Scope centering - 3...
 

Scope centering - 3 methods  

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nervoustrigger
(@nervoustrigger)
Joined: 2 years ago
Posts: 184
2018-01-23 18:58:20  

Okay, let's see if we can get this new forum off to a start.  Here's a scope centering article I drafted that may be of some use.
SCOPE MECHANICAL CENTERING
1.0  Centering methods
Firstly, let’s have a quick introduction to the 3 common methods for centering:

  1. Mirror method – This method involves aligning the reticle with its reflection in a mirror.  The caveat is that it assumes the objective bell is perfectly perpendicular to the tube axis.
  2. Counting clicks – This method involves—you guessed it—counting clicks!  The idea is locate the turrets’ end stops and then return to the center.  The caveat here is that it assumes the mechanics of the scope are perfectly balanced.
  3. V-block method – This method involves resting the scope in a cradle-like device called a V-block.   Then what you do is rotate the scope in the V-block until the reticle spins against your background in one spot.

Now let’s talk about how to do each one and look at some advantages and disadvantages.

 

1.1  Mirror method
This method is the quickest and easiest of the three.  All you need is a strong light and a mirror.

Start by setting the magnification to its minimum and the yardage to infinity.  Now press the objective bell against the mirror, taking care to hold it firmly against the glass.  A bathroom mirror is typically a good choice.  The thicker the glass the better because it allows more room for light to enter where you need it, where the scope meets the glass.  And since bathrooms are usually well lit, you may not need additional light but if you do, a bright flashlight is a good choice because you can play with the angle the light enters the glass.

Now look through the scope and you should be able to see two reticles; the actual reticle and its reflection:
<p style="text-align: center;">
mirror methodreticle and its reflection in a mirror</p>

Simply twist the turrets until they lie on top of each other.

Another advantage is that you can also use a small mirror and do it without removing the scope from your rifle.  Naturally, this method can only be used on scopes that have an objective bell that is a simple round shape that can be held flat against the mirror.  If instead it has a hooded shape, one of the other methods will have to be used.

 

1.2  Counting clicks method
One, two, three, four……………three hundred and thirty!

Okay, not quite that literal.  Rather than counting clicks one by one, count complete turns.  That is, start by turning the turret all the way to its end stop in one direction.  Take care not to strip it out by going too far.  Then count complete turns until you reach the end stop in the other direction, including whatever final partial turn you get.  Then simply go back to the halfway point.
<p style="text-align: center;">counting clickscounting turret clicks</p>
Like the mirror method, this technique can be performed without removing the scope from the rifle.

 

1.3  V-block method
Of the 3 methods, I prefer this one because it registers off of the scope tube, same as the scope mounts.  I made a V-block of wood but for a quick one-off, a suitably sized cardboard box with a couple of V-shaped cuts will do.
<p style="text-align: center;">V-blocksimple V-block made of wood</p>
<p style="text-align: center;">
V-block with scopescope placed in V-block</p>
When you are ready to center your scope, clamp the V-block in a vise (or something else sufficiently stationary) and point the scope at a target.  I say a target...anything with a grid or pattern that you can use as visual markers is fine.

What you want to be able to do is spin the scope and have the intersection of the crosshairs to remain in the same place throughout the rotation.  Think the spinning propeller of a helicopter.  It can be tricky to home in on the center if you start fiddling with both turrets.  Instead, take one at a time.

It doesn't matter which one we start with so let’s just pick the windage.  Orient the scope normally and take note where on the X-axis of your target the crosshairs rest.  Then rotate the scope 180° and take note again where the crosshairs end up on the X-axis relative to the previous spot.  Now adjust the windage turret to an imagined midway point between those two spots.

Repeat this back and forth until the spot on the X-axis ends up in the same place when you rotate the scope 180°.  While you are rotating the scope back and forth, you will probably notice the crosshairs orbiting against your target.  You want to ignore that.  All you care about right now is the X-axis and where the crosshairs end up left-to-right when the scope is oriented normally versus when it's upside down (rotated 180°).  What it does in between those two points is of no interest.

