Centering a scope - 3 methods

Hey guys, I drafted up a little tutorial on centering a scope.  I'm putting it up here for review and critique so please let me know if you have corrections or suggestions for improvements.



1.0  Centering methods
Firstly, let’s have a quick introduction to the 3 common methods for centering:

• 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.
• 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.
• 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:


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.


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.



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.


[edit]Added pictures.[/edit]

Great write up Jason, can I ask what is the benefit to optically centering a scope?

The benefit of centering a scope is that spring tension and turret travel are at their midpoint. This is the most stable and strongest position for the reticle. It also allows the most adjustment.

There is no benefit to speak of optically centering the scope versus either of the other two methods except that it is generally the quickest method of the three provided the scope objective bell is one that is round and flat.
....
I prefer the optically center method and perform it before I mount the scope.

Hi Mike, if I'm reading your comments correctly, you are distinguishing the use of a mirror to be "optically centering" the scope.  Actually, none of these methods necessarily equates to optically centering.  Rather they are three methods to mechanically center a scope.  Granted, it seems commonplace in shooting sports to use these two terms interchangeably, and ideally the mechanical center is also the optical center but they are often not the same.

Sorry to nitpick.  I'm just trying to be clear and consistent with my terminology because during my research, I kept finding inconsistencies that made the subject difficult to digest.   I know I've repeated some inaccurate things I've read, and I won't be surprised if someone finds inconsistencies in my tutorial but that's why I wanted to put it up for critique.

On a more positive note, I think you are spot on with this important point:

The benefit of centering the scope before sighting the gun in is that you will know how far the reticles are from the center of the scope tube once sighted in. This lets you know if you are close to the end of one of the adjustments which can cause reticle float when shooting and make it impossible to get sighted in correctly or cause flyers all the time. It allows you to decide if the gun requires a drooper mount or adjustable mount to correct for the large misalignment of the barrel to the scope dovetail or piccatinny rails on the action.

Barrel bending 101:  http://www.gatewaytoairguns.org/GTA/index.php?topic=49873

"Rather they are three methods to mechanically center a scope."
Agreed, however I do refer to the "mirror method" as "optically centering" and the other two methods as "mechanically centering" the scope simply because the "mirror method" uses the optics to project a shadow of the reticle onto the mirror instead of centering relative to the scope tube, however, all 3 methods mentioned are indeed "mechanical" centering of the erector assembly.

Is any method "perfect"? Even if I "spin the scope in vees" and have "perfect alignment", all I've done is "mechanically center" the reticle in the scope tube. The scope still might not be "optically centered" depending on the precision of the scope's construction. A while back I had a 4-12x40 Hawke AirMax that would shift the poi about 1 1/2" horizontally simply be aiming at an 18 yard bull, then moving up to 10 yards, refocusing and aiming at the same bull. The two groups should have been stacked vertically about 1/2" apart if aiming at the same bull, however the two groups were aligned horizontally about 1 1/2" apart. Here is the target after testing the scope straight from the box without zeroing......


Then I proceeded to "optically center" the scope via the "mirror method" and found that the poi still shifted when focusing after the adjustment. Next I used the Hawke recommended method of "count clicks stop to stop and divide by two". The scope still shifted the poi with focusing. "Just for grinns" I took the scope to the mirror and found that the Hawke recommended reticle method wasn't even within "hand shaking" distance from the "mirror adjustment". The final test was to spin the "scope in vees" and after the tedious (for me) adjustment I found that the poi would still shift with focusingt. Again "just for grinns" I took the scope to the mirror with the "spin in vees" adjustment and the reticle wasn't even in the same quadrant as it was with either of the other two methods! Three different "reticle centering methods" and I got three vastly different results! The scope was returned to Hawke for a replacement.

Taking my 3-12x44 Optisan Viper which had been centered using the mirror method  I "counted clicks/divided by two" (a lot of clicks with a 30mm tube). Checking the alignment vs "the mirror method" the reticle was centered within a few clicks using either method. Obviously the Viper erector assembly was closer to the "optical center" than the Hawke based on the reflected shadow if the reticles in the mirror. LOL....been using the simple "mirror method" ever since with the Viper.

I bend my barrel to agree with my "mechanically zero'd scope via the mirror method" so the poi is within about 1/2" of the aim point at 30 yards. The fine adjustments are done with the scope turrets. Some have concern about bending an airgun barrel, however barrels are often bent at the factory to make the BORE straight.

LOL.....concerning straight barrels.........

I guess this shows that only the muzzle needs to be aligned properly and it doesn't matter too much what happens between the muzzle and breech!

My simple "barrel bender"..........
Decades ago I messed with a couple adjustable mounts to deal with "optics outta line with the bore" and found them to be much too tedious to set up, especially when the B-Square and RWS "C" mount adjustments tended to "shift around" after a while mounted to my .177 R9 that was tuned to almost 15fpe at that time. Maybe I didn't set them up properly but I ended up trashing the "adjustables" and simply started bending my barrels. I've been bending my HW barrels ever since and I even made up a simple "barrel bender" like this........




