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Adjusting reticle vs. Adjusting mounting - Burris Zee rings

 
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  #8  
Old 09-29-2012, 09:50 PM
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Re: Adjusting reticle vs. Adjusting mounting - Burris Zee rings

Quote:
Originally Posted by Shootin4fun View Post
Yes, the part about optical quality changing at the scope's adjustment extremes is what I was really wanting to hear peoples' opinions about. I don't see why there should be a change in image quality when adjusting the retcile to the extremes and don't notice it on my scopes.
Take a scope - any scope - and use a mirror to optically center the reticle:
Place the mirror on a sturdy table then place the scope on top of the mirror with the eyepiece up. Then look through the eyepiece and observe the image.
You will see the crosshairs and you should also see a reflection of the crosshairs as well (ghost image). All you have to do is to turn each of the knobs until they match up perfectly. If you can't see the reticles that well try placing a light near the mirror shining at its edge.


Now, take a cardboard box and cut a "V" notch in each end to hold the tube of the scope similar to where your rings would hold it. Rotate the scope on its axis while looking through it and notice the quality especially towards the edges.

For step 2 take the same scope and move one of the reticle adjustments to its limit. Move both adjustments to a limit if you want. Now place the scope back on the cardboard box and rotate it again on its axis. You'll now see a significant difference in optical quality from the first test and will understand why some try to keep a scope in the middle of its adjustment range whenever possible.
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  #9  
Old 09-29-2012, 11:23 PM
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It's really a matter of degree (pun intended). It sounds like trebark is looking for a technical or at least plausible explanation, so here it is.

The objective lens on most scopes is a simple doublet achromatic design. This type of lens exhibits optical aberrations as the incidence angle of the light increases (i.e., as the viewing angle increases). As this angle increases, various aberrations, such as spherical, coma, field curvature, and off-axis chroma, will blur the image. Other aberrations, such as barrel or pincushion distortion, will cause the image to contract or expand. These effects become worse as the incidence angle increases. So, yes the image is clearer and the adjustments are more accurate near the center of the field of view.

For most scopes, the aberrations in the erector lenses are more noticeable than those in the objective. The erector aberrations are observed as you look from center to the edge of the field of view. The aberrations in the objective are difficult to "see" because the manifest themselves only as the elevation/windage knobs are rotated large amounts.

The first aberrations you will likely notice are field curvature and field distortion. Field curvature is a change in focus and can be removed by adjusting the AO or SF knobs. Field curvature causes an error in the adjustment at large viewing angles.

For viewing angles up to about 15 MOA, I have not been able to detect these aberrations visually on any but very cheap scopes. For viewing angles of 15-30 MOA (one-quarter to one-half a degree) the aberrations start to become visible to the trained eye. Above 30 MOA the aberrations become more obvious. At these angles image blurring occurs that cannot be corrected with the AO or SF knobs.

So, I recommend getting the scope tube axis aligned with the rifle bore axis to within +/- 15 MOA. This can be done with Burris Signature rings, or by using shims and windage adjustable bases. I also recommend using biased bases (or Signature rings) on long range rifles to minimize the elevation adjustment for long range shots.

FYI, in about a week HighPowerOptics will start renting rifle scope installation kits that allow you to align the scope to the rifle bore in this way.
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  #10  
Old 09-30-2012, 02:32 AM
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Re: Adjusting reticle vs. Adjusting mounting - Burris Zee rings

This is all very enlightening, as I thought that in order to utilize the full range of adjustment of a scope for long range shooting (my scope only adjusts a total of 35 MOA), it would be best to mount it in such a way that a 100 or 200 yard zero would be at a very low elevation setting.
On the other hand I was concerned that a 20 MOA rail would raise the scope too much so that I couldn't adjust the reticle down far enough to zero at 200.

So I figured that a 0 MOA rail and Burris Zees which allow as little as 10 MOA would be a better option....I guess I bumbled into a good solution. Given that, what would be a good mounting position with respect to the scope reticle setting to zero at 200 if I need to go up 30+ moa to reach 1000 yards? If the reticle is set around 10 MOA for a zero at 200 wouldn't it run out of adjustment to reach 1000?
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Old 09-30-2012, 08:58 AM
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I recall that 30 mm Signature rings include two pairs of .010" offset inserts. Using both offset inserts will provide up to about 17 MOA of elevation bias (for rings separated by 4"). The set in the back raise the scope, while the set in the front lower it.

