A telescope equipped with a camera adapter and video camera is set up at the front of the lecture hall. The telescope
An adjustable aperture is now placed in front of the telescope. With a telescope magnification of approximately 125,
it is necessary that this aperture be mounted on a separate tripod and not touch the telescope—even so, walking around
on a cement floor still jiggles the image, as do air currents in the room! 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-diam aperture happens to be the Rayleigh resolution limit
aperture for our particular geometry. Not visible in these im-
ages 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 magnifi-
cation ~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.