Once you have the windage fine tuned, now turn your attention to the elevation turret.  Same idea except you're now focusing on the Y-axis.

When you're done, you should be able to rotate the scope 360° and the crosshairs will remain in the same spot on the paper.

Note:  Be sure your scope's parallax (range) is set correctly or you'll drive yourself crazy trying to get the crosshairs to stop orbiting.  It's one thing to have a consistent cheek weld when you're holding a rifle, but here you have nothing to help you keep  your eye in exactly the same place behind the ocular bell. 

For that reason, if your scope does not have an adjustable parallax, you need to do this process at the range at which its fixed parallax is set...not what it says on the spec sheet (it's a lie!) but at the range where you can bob your head and not have the crosshairs move against your target.


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nervoustrigger
(@nervoustrigger)
Joined: 2 years ago
Posts: 184
2018-01-23 19:00:12  

Well, it was formatted great in the editor window but the centering is jacked up when I submit it 🙁


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Harry Fuller
(@yrrah)
Joined: 2 years ago
Posts: 7
2018-02-02 12:09:12  

The mirror method was first described by J Bruce Aurand elsewhere but later in 2008 on The Yellow Forum (See quote below).

Thank you NT for your clear summaries of the three methods. Scope centering is of course most useful for getting the best from the scope when used in conjunction with adjustable mounts. .....   Kind regards, Harry.

Reference:     Quote:
"Optically center your scope easily - with a mirror!
May 8 2008 at 1:20 PM
J. Bruce Aurand  (Login Revwarnut)
AR&P

I have posted this elsewhere but not here, and I see that the subject of optically centering a scope comes up once in a while, so thought I would post it here also.

I used this mirror technique on my 2 spare scopes, a Bushnell Banner 3x9 and an Optronics 4x. It took me less than 3 minutes from removing the caps on the adjustment knobs to centering BOTH scopes!.  I also have checked my 3 other scopes and did so while they were still on the guns!

I used a 4" mirrored candle coaster that had fairly thick glass to do this.  Also, good lighting makes all the difference when doing this.
I found by experimenting that up to a point, the thicker the glass is, the better, you not only get more light in to see what you are doing, but the farther out you are from the mirror surface (which is actually on the back side of the mirror glass), the more it amplifies the deviation.
I also found that you need to hold firmly to the end of the scope on the glass because any little movement causes the image to shake.

With that said, you will need to also remove any rubber pads that might be on the bottom of the coaster. Best thing is, you can get these things cheap at any Walmart, Goodwill, Hallmark, or candle store.... etc., and you will not have to cut any glass or worry about sharp edges! I also found that clear plastic gives a false image, so use glass.
You might even be able to find a make-up mirror that will work, but I don't think that the "magnified" sides of those types of mirrors will work for this.

Edit - this portion was added to the original message to keep it all together.

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.  All you have to do is to turn each of the knobs until they match up perfectly!  It is that simple!  Very effective, fast and easy to explain and to setup etc.

A similar mirror technique is used to align telescopes.

Give it a try!  You can also place 3 washers on the glass to raise the scope up a bit to increase the amplification of the alignment difference.  Just place them so that the end of the scope is firmly on them and is stable.  It won't matter if they protrude into the image.

Below are some questions and answers from responses to the original post and threads from that post.  Thanks to all that contributed!  (I admit to plagerizing your posts!!  LOL!)

Q. Does the power or AO setting matter?

A.  It  shouldn't really matter what the power setting is, but you do get a better view at the highest setting because then the whole image is a white background instead of just a portion in the center (which works well enough).

Q. Why not turn to max both ways, count clicks and turn back half way?

A. The reason you would NOT want to turn each knob to the max each way and then count the clicks and turn back half way is that you may run past the last click point on some scopes and not be able to turn it back!  Probably only on a cheap scope, but why take the chance!  Also, that is a lot of clicks and it takes more time and is not as accurate either.  And on some scopes, they will stop moving, but will continue to click, click etc.

Q.  What exactly does this accomplish once it has been done?

A1.  This puts the crosshairs back to the center of  the scope.

