16 AM

If I'm understanding your question and your drawing correctly, I think the think what is confusing you is where you're measuring the 35 degrees from. In the drawing it looks like you're measuring a 35-degree angle between the base and wedge. That angle will actually be 55 degrees for you.

 

The "35 degrees" on the lattitude scale measures the other direction. Facing south with the scope parallel to the ground (i.e. the wedge flopped all the way over so that the scale reads "0 degrees") you raise the scope 35 degrees to have it pointed at the South Celestial Pole. If you look at the scale on the wedge you'll see that it runs backwards: the more you "open it up" the smaller the angle reading becomes. So when the wedge is set at "35 degrees" and the angle it makes with the base is 55 degrees then the scope is parallel to the earth's N-S axis. It took me a while to wrap my head around that.

 

Jim Davis's modification to your drawing above shows this correctly though he didn't spell it out in detail.

Thanks so much, that makes PERFECT sense. So am I right then in understanding the scale on my wedge has been screwed on backwards? Please see photo's attached?

Attached Thumbnails

  • Wedge.jpeg
  • Scale.jpeg

Edited by burtba, 20 July 2021 - 09:27 AM.


#12 michael8554

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Posted 20 July 2021 - 03:43 PM

"So am I right then in understanding the scale on my wedge has been screwed on backwards?"

 

Yes !

 

Think of this another way.

 

If you were at the Equator, Lat = 0, the tilted mounting plate should be at 90 degrees to the base.

 

And your scale should read Lat = 0,  but would actually read Lat = 90.

 

Reminds me of my first scope, a Russian TAL-1.

 

Many frustrating nights until I discovered the RA setting circle was for the southern hemisphere.

 

And unlike your scale, taking the circle off and turning it over didn't work, it was still incremented backwards.........


Edited by michael8554, 20 July 2021 - 03:43 PM.

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#13 burtba

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Posted 20 July 2021 - 08:53 PM

Thanks all for your responses, I have an accurate understanding now. I must admit though, bought this wedge new and the manual says set the wedge scale to your local latitude(as does much online) is all fine if they had screwed the scale on according to that. So I have reversed my scale now so I now set my scale to 35 degrees and it is correct. 

BUT

As I dont like to just do things because it says so, I want to understand why, hence my question and this didn't make sense to me. There should be far more descriptions and articles that say, 1/ The angle is the from your tilted plate towards the celestial pole, NOT the angle the tilted plate is up from flat and 2/ The actual angle is 90-Latitude.....


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#14 Ulmer Spatz

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Posted 21 July 2021 - 07:09 AM

There should be far more descriptions and articles that say, 1/ The angle is the from your tilted plate towards the celestial pole, NOT the angle the tilted plate is up from flat....

I'm not sure what you're saying here. You're not confusing the observer's zenith with the celestial pole, are you?



#15 DSOGabe

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Posted 21 July 2021 - 11:16 AM

You need for the base of your scope to be on the same plane as your latitude so that its base will be parallel to the celestial poles. That way as the motor rotates in RA, the scope can track accurately with little to no drift- depending on how accurately you do to your alignment. 

 

Since you are at 35S, if the wedge is set to 35 degrees and the top plate of the wedge is angled towards the southern celestial pole, you will be lined up to the southern celestial pole. You will see this when you rotate the scope in declination so that the tube is parallel with the fork arms- it should be pointing straight to the SCP. You are at 35 degrees; don't think about 35+35=70. That is not the case; think instead of 35=35 - you are at 35S and the wedge is also at 35S. 


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