Poor tracking? Go North young man !

[Guest peepshow]

This is me thinking aloud again..........

 

I have not read any mention of how much easier it is, ( I think ?? ) , to track a DSO etc. near to the pole star, Polaris, compared with tracking a DSO near to the Celestial Equator.

 

Although both move through the same angle per hour/day/unit of time,  it seems to me that those near the pole move an 'effective' shorter distance per unit of time compared with those near the Celestial Equator.

 

So, any tracking errors will show up less, the nearer to the pole, the DSO is. 

 

Going to extremes, if a DSO was exactly on the NCP or within a few arc second of it, then no tracking would be needed at all !

 

I hope my little sketch makes this clear ?

 

 

 

So, if one is a beginner to DSO tracking and doesn't have the best equipment, (who does?.  ) is it better for them to, "Go North young man?."

 

Yay or Nay?

Edited September 18, 2013 by peepshow

[+Graham]

I would say Nay.

I can only speak for myself using a guided set up on an old but much modified EQ5.

The best tracking I get is between 40 and 70 degrees when the mount is 'loaded' against the weights and camera.

Once you get towards the zenith strange things begin to happen.

I develop serious problems when the scope is getting towards the vertical position as the backlash becomes a real PITA.

[+RonC]

Got to agee with Graham for the same reasons + the Meredion flip!!

[dawson]

I don't have enough imaging experience so i'd listen to graham and ron on that front, but i see your logic and can see an imaginary DSO next the NCP would appear to move more slowly as it would have apparently less distance to travel in the same amount of time as one on the celestial equator.

There is a lovely diagram in one of my imaging books showing something like this, with the maximum times one can image without a motorised mount (i think), but the younger of the two young tobias's has it.

James

[Perkil8r]

I'm with Graham.

 

As for not needing tracking directly on NCP, not true. The target would still rotate in the FOV unless of course it was a single point of light such as a star and an incredibly small FOV.

[Guest peepshow]

OK guys on all that, but things do move at a slower pace at the pole as my diagram shows.

 

Re backlash when near NCP.   Springs are a good way of dealing with backlash.

Anyone tried using those long elasticated luggage holders as along spring and load their

RA axis against the gearing when working at high DEC angles ?

 

Ironically, the old barn door tracker system is as good tracking near the NCP as it is tracking

 at the Celestial eq.     No gears, no worms, no backlash.   

[Perkil8r]

Angles per hour will not alter no matter where the object is, the effective distance across the sky will alter as you point out, so in effect you are correct as the perceived speed (distance over time) will appear to be slower near the pole, however as I said, it's the angles per hour that really matters. To be truely effective a barn door tracker would need to be polar aligned.

[Guest peepshow]

. To be truely effective a barn door tracker would need to be polar aligned.

Oh, I think that goes without saying.  It needs, as all scopes do, the very best polar alignment one can attain, by any means.

 

I'll throw something else into this ring........polar alignment is more important in the East/West direction than in the North/South direction from the NCP. 

Yay or Nay, guys? 

[Perkil8r]

Oh, I think that goes without saying.  It needs, as all scopes do, the very best polar alignment one can attain, by any means.

 

I'll throw something else into this ring........polar alignment is more important in the East/West direction than in the North/South direction from the NCP. 

Yay or Nay, guys? 

 

No, in theory exactly the same. If you were 90 degrees farther east from here your north/south would be my east/west...... sort of, almost, not easy to explain.

 

Say polaris were exactly in the 12 O'clock Position when perfectly aligned (for me sat here), at that exact same time it would be 90 degrees out showing at 3 O'clock (for you 90 degrees farther East) if that makes it easier to understand?

[+Graham]

Simply put Polar alignment is just that.

Alignment to a single star there is no east/west or North/south about it.

If you are off in any direction then you will not be Polar Aligned.

Edited September 19, 2013 by Graham

[Guest peepshow]

This is all very interesting but its difficult to explain what I meant. 

 

I believe we are at cross purposes, but I am not sure.    Nor why.

 

I was trying to  refer to a telescope's RA axis pointing being in error.  Pointing either higher or lower in the sky than the NCP. 

( that was my N/S reference) 

 

Or being in error  to the left or right pointing in the sky of the NCP. ( that was my E/W reference).   

 

Also assume that each error to be of the same magnitude.

 

So is the E/W error is the more serious when tracking,( again assuming that either of the errors to be the same for the sake of argument. )  

 

All these errors taken from one place at one telescope, BTW.

 

I am, of course, talking about tracking for imaging and not just for general viewing where  alignment is not so critical.

 

O dear, I bet I haven't explained myself again.........but don't know how to !    (maybe get two balloons, one rotating inside the other. One is the Celestial sphere and other the telescope's track whose axis is in error because it's not in line with the other balloons axis)

 

AW, let's forget it.    

[+Graham]

Richard.

For imaging both are as critical as each other.

If the RA is out (east /west) the target will drift off to the top or bottom of the frame.

If the DEC is out (north/south) the target will drift off to the left or right of frame.

Or vice versa depending on the orientation of your camera.

Of course that is where the guiding comes in but even so the better the PA the less your guiding has to do so better pictures.

I find when guiding with a PA error every time the guiding adjusts either the RA or DEC it will effect the other so setting up a continuous cycle of adjustment and counter adjustment.

Edited September 20, 2013 by Graham

[Guest peepshow]

Richard.

For imaging both are as critical as each other.

If the RA is out (east /west) the target will drift off to the top or bottom of the frame.

If the DEC is out (north/south) the target will drift off to the left or right of frame.

.

Yes, Graham,  agreed with all that. 

But as I imagine it, the N/S track error is in line with the star's track and so error is not as large as the

E/W track error where the scope's track is at an angle with the star's track and so is more marked.

 

One in line, the other at an angle ?  Leading to same error in each case?

I daresay you will say yes, but I am doubtful.

 

I think we have exhausted this thread........well, I feel exhausted.    Whew.

Edited September 20, 2013 by peepshow

[Guest Kheldar]

Does this help or confuse?

 

Whether the error is in ALT or in AZ - you can see that at 6h, 12h and 18h post starting the offset for either incorrect PA is the same (straight line distance) - but this is true of ALL points on the circle.

 

If these lines are equal, then the amount of drift in your image (RA or DEC) is also equal

 

Or I'm wrong and this is all crap

 

20130920134110929 by Kheldar UK, on Flickr

Edited September 20, 2013 by Kheldar

[Guest peepshow]

Well Stephen, your argument and drawing looks very convincing and thanks for it.

I think I will have nightmares over this, as I am still wondering about it.  

[Perkil8r]

No matter which direction your PA is out it is just as bad as any other direction.

[dawson]

Whilst hunting for something else, I came across this website, which may (or likely may not) contribute something to this thread:

 

http://calgary.rasc.ca/field_rotation.htm

 

James

[Guest peepshow]

It's very colourful, James, and thanks for link, but  field rotation is just making me giddy.
Seems as though even more problems to add to PA.