I like this simple arrangement because it bends the barrel without putting stress on the joint between the barrel and barrel pivot block. It also bends the barrel in roughly the same location making it easy to "adjust" the bend incase there was some "over bending" in one direction or the other. The Delrin sleeve gets snapped into place in the eyebolt after slipping the loop over my R9 & HW95 muzzle break. The Delrin sleeve doesn't damage the barrel finish, plus it "spreads out" the pressure on the barrel so the barrel is bent rather than "kinked a bit". It's interesting to me that bending the barrel enough to move the poi several inches at 30 yards can't even be seen "by eye".   
While I usually bend my barrel so the poi is about 1/2" from the aim point at 30 yards, I really don't think that as long as the turret adjustment isn't more than 1/4" turn from mechanical center all is well. After I "mechanically center my scope via the mirror method" (LOL...easier to write "optically center") I set the turrets on my Optisan Viper scope to "zero" like this..........

Doing this allows me to "fine tune the poi" and as long as the turrets are less than "1/2 turret rotation off" after zeroing I can always return the scope to "mechanical zero" without revisiting the "mirror".

Anywhoo......perhaps it's too easy to "get wrapped around the barrel" with minutia like "Perfect Optical (or mechanical) Centering". Seems to me that as long as the "erector" is close to being centered and the erector spring has enough tension to prevent "shifting with recoil"...........all is well. [/b][/i]

This would make a great sticky! I used the second method on my a couple of my scopes, but on my cheaper ones that had no clicks I will try 1 or 3.

This would make a great sticky! I used the second method on my a couple of my scopes, but on my cheaper ones that had no clicks I will try 1 or 3.

+1!

Jim

Quote from: buldawg76 on November 03, 2016, 01:40:40 AM

There is no benefit to speak of optically centering the scope versus either of the other two methods except that it is generally the quickest method of the three provided the scope objective bell is one that is round and flat.
....
I prefer the optically center method and perform it before I mount the scope.

Hi Mike, if I'm reading your comments correctly, you are distinguishing the use of a mirror to be "optically centering" the scope.  Actually, none of these methods necessarily equates to optically centering.  Rather they are three methods to mechanically center a scope.  Granted, it seems commonplace in shooting sports to use these two terms interchangeably, and ideally the mechanical center is also the optical center but they are often not the same.

Sorry to nitpick.  I'm just trying to be clear and consistent with my terminology because during my research, I kept finding inconsistencies that made the subject difficult to digest.   I know I've repeated some inaccurate things I've read, and I won't be surprised if someone finds inconsistencies in my tutorial but that's why I wanted to put it up for critique.

On a more positive note, I think you are spot on with this important point:

Quote from: buldawg76 on November 03, 2016, 01:40:40 AM

The benefit of centering the scope before sighting the gun in is that you will know how far the reticles are from the center of the scope tube once sighted in. This lets you know if you are close to the end of one of the adjustments which can cause reticle float when shooting and make it impossible to get sighted in correctly or cause flyers all the time. It allows you to decide if the gun requires a drooper mount or adjustable mount to correct for the large misalignment of the barrel to the scope dovetail or piccatinny rails on the action.

Kirk
I see where you are at and yes all three methods are indeed mechanically centering the scope, but as ED states using a mirror is using the scopes optics to determine the center of the reticles hence my reference to it being optically centered. Agreed that none of the methods can be used to perfectly center the reticle like it should be done at the factory with very sophisticated equipment that we assume they have and use. That sad part is that likely very few scope are actually centered before packaged and shipped to distributor's for sale to the public. We do need to keep the terminology consistent so beginners do not get confused to easy.

I have found that centering the scope first before mounting to have been a big benefit to me with my D48 and one of my 300s. the 48 would shoot 11 inches low at 30 yards before any scope adjustments so I knew it had to be a real drooper and decide on a Sportsmatch mount with the right correction needed to get the 48 hitting at 1/2inch low at 30 yards for very little scope correction to get it dead on, the 300s was about 4 inch low at 30 yards and since it is a Junior with no barrel weights I chose to bend the barrel to get it on the bulls at 30 yards. 

I used a piece of pvc pipe that the barrel would just slide into and clamped the pipe in my vice and then inserted the barrel in the pipe and tweak it the opposite direction from where the pellet was hitting.
After several tweaks I had it hitting 1/4 inch off of dead center of the bull so left well enough alone and sighted with the scope. It is a one hole shooter if I do my part.

Thanks for correcting me so we use the same terminology for the benefit of new shooters.

Mike

You're correct! I'm in error since shooting with an unzero'd scope at both 10 yards and 18 yards wouldn't give two groups "stacked vertically"!

To clarify I just took my .177 R9 with 30mm tube Optisan Viper scope mounted in a high BKL mount, and  after deliberately turning the windage turret 3 turns out of zero I shot two 5 shot groups (one at 18 yards and one at 10 yards).
This scanned target and shows the two groups separated vertically by about 3/4" after shooting at the same bull, but not aligned horizontally like I got with the Hawke scope.
 

Anywhoo........thanks for your feedback and clarification!