As the aperture is slowly closed, the students can see the images of the two points of light grow in size as they smear out @Fig. 1~b!#. Closing the aperture further results in the smeared images melding together @Fig. 1~c!#. The 2.8 cm-diamaperture happens to be the Rayleigh resolution limit aperture for our particular geometry. Not visible in these images are the diffraction rings around the central bright area—one or two of these rings are clearly visible in the live video
images, probably because viewing live video integrates 60 frames/s whereas the images in Fig. 1 are single video
frames. Closing the aperture even further results in a large blurry spot @Fig. 1~d!#, and by no stretch of the imagination
can one claim to see two sources of light. At this stage the students can fully appreciate that further telescope magnification ~in hopes of resolving the two sources of light! would be absolutely futile. Two points should be noted: all four
images in Fig. 1 are the same scale and, in the live video
images, one can see the diameter of the diffraction rings
increase as the telescope aperture is decreased.