A2  Provides a starting point for getting a rough zero with an adjustable mount, or for taking the measurements needed for ordering a "drooper" mount with a preset angular correction machined in.

JBA

This message has been edited by Revwarnut on May 8, 2008 7:12 PM
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J. Bruce Aurand
(Login Revwarnut)
AR&P
The method explained
May 8 2008, 2:39 PM

OOPs! sorry!

The previous time I posted this was a lengthy discussion of the method and had already discussed the method.

Here it goes.

You 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.  All you have to do is to turn each of the knobs until they match up perfectly!  It is that simple!  Very effective, fast and easy to explain and to setup etc.

The reason you would NOT want to turn each knob to the max each way is that you may run past the last click point on some scopes and not be able to turn it back!  Probably only on a cheap scope, but why take the chance!  Also, that is a lot of clicks and it takes more time and is not as accurate either.  And on some scopes, they will stop moving, but will continue to click, click etc.

A similar mirror technique is used to align telescopes.

Give it a try!  You can also place 3 washers on the glass to raise the scope up a bit to increase the amplification of the alignment difference.  Just place them so that the end of the scope is firmly on them and is stable.  It won't matter if they protrude into the image.

Bruce".  .......   End quotes.

 

 


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nervoustrigger
(@nervoustrigger)
Joined: 2 years ago
Posts: 184
2018-02-03 04:48:28  

Harry, thank you for finding Mr. Aurand's writeup and posting it here.  Always a pleasure to hear from you.


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nervoustrigger
(@nervoustrigger)
Joined: 2 years ago
Posts: 184
2018-02-03 14:27:15  

Harry reminded me that it would be useful to explain why one would want to center a scope.

For me, the biggest thing is so that when I mount it on a rifle, if the POA and the POI differ substantially left/right (windage), I know something is misaligned*.  Such misalignment can cause accuracy issues that are often associated with cant error...pellets landing left or right of where you wanted when you're shooting at varying distances.  Could be the barrel, the way the receiver is bored out to accept the barrel, how the dovetail was machined into the receiver, the scope rings, or some combination thereof.

Identifying how to address it usually takes me some trial and error to resolve.  If it's not off by more than, say, 2-3" at 25 yards, I'll usually shim the front or rear dovetail as shown here:

windage shimming
diagram from http://www.fekete-moro.hu/bfta-setup-manual/index-en.php

A strip cut from a soda can (typ. 0.005") works well.  Most 3/8" or 11mm dovetails are pretty shallow so I don't like using more than about 3 layers (long strip folded back onto itself).  If the discrepancy is larger, I usually move on to either bending the barrel or using adjustable rings.  Then I'll check how well the windage tracks at disparate ranges.  Let's say I've zeroed at 25 yards, I would then check the lateral POI at 10 yards and 40 yards to make sure it stays consistent.

Secondly, if you have to use nearly all of a scope's adjustment range to get it zeroed, it can sometimes lead to problems holding the zero because the erector spring(s) is at its limit.  And on a heavily recoiling springer or gas ram, it may put undue stress on the erector tube and reticle and lead to a premature failure.  With top shelf scopes, the former is less likely to be a problem but the latter is worth avoiding regardless of the quality/robustness of the scope.

* This is an important distinction, that I'm really only interested in the windage discrepancy at this point.  That is, there will always be an elevation discrepancy due to the vertical separation between the scope and the bore, and the fact the pellet begins a downward arc the instant it leaves the barrel.  In order for the elevation to converge (POA = POI) at any distance, the scope and the barrel must be tilted toward one another.  So you could say every rifle needs a drooper mount if you want to keep your scope centered...or some other means of compensation like shimming, bending the barrel, adjustable scope rings, etc.  However in many cases there is sufficient adjustment available in the scope's elevation turret...which brings us to another important distinction.  Keeping the elevation turret centered isn't as useful as keeping the windage centered.  That's because when you develop your holdover card, you'll have sufficiently characterized any error associated with the elevation.  But if the windage error is bad enough, you may find yourself chasing the windage when you're shooting at varying distances.  I suppose one could make a windage doping chart to know how much to hold off left or right but who wants to do that? 🙂 


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