How Do Star Addresses Translate On An Equatorial Mount?

[bmcentee148]

I recently bought a reflector telescope with an equatorial mount and have done the research into how to balance, polar align, etc. I initially I thought I understood how declination worked with south of the celestial equator being deemed negative and those north deemed positive. But when a star is say at  + 70 degrees declination and I have my telescoped aimed at the celestial pole, I don't understand which way to turn the telescope. Left or right? either way I can have the telescope point at + 70 declination it appears to me. Then if I move to the star addresses R.A, it seems I would be looking at two entirely different sections of the sky with my choice. Any help would be greatly appreciated! 

[BeanerSA]

You polar align your mount using the latitude setting (~40 degrees in your case) and leave it there. On the bits of the mount that now can be unlocked and moved, are (ineffective) setting circles. The one that rotates around the polar alignment is the RA, and the one that would allow you to look behind the mount, is the Dec.

[bmcentee148]

Yes, I have watches both of these videos previously and like I said in the post, understand this information. My question pertains to how to theoretically aim at specific stars after polar aligning and calibrating the setting circles on the mount. If I am given the coordinate of a star that that is at + 60 degrees declination and my scope is now aimed at the celestial pole ( + 90 degrees declination) which way do I turn my scope for the CORRECT +60 degrees. There are a total of FOUR 60 degrees on the declination circle, two positive and two negative.

[BeanerSA]

Ok. The two positive 60° are actually pointing to the same line, one in the east direction, and one in the west direction. The two negative 60° are both pointing to the same line, one in the east direction and one in the west direction. Depending on which side of the meridian (east or west) your object is, will determine which way you need to point it.

Are you able to navigate your way around the night sky at all, because a lot of this will make a lot more sense if you can make some naked eye observations first. As the last video I linked states, the setting circles are a really poor way of finding objects. Your best way to locate objects will be starhopping with the finderscope.

Keep asking questions if I haven't made myself clear.

[bmcentee148]

Thanks a lot for your responses. I am currently learning to navigate my way around the night sky with the naked eye and understand that using the setting circles to actually locate objects but was more wondering how it should work in theory. 

For instance, if there are two positive 60 degrees one in the east and on the in the west then it seems to me two stars could have the same given "position" in declination and R.A. but be on opposite sides of the meridian. Then it truly isn't a way to uniquely identify positions of stars in the sky is it?

[BeanerSA]

We need to find a way to help you understand the Celestial Coordinate system, or Celestial Sphere.

All celestial objects rise in the east and rotate towards the west over a roughly 12 hour period. Whether we can see them or not depends on whether the sun is up at the same time. The objects that we can see when the sun isn't up, changes throughout the year.

Take a star that rises in the east just after sunset tonight. It's RA and Dec is fixed, but that line of RA travels across, from east to west, during the night. The stars RA and dec never change, but as earth rotates, the line of RA travels across the sky.

Try this video from the wonderful Phil Plait

[kens]

Thanks a lot for your responses. I am currently learning to navigate my way around the night sky with the naked eye and understand that using the setting circles to actually locate objects but was more wondering how it should work in theory. 

For instance, if there are two positive 60 degrees one in the east and on the in the west then it seems to me two stars could have the same given "position" in declination and R.A. but be on opposite sides of the meridian. Then it truly isn't a way to uniquely identify positions of stars in the sky is it?

No because the one in the west has a different RA to the one in the East. RA and Declination are analogous to terrestrial Longitude and Latitude.

[bmcentee148]

Then I am sorry but my question still stands. How does the celestial coordinate system tell me which positive 60 degrees to turn my telescope (left or right) to look in the direction of my target object of declination positive 60 degrees and an arbitrary Right Ascension?

[bmcentee148]

No because the one in the west has a different RA to the one in the East. RA and Declination are analogous to terrestrial Longitude and Latitude.

Then why can I lock my declination at any one of these two positive 60 degrees and rotate completely around in R.A.? It seems to me there would be stars in my field the entire time and I am rotating around 24 hours in two different points designated the same positive declination. In my head, two stars would then end up with the same declination and Right Ascension according to my dials. 

[Ruud]

It's a while since I've done this!

On a manual EQ mounted scope with setting circles you first have to make sure  that the telescope is polar aligned.

Then you point the telescope to a star with know coordinates (right ascension and declination). Once you find it you adjust the RA circle so that the indicator for that circle points at the RA of the star of with known coordinates.

Now you're ready to find your object with the circles. (Or a nearby bright star and from there you star hop to your target.)

[miguel87]

PLEASE READ!

Sorry for the capitals but the explanation is quite simple.

You need two coordinates to find a unique location in the sky.

Dec, you seem to understand this. But 90 is the only one that is a single point. Think about 0dec, a line around the whole sky. 60 is also like this.

Any point on this imaginary line will have a different RA coordinate depends how far around the circle you go.

