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Question about optical center

 
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  #1  
Old 10-20-2012, 02:20 PM
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Join Date: Dec 2009
Location: Buckley, WA
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Question about optical center

Can someone please bring me up to speed here... I have a Leupold 4.5X14 LR BC.
I've been concerned lately that there may be something wrong with it. It seems like every time I go to the range, I need to make a windage adjustment from left or right (nothing consistent). Once I redial the center it seems to shoot fine during that session, until the next trip out, when I go through the same exercise to center it again.
I contacted Leupold's tech line. They suggested that I mechanically center the scope, then check the optical center, "these should be fairly close" according to the tech.
What I found was they were not even close. After optically centering the scope I bore sighted using a Leupold bore sighter, have not been to the range since.
My question / question's...

1. Should the mech. center & optical center be very close?
2. Is it critical that they are? With a "multiple aim point" (Bone & Crocket) reticle, where once its zero'd no adjustments are necessary, how critical would a vast variance between mech. zero & optical zero be?
3. Which would be the more important one to set the scope to, mech. cntr or optic. cntr.?
4. Finally, would having a drastic difference between the two be a clue of internal issues that would warrant sending it to the manufacture for repair?
I know this is a lot to consume but if anyone could school me I would appreciate it.
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  #2  
Old 10-20-2012, 02:30 PM
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Re: Question about optical center

If your mount is straight on the receiver, and barrel straight... then yes, the scope's mechanical center should be pretty close to your windage zero.

If you have to dial a ton of windage into the scope to get zero'd, something is off.

Do you have a Leupold (Redfield) type rear base, with the windage screws? If so, you need to set the scope to mechanical center (total number of windage clicks from one end to the other, then half way back)... then use the windage adjustable rear base to get your rough windage.

Some folks with the Redfield type windage adjustable rear bases just mount the ring, and tighten both screws... and commence to dialing a ton of windage into the scope to get zeroed, if even possible.

If you are dialed way over to one side or the other, not only will your "headroom" for elevation be affected, but your windage zero *might* be a little shifty.

Dan
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  #3  
Old 10-20-2012, 10:16 PM
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Re: Question about optical center

Something is moving, could be the stock if it is wood.
db
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  #4  
Old 10-21-2012, 01:57 PM
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Re: Question about optical center

Quote:
Originally Posted by dbhostler View Post
Something is moving, could be the stock if it is wood.
db
That was my thought as well. I would not be surprised if the rifle is moving in the bedding or it is a technique issue.
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  #5  
Old 10-21-2012, 08:22 PM
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Re: Question about optical center

Quote:
Originally Posted by Dr. Vette View Post
That was my thought as well. I would not be surprised if the rifle is moving in the bedding or it is a technique issue.
Well... went to the range today, zero'd in @ 200 yrd a nice dime sized group. Torqued the ring screws. Doubled checked the optical center after everything was game tight, the windage hair was lined up perfectly, the elevation line (reflection) was probably about 6-8 MOA above the actual cross hair, which I think would be about right if zero'd @ 200.
I have deduced that on a BDC scope, mechanical center (IMO) is not critical. The test will be if the damn thing is still on point next weekend.
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  #6  
Old 10-21-2012, 10:17 PM
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Quote:
Originally Posted by Rickyrebar View Post
...I contacted Leupold's tech line. They suggested that I mechanically center the scope, then check the optical center, "these should be fairly close" according to the tech.
What I found was they were not even close. After optically centering the scope I bore sighted using a Leupold bore sighter, have not been to the range since.
My question / question's...

1. Should the mech. center & optical center be very close?
2. Is it critical that they are? With a "multiple aim point" (Bone & Crocket) reticle, where once its zero'd no adjustments are necessary, how critical would a vast variance between mech. zero & optical zero be?
3. Which would be the more important one to set the scope to, mech. cntr or optic. cntr.?
4. Finally, would having a drastic difference between the two be a clue of internal issues that would warrant sending it to the manufacture for repair?
I know this is a lot to consume but if anyone could school me I would appreciate it.
I think the advice you got from Leupold is odd. You indicated that you have a zero that drifts over time. It seems unlikely the problem is the scope, so why investigate the mechanical center? Unless that model has a history of related problems, it doesn't seem germane.

To answer your questions, the optical and mechanical centers are usually within 5 MOA of each other both vertically and horizontally.

The only exception is when a SFP reticle is vertically offset in the erector tube to increase the elevation range. Then it can be 5-10 MOA away from the vertical mechanical center. When the reticle is correctly offset there should be more bullet up adjustment range than bullet down. If otherwise, then the scope has a problem.

By "optical center", I assume you mean the reticle position when centered with a mirror against the objective.

Which one you should use depends on what you're trying to do. When aligning a scope to the rifle bore (during scope installation), I use the optical center. After the scope is installed, the only thing that really matters is mechanical adjustment range.
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  #7  
Old 10-22-2012, 07:24 AM
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Join Date: Feb 2005
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Re: Question about optical center

and we didn't get an answer as to the rear base type... I've seen dozens of scope "problems" go away when the windage turret was centered and the rear adjustable base was tuned for the coarse windage adjustment.
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