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Atlas axis seems out of true, not concentric!

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#1 telfish

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Posted 13 December 2012 - 09:59 AM

I recently mounted my Atlas on a pier. On polar aligning I found the polar scope to be out as Polaris moved all over the place when the axis was rotated.

I can't get the head low enough to do an adjustment on the pier so I took the scope out and put it in a precision V block. I adjusted it perfectly central with no movement at all when rotated through 360 degrees.

Put the scope back in the mount expecting all to be fine. When I tried to polar align the issue was still there!

So I bit the bullet, took the mount off the pier, stuck it on a tripod and adjusted the polar scope in the mount.

It was impossible to get things exactly right. It took 3 hours to get it close the center cross still moves a quarter inch when the axis is rotated and there is nothing I can do to take the "wobble" out.

My question is. Is this normal? Second question will it effect tracking accuracy?

If so can I do anything about it.


Thanks

Terry

#2 rmollise

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Posted 13 December 2012 - 10:41 AM

Not sure what you are adjusting. Have you tried adjusting _the reticle_?

#3 Eddgie

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Posted 13 December 2012 - 10:46 AM

I don't think you will find a polar scope that will give the precision necessary to do long period tracking, or for even moderatly short period imageing.

A Polar Scope at best is just a quick way to get the scope into basic polar alignment. IT really isn't a substitute for doing drift alignment.

And if you mount the head on a pier and do a drift alignment, why would you care about the polar scope after that?

Just an example. If I align my CG5 using the polar scope, I get quite good pointing, but if I leave a target on a star of even a medium sized field and go in for 20 minutes, when I come out, the star will have drifted out of the field.

As to whether the condition is normal that your scope is not in alignment with your RA axis, I cannot say.

But even if it were, the scope will likely lack the precision to allow long period imageing or even visual observing.

If you need perfect tracking accuracy, my advice, especially if the scope is going to be mounted on a pier, is to ignore the polar scope and drift align. This will give you the best possible tracking.

#4 telfish

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Posted 13 December 2012 - 10:51 AM

Yes I have. But I can't get it to stay perfectly in line with an object no matter how many times I use the 3 screw to adjust it. The closest I can get it is a quarter inch of movement through 180 degrees.

I am just concerned that the RA axis of the scope is not concentric.

#5 telfish

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Posted 13 December 2012 - 10:53 AM

I think you may be right. I will try the routine in the new firmware to polar align to a better degree of accuracy then do a drift align.

As you say once I am decently aligned I can forget Polar alignment altogether unless we have an earthquake and the pier moves!

#6 rmollise

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Posted 13 December 2012 - 12:33 PM

I don't think you will find a polar scope that will give the precision necessary to do long period tracking, or for even moderatly short period imageing.


I'm not sure how long "moderately short period imaging" is, but the EQ-6 polar scope when used properly will easily allow 5-minute subs at the focal lengths most folks work with. ;)

#7 jrbarnett

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Posted 13 December 2012 - 01:16 PM

Are you imaging with the mount?

If "no" then the error you describe is largely irrelevant for visual use GOTO and tracking purposes.

If "yes", harder to say, though I would drift align instead of relying on a polar scope for polar alignment of a permanently pier mounted EQ head, if imaging was my goal.

Good luck,

Jim

#8 telfish

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Posted 13 December 2012 - 01:34 PM

I will be. I used to image with my C11 and hyperstar on the tripod and wanted a more permanent setup. I have been using the pier for a while but never thought I had a great polar alignment.

I will do a drift align and get it sorted.

#9 astro_baby

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Posted 13 December 2012 - 02:07 PM

Maybe my mount was made in Heaven then because my HEQ5 will keep a target in the EP of a 32mm Plossl for 40 minutes easily, I normally use the setting circles for a polar alignment.

I can be pretty sure of that because a year or so back the mount was left tracking M57....I wanderd off to have a look at someone elses kit, got nattering and came back about 40 minutes later, may have been close to an hour...Peeked in the EP and M57 was still in there about halfway to the edge of the field in a 32mm Plossl.

Normally mine will track a planet at high power for 20 minutes at a stretch with only the occasional manual bump on a bad night to keep the planet dead centre.

I am fussy with polar alignment, though not obsessive as I only observe and dont do imaging.

#10 gonzosc1

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Posted 13 December 2012 - 06:21 PM

I just got my atlas little more then a week ago. what you describe sounds like what I'm dealing with almost.
after perfect reticle alignment I discovered that my polar scope itself is not lined up with the RA axis.
when I rotate in RA while looking through the polar scope my whole FOV moves in an egg shaped oval pattern while at the same time the reticle stays in perfect alignment on target.
I chatted with Orion tech support for a while and we both agree that the polar scope itself is not in alignment with the RA axis. they said it can be adjusted with the four allen screws surrounding the polar scope.
they also suggested that I try to site a more distant target and see how much movement there is before making any adjustments. said adjusting mechcanical polar scope axis can be a PITA!

#11 telfish

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Posted 13 December 2012 - 10:11 PM

Thanks, that is what I thought was the issue, either the RA axis was not concentric or there was another adjustment on the polar scope to adjust for eccentricity in the mount.

There are no instructions regarding this adjustment in the manual!

#12 EFT

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Posted 14 December 2012 - 10:27 AM

I would say that this is fairly standard for these scopes, that's why the polar scope is generally only useful for rough polar alignment. If you think about the geometry of the situation, misalignment of the axes will have impacts on the gotos over the entire sky, but they will usually be fairly small. But if you expect to go from an object on one side to one other the other with a high power eyepiece, you will have problems. When it comes to tracking, the impact will also be fairly minor and result in a slow drift over time that should be easily guided out.

I can pretty much guarantee that if you check the position of the axis shafts on these and other mass produced mounts in reference to the housings and axis heads, you would find that they are rarely, if ever, true (I can't remember a the last CGE mount that I worked on that did not have at least one wobbling axis head). That's why unguided, long exposure AP is simply waste of time. The precision is simply not there.

#13 Cliff Hipsher

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Posted 14 December 2012 - 11:10 AM

I had similar issues with my Meade LXD-75.

To get you PAS squared away:

Rotate the mount to one side and get a target centered.
Rotate the mount to the other side.
Use the adjustment screws to take out HALF of the error.
Rotate the mount back to the other side.
Take out HALF of the error.
Rinse, Pepeat..

This is called BORE SIGHTING.

#14 John Carruthers

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Posted 15 December 2012 - 09:35 AM

with a distant target and patience you will get your polar scope centred. The key is to correct by 1/2 the error each time. Dion has done a quick tut here;
http://www.astronomy...php?f=19&t=4006
the thumbscrew mod saves hours of fiddling with a 0.5mm Allen key.

Alan has a 'how to' here;
http://www.awrtech.c....htm#POLARSCOPE






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