Nobody using settig circles to find stars really. Settig curcles will not move on their own so unless you reset them every time you want to find something new.

90 percent of people with EQ mounts never look at the setting circles. Me included.

Hope it helps

[Ruud]

The big complication is that the sky 'turns around us'. Right ascension is fixed with respect to the stars but not to us. How far the sky has turned at any given moment is given by the hour angle.

To calibrate the RA circle for the hour angle you can adjust turn it.

[bmcentee148]

I appreciate all the answers but like... no one is actually answering my question. How do I know which 60 degrees on the the declination setting circle? There are two positive and two negative making a total of four choices. 

[bmcentee148]

And I understand that its not going to work correctly, I just want to know how it would be done in THEORY. 

[bmcentee148]

PLEASE READ!

Sorry for the capitals but the explanation is quite simple.

You need two coordinates to find a unique location in the sky.

Dec, you seem to understand this. But 90 is the only one that is a single point. Think about 0dec, a line around the whole sky. 60 is also like this.

Any point on this imaginary line will have a different RA coordinate depends how far around the circle you go.

Nobody using settig circles to find stars really. Settig curcles will not move on their own so unless you reset them every time you want to find something new.

90 percent of people with EQ mounts never look at the setting circles. Me included.

Hope it helps

R.A. has unique setting circles. Its like a clock that runs 24 hours, so each setting on my R.A. circle will be unique. My problem is the declination setting circles have two 90 degrees, one positive and one negative for north and south. It then has 4 different 60 degree lines 4 different 50 degree lines etc. How do I know which one of these declination settings to use when given star coordinate? R.A. will be easy since it is always unique but this is NOT THE CASE WITH DECLINATION!

[Ruud]

I appreciate all the answers but like... no one is actually answering my question. How do I know which 60 degrees on the the declination setting circle? There are two positive and two negative making a total of four choices. 

One of the axes has to point to the pole star. If you live at 60° North, that's at an angle of 60° with the horizon. See the first image here.

EDIT: If you lived at the North pole the same axis would have to point straight up.

[miguel87]

There are thousands of objects at 60 declination. So it cant be a unique position.

4 different 60 degrees. Well it wont be either of the southern ones else your telescope will be pointing at the floor. It doesnt matter which direction the scope points in when you sit it at 60 dec so you can use either northern hemishere 60's.

Once the scope is pointing 60 degrees up, you can whirl it around 360 degrees through all RA positions. So ANYTHING that is located at 60dec would be seen in this rotation. There cant be two different 60degree norths.

[miguel87]

Going from one of your 60 marks to the other 60 mark is exactly the same as setting it to ONE of those marks then rotating 180 degrees through RA only.

You would end up in the same position both times.

[Ruud]

60 degrees declination is thirty degrees from the pole star. Anything at 60 degrees declination sits on the same circle around the pole star and that whole circle is above the horizon.

[BeanerSA]

Going from one of your 60 marks to the other 60 mark is exactly the same as setting it to ONE of those marks then rotating 180 degrees through RA only.

You would end up in the same position both times.

That's a great answer.

[bmcentee148]

Going from one of your 60 marks to the other 60 mark is exactly the same as setting it to ONE of those marks then rotating 180 degrees through RA only.

You would end up in the same position both times.

Then in order to aim at a star given its declination and R.A. ,  which one of the 60 degree marks is the one that will allow me to turn to the GIVEN R.A. from the star atlas instead of having the rotate it 12 hours ( 180 degrees ) around? And thank you this is much more along the lines of what I was asking.

[bmcentee148]

Thank you all for your responses and putting me in the right direction!

[BeanerSA]

Theoretically, you would need to know which line of RA is on the meridian, and whether your object is east or west of it.

Practically, you look skywards until you see a star you can identify, and you can only swing the DEC one way and put it in view.

[bmcentee148]

Theoretically, you would need to know which line of RA is on the meridian, and whether your object is east or west of it.

Practically, you look skywards until you see a star you can identify, and you can only swing the DEC one way and put it in view.

Thank you for clearing that up

[kens]

Then in order to aim at a star given its declination and R.A. ,  which one of the 60 degree marks is the one that will allow me to turn to the GIVEN R.A. from the star atlas instead of having the rotate it 12 hours ( 180 degrees ) around? And thank you this is much more along the lines of what I was asking.

The simple way is to first move your scope to the right RA and then move in declination. The direction is then obvious. Make sure the scope is pointing above the pole first or you risk crashing it into the tripod.

BTW, if you want to use setting circles I've found it easier to convert RA to "Hour Angle" (HA) - you can configure Stellarium to show HA. Unlike RA, HA stays fixed relative to your mount but you do need to look up the HA of your target just before looking for it as its HA is continually changing. If you can't use Stellarium whilst observing there is a reasonably simple way to calculate it after one calibration.