Recently I had a really nice and informative conversation with Mike Kurtz, technical guru, over at Hawke Optics (www.hawkeoptics.com). He told me that well over 90% of their customer service issues have to do with scope owners improperly adjusting their scopes, and I found out that I have been doing some things wrong from time to time. Perhaps you have too, but before we get to that, let do a quick overview of how a scope works.
Take a look at the picture of the scope below. Light comes in through the big end (on the left) and is deflected down by lenses so that it will pass down the main tube of the scope (which in this case is a 30mm tube). As the light reaches the tube, the image is inverted by a lens.
The light then passes into the erector tube, which holds the crosshairs and is roughly in the vicinity of the elevation and windage knobs. The erector tube is fastened to the main body of the scope at the rear, but the front is free to move. When you adjust the elevation and windage knobs, you are moving the erector tube up and down, left and right, until the point of aim corresponds with the point of impact.
(A side note: theoretically, you could zero your scope in a single step at a given distance by clamping your airgun in a vice, firing a single shot, and then adjusting the scope so that the crosshairs intersect at the spot where the pellet hit.)
When the light comes out of the erector tube, it passes through another set of lenses, which flip the image back to right side up and focuses the image for the ocular lens where, you, the shooter, look through the scope.
Now, before we go any further, please notice this one key point: the erector section of the scope is a tube, and it lives within the outer chassis of the scope, which is also a tube. So the erector is a tube within a tube – got that?
The picture below shows an internal view of the scope as it comes from the factory with the outer chassis and the erector tube in perfect alignment, and the elevation turret and the windage turret are adjusted equally.
Okay, now let’s look at some ways folks get into trouble with scope adjustment. The picture below shows a scope that has been adjusted too far left. The erector tube is pinned against the wall of the outer chassis, and, as a result, the elevation adjustment is severely limited because there is no room for the erector tube to travel.
The picture below shows a scope that has been adjusted too far down and right, pinning the erector tube against the erector spring, the windage turret, and the outer chassis. Again, windage adjustment is very limited since there is little room for the erector tube to move.
Finally, below is a scope that has been adjusted too far up and right, and the erector spring has lost contact with the erector tube to support it. When this happens, the reticle free floats, and you will have point of impact issues. Mike tells me this is the most common of scope mounting issues.
So, how do you avoid these problems? First, don’t use up all of the adjustment in any direction with either the elevation or windage turret. Second, if you find yourself using up all of the adjustment, get yourself an adjustable mount. Put the scope back in optical center, then use the adjustments on the mount to get you pretty close to where you want to be zeroed, and use the elevation and windage adjustments on the scope to do the fine tuning.
Til next time, aim true and shoot straight.
– Jock Elliott