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.