Aperture Diffraction?
From: rjb_ks (rjburdge.ks_at_netzero-dot-net.no-spam.invalid)
Date: 07/27/04
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Date: 27 Jul 2004 11:29:27 -0500
My background is Electronics and not optics, so I am hoping that I can
find the answer to a question in this forum. I compete in target
shooting competitions and have come across a strange phenomena when
using the sights on my rifle that I think is due to defraction, but I
am not sure.
The sights on the rifle consist of a 1.0 mm circular apeture rear
sight that is approximately 1.0 inch from the front of my eye when
aiming. The front sight is a rectangular vertical post that is 0.052
inches in width and is located 20 inches in front of the rear aperture
sight. When viewed through the rear aperture the apparent width of
the target (black circle) is about the same as the apparent width of
the front sight post. The sight picture of the front sight and
target, when aimed properly and looking through the rear aperture,
looks like the small letter "i".
To achieve a high score when competing, proper sight alignment is, of
course, very important. The sights are aligned by locating the top
surface of the post front sight in the center of the field of view
presented by looking through the rear aperture. The rifle is then
aligned so that the target "sits atop" the front sight post.
Once the sights are properly aligned (the rifle is aimed directly at
the target) any lateral movement of the eye causes the image of the
front sight to shift within the field of view of the rear aperture.
There are three distinct "pieces" in the viewed image: the boundary
of the field of view which is determined by the aperture, the front
sight, and the target. All of these objects are at a different
distance from the eye. When the eye is moved laterally, I expect the
image of the front sight to shift within the field of view set by the
aperture. This, in fact, does happen. I also expected the perceived
alignment between the front sight post and the target to change. This
does not appear to happen. Regardless of the displacement of the eye
relative to the rear aperture, the post front sight and target seem
to maintain alignment. This result is not predicted by geometric
optics.
The amount of diffraction expected for visible light passing through a
1.0mm aperture would be (according to my college physics textbook)
550nm/1.0mm = 550urad = 0.03151 degrees = 1.89 minutes of angle (3 dB
beamwidth, I assume). The amount of apparent lateral displacement of
the rear aperture to cause the front sight to be viewed at the edge
of the field of view of the 1.0mm aperture is 0.5mm/21" = 937 urad =
0.05371 degrees = 3.2 minutes of angle. Beyond the 1.89 moa
diffraction angle the image from the aperture will grow dimmer, but
will still be viewable over a slightly larger angle. This would seem
to support the conclusion that the effect I am seeing is due to
diffraction since the 3.2 moa viewing angle is not much larger than
the 1.89 moa diffraction expected.
Any opinions? Thanks.
- Next message: Bob May: "Re: Aperture Diffraction?"
- Previous message: Ol' Bab: "Big old aerial camera lens -14 lbs"
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