The 1" Signature rings come with no offset inserts, but i recall that the kit includes both .010" and .020" offset inserts. If so, then they will allow up to about 26 MOA of elevation bias.

Does your scope have 35 MOA or +/- 35 MOA of adjustment? If its only 35 (+/- 17.5), then you will need to shim your base as well to get the 200 yd zero at a scope elevation of 0-2 MOA from the limit of adjustment (the extreme bullet down limit). This would require an accurate boresight collimator, or a lot of time shooting at the range. You should also reduce your ring separation to increase the elevation bias available from the Burris rings.

If its +/- 35 MOA, then just use both .010 offset inserts to get 17 MOA of bias. Then you will have plenty of up adjustment for a 1,000 yd shot.
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Old 09-30-2012, 10:05 AM
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Re: Adjusting reticle vs. Adjusting mounting - Burris Zee rings

Are you going to notice the difference on say.... a Vortex Viper or better scope?

The thing I would be worrying about would be inducing torque in the scope, Zee rings in theory should prevent that though.
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Old 09-30-2012, 03:52 PM
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Quote:
Originally Posted by Joe King View Post
Are you going to notice the difference on say.... a Vortex Viper or better scope?

The thing I would be worrying about would be inducing torque in the scope, Zee rings in theory should prevent that though.
Any scope below about $1,400 price point will probably have a simple doublet objective lens. Above that you can expect to see better objective designs in some, but not all scopes. I know that some Leica scopes, for example, use an air-spaced triplet objective design. Probably the high end Zeiss, Kahles and Swarovski scopes do as well.

I don't see how it's possible for Signature rings to stress a scope tube.
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  #14  
Old 10-01-2012, 10:21 AM
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Re: Adjusting reticle vs. Adjusting mounting - Burris Zee rings

Quote:
Originally Posted by bruce_ventura View Post
Does your scope have 35 MOA or +/- 35 MOA of adjustment? If its only 35 (+/- 17.5), then you will need to shim your base as well to get the 200 yd zero at a scope elevation of 0-2 MOA from the limit of adjustment (the extreme bullet down limit). This would require an accurate boresight collimator, or a lot of time shooting at the range. You should also reduce your ring separation to increase the elevation bias available from the Burris rings.
Thanks for all your feedback so far, guys.

The scope has a total of 35 MOA adjustment. The Burris ring set has the offset shims.

However I still don't have a full understanding of how a 20 MOA base or shims affect the impact point / reticle adjustment from 100 to 1000 yards. Furthermore there is the issue of adjusting a scope to its limits and the resulting effect on image quality.

So, let me understand. If I had a 20 MOA base OR installed 20 MOA ring shims on a set of high rings, and say the bullet drop at 100 yards is 2" @ 100, 8" @ 200 yards (similar to many calibers), wouldn't the impact point be too high even at the lowest reticle setting at 100? How many MOA from the bore line vs. sight line does that 2" or 8" bullet drop represent? (I supposed this is affected by the ring height - low med or high.) I bought a 0 MOA base (picatinny rail) because I was concerned I wouldn't be able to adjust down enough to zero at 100 or 200 if I used a 20 MOA base.

Next comes the issue of image quality when adjusting the reticle to extremes. If I do use a 20 MOA base or 20 MOA of ring offset shims, and then need to adjust the reticle down to 3 - 5 MOA from the scope's lowest setting for a 100 or 200 zero, how much image degradation will I incur? Much of my shooting is between 200 - 500 yards. Do I have to give up image quality for the ability to reach out to 1000 yards?

Up til now I have mostly used holdover to adjust for drop. The duplex post, or Nikon BDC. The Nikon BDC system has worked great for my most common conditions, but won't cut it for shooting over 700 yards. I've had a lot of fun chrono-ing my loads and programming them into the software, then seeing how that matches up to the physical reality of holdovers and impact points. Now I want to get into adjusting the reticle for distances.
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