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Centering adapter, theoretical problem

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#1 SpaceFreak131

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Posted 30 January 2013 - 08:16 AM

I was looking into getting a 2" to 1.25" precision centering adapter. Then I thought of a problem associated with it. Yes, the eyepiece fits into the adapter perfectly centered, but the 2" back fits into the focuser and is tightened off-center using a thumbscrew. Doesn't using a thumbscrew render the whole purpose of a centering adapter useless?

Are there any dual adapters? I.e., ones that have a 2" back that centers into the focuser, and a 1.25" eyepiece centering section?

#2 Lt 26

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Posted 30 January 2013 - 09:11 AM

Very good space freak. I got into this hobby about four years ago and like you found this design flaw right off. Why the smart folks making scopes don't do more to address this is a mystery.


I am only a garage engineer but if I had their machines and tools .I think a chimp could hammer out fix.All scopes and diagonals should be made with a centering device.

Dereck

#3 Fred1

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Posted 30 January 2013 - 10:05 AM

Howie Glatter already thought of it with his Parellizer. I own one and use it regularly but I've yet to test it with my 2" laser vs a buddy's 1.25" laser to see if both beams hit my center spot in the same place. I intend to do so at first opportunity. Glatter's tools are high precision instruments, so I have no doubt that I'm getting the proposed results. However, my curiosity has me intrigued.

#4 Eddgie

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Posted 30 January 2013 - 10:07 AM

Of course the question begs to be asked.. Why?

There is absolutly no beneift in improving the centering by a millimeter or two.

infinitly more important than centering the eyepeciece on the focal plane (which is completely unnecessary unless the scope has a hugely curved focal plane, with a far steeper curve than any telescope produced today) is ensuring alignment of the focuser tube to the optical axis for tilt. Even a millimter of tilt of the focuser tube will likely shift the centering of the eyepiece on the focal plane by more than the amount of the eyepeice not being perfectly centered in the holder, or the diagonal not being perfectly centered in the focuser barrel.

Tilt in the focuser tube would be much more serious than an eyepeice that is square to the focal plane but slightly off center.

Tilt in the focusr tube does real damage. An eyepecie off center a millimeter or two does no damage at all. Not a bit.

No offense intended, but frankly, this is a totally pointless theoretical problem.

I guess if there was some specific benefit to this, you could get an expanding self-centering dipoter from a binoviewer. I am willing to bet that Denkmeier would sell you one.

Of course mounting it to the focuser or diagonal with the precision required could in itself become something of an engineering problem, but since it is theoretical, this doesn't matter I guess.

But that would theoretically get you what you wanted.

But if you want to worry about something, worry about your focuser tube being tilted. It is a far more likley and far more serious condition.

#5 Eddgie

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Posted 30 January 2013 - 10:24 AM

Well, the focus of the "Parallizer" is not centering, but rather exactly what I was talking about in my previous post, makeing sure that things are parallel to the focal plane (no tilt in the focuser tube).

So, the Parallizer dosent worry about "centering" as much as tilt.

A direct qoute from the web page you referenced:

The bottom line is that centering is nice, but parallelism is golden.


And that was my going with my previous post. Centering is nice, but having an eyepeice slightly off center does not damage to the image.

Tilt is bad. The Parallizer is designed to ensure that there is not tilt in the system.

Cenetering is simply not something to be concerned about unless the decentering is quite severe.

#6 nevy

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Posted 30 January 2013 - 10:41 AM

Yes the centering adapter might offset slightly , but so does the collimater probly by the same amount , so I don't see it as a problem.

#7 Jon Isaacs

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Posted 30 January 2013 - 11:13 AM

Tilt is bad. The Parallizer is designed to ensure that there is not tilt in the system.



In my experience, tilt is only really a problem when collimating a Newtonian. The allowable tilt to the focuser axis is actually surprisingly large, about 3% of the mirror diameter, that's 3/10ths of an inch for a 10 inch. The important thing is to be using a collimator that is itself collimated and has flange that fits squarely against the drawtube. Howie makes those.

The primary tilt is critical but when one is adjusting the primary tilt that should be done with a Barlowed Laser (Tublug, Blug etc) rather than the return beam of the laser so again, the actual tilt in the focuser is not important.

Jon

#8 Eddgie

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Posted 30 January 2013 - 04:43 PM

well, in fact for visual only use, it (like eyepeice centering) tilt as you say is not at all a problem.

For imaging though tilt (if bad enough, and it doesn't have to be all that bad for todays wide CCD chips) will cause one side of the field (in the direction of the tilt) to be in focus while the other side is out.

Or, if the system is a flat system, if you focus on a star at the center, stars on two sides will be in focus (lets say "Top" and "Bottom" while stars on the "Left" and "Right" of the focal plane will be out of focus, one inside, and one outside.

Again, like eyepeice centring, for visual use tilt is usually not a problem, but it can do far more damage to have the focal plane tilted a couple of millimeters than having an eyepeice off center by double that distance.

#9 Don Taylor

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Posted 30 January 2013 - 09:20 PM

I must admit I can't understand using a centering adapter for the Collimation tool unless the eyepiece or camera is then mounted in exactly the same manner - also in the centering adapter. There are laser collimators with integral centering devices - which is a bit of a waste if the eyepiece or camera is then offset when installed by affixing to the focuser with set screws or normal compression ring.

Btw: the Baader clicklock claims to maintain centering - but I have not tested mine with an indicator.

#10 nevy

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Posted 31 January 2013 - 12:07 AM

I collimate with howie glatters 2" tublug & laser , I mostly use 2" eyepieces but when I use 1/14" plossls I use the centering adapter , so I'm reasonably confident that its still collimated with the plossls , as it confirms with the star test.

#11 Jon Isaacs

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Posted 31 January 2013 - 06:15 AM

I collimate with howie glatters 2" tublug & laser , I mostly use 2" eyepieces but when I use 1/14" plossls I use the centering adapter , so I'm reasonably confident that its still collimated with the plossls , as it confirms with the star test.


What does the star test look like without the centering adapter? Howie says the centering adapter is useful for actual collimation, for cameras and for use with binoviewers.

Jon

#12 nevy

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Posted 31 January 2013 - 12:37 PM

Without the adapter the plossls would fall through the focuser and end up smashing my primary. Ha ha only joking :-) ,
If you mean using a 2" - 1 1/4 " adapter instead of the centering adapter , then it would look slightly out ( doughnut not perfectly centred ) with the cheap adapter that came with the scope ( sky watcher) but it is still good if I use the one that came with my lightbridge , I suppose it would depend on the quality and how well it was made.
I don't collimate with the adapter as I got the 2" tools and I ALLWAYS make Sure I'm pushing down on the collimators or adapters or eyepieces before I tighten the set screw of the focuser to make sure they are seated flush on the focuser top.






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