How to optically center a scope

[snyper]

How to optically center a scope
By B.B. Pelletier

Today's post answers a question I received yesterday. To get the best performance from a scope, its adjustment knobs need to be in the optimum range, which is very close to the scope's optical center.

What is the optical center of a scope?
The optical center refers to the reticle and the field of view. An optically-centered scope shows zero reticle movement against a distant backdrop when the scope tube is rotated in a complete circle. Theoretically, that's possible to achieve, but I've never seen it. The best I've seen is a reticle that moves about a quarter inch against a target 20 yards away when the scope tube is rotated in a complete circle

Use a box instead of a gun to optically center your scope
I suppose you could rotate the scope in its rings if the rings' top caps (the top half of the rings) were removed and the turrets would clear the top of the gun when they came around, but I like another method.

With a small box sitting on a shooting bench, cut two v-shaped slots into the top edges and rest the scope on those two slots. Once the scope is focused on the target (which is 20 yards away), look through the scope and put a dot or mark on the target close to where the crosshairs are. It helps to have two people when you do this - one looking through the scope without moving it and the other downrange to draw the mark.

Rotate the scope tube around a complete circle and watch how the center of the crosshairs moves against your mark. It will probably move several inches at first. Figure out which adjustment knob to move and make your correction to reduce reticle movement against the target. This takes some time.

On the best day, I've adjusted a scope in 45 minutes. At worst, it took almost two hours to get it as good as it would get. Remember, it's almost impossible to remove the last bit of movement out of the reticle. Both the windage and elevation knobs will probably need some adjustment.

DO NOT think you can count the clicks of adjustment between lockup and complete spring relax and go to the center of that number for the optical center. It sounds good, but that way never works. Optical center is more precise than the center of the click adjustment range.

Once the scope is optically centered, put it in an adjustable mount like B-Square's AA adjustable mount and use the mount's adjustments to zero the scope and gun. Once you're zeroed, you'll still have all the scope's best range of adjustments remaining.

Is optical centering nesessary?
Absolutely not! You can just mount a scope and use it without going through this drill. But, if your scope runs out of adjustments or if you find yourself shooting to the right of the target at close range and to the left at long range, centering will cure the problem.

Source: PyramidAir Archives

[snyper]

Here another method I have come across, haven't tried it though

Optically center your scope easily - with a mirror!
poste by: J. Bruce Aurand

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

Source: The Yellow Forums (click link for original article)

[satclic]

sir, i'd like to share i read at airgunadvice forum regarding the subject. its a copy n paste info.

This proceedure comes from a Tech-Rep @ Leupold...
-----------------------------------------------------

{The difference between optical center & mechanical center stems from the fact that the turrets protrude into the tube [@12 & 3 o'clock]...so when you divide the total adjustment available in half and apply it, you end up off center towards 9 & 6 o'clock}

The 'Box-Dot-Rotation' method will work, but is frought with error & pitfalls.
-------------------------------------------------------
An easier, more accurate way is to;

First, set scope on LOWEST power...
Next, Set parallax to infinity...
Using a room with BRIGHT lighting [Such as bathroom]...
Press the objective end flat to a mirror...[Do not block light with hands]...
Move your eye in & out until you see the Reticle & it's Shadow...[Looks like 2 reticles...]
Now you just adjust knobs until reticles coincide [Become one]

WAHLAH !!!! You are now OPTICALLY CENTERED

hope this also add additional info to the above topic. i verified this method with my cloned 6-24x50 aoeg scope and 5th gen leapers 6-24x50 ao. you can use this method to verify also how many clicks between mildots in your scope or if your cloned scope is up to its adjustment knob.

[snyper]

thanks bro satclic, its actually similar to the one I posted, But the one you posted has more details. Though I used a different method on my scope, I will try the mirror technique when I get a new scope.

[satclic]

bro snyper, i experienced ko kasi na mag sight in the scope na bago at napansin ko na hindi nagana ang mga adjustment nito, pala nasa extreme end ng adjustment range ang windage at elevation kaya ang mga small adjustment ay parang walang response. i conclude na sira itong new leapers scope na galing sa state. but with the above test prove that the scope is working 100% ok kaya hanap ako ng info tungkol sa scope sight-in at eto ang magandang info from BSA website www.bsaguns.co.uk/scopes_settingup.php, tungkol sa problem ko. I followed to the letter ang pag gawa ng shim na alum foil at sakto agad ang vertical elevation ng scope sa poa to poi. windage na lang ang adjustment..kaya nasa range ang scope na ang response ng knobs is ok. ang gamit kong mirror eh iyun vanity mirror ni mrs. na bilog na covered completely ang objective lens. hope this info help others. mahal kasi ang mga adjustable mounts sa internet..

[heightaddict]

after learning WHY we want our scope to be optically centered, the next step now will be mounting your scope. a SMALL difference in you elevation can easily be solved with shims...but not too much since it will dent your scope. BIG difference in elevation requires a droop mount. (try to search and see the difference)

but most problems come with canting. you cant use shim for windage. some would try adjustable mounts, the problem is those mounts have limits. it is best shown in this illustration





a better solution would be re-do the dovetail

[snyper]

Nice post, as usual bro H. And I agree, re-doing the dovetail is a better idea.

[snyper]

The method I used BTW to prep my scope before mounting it is the one the "A team " is using. I'm sure it is posted here somewhere.