 The human eye has the collecting area of a 2-5 millimeter diameter aperture depending on its state of dilation.  A telescope’s function is to collect light from a much larger aperture and squeeze it down to fit your eye.

 Ask visitors what they think will happen when you vary the aperture size.  You should note that only the brightness of the image changes with different apertures. The field-of-view stays the same.  The 4-inch aperture collects 16 times more light than the 1-inch aperture and is therefore 16 times brighter.  Note how the image is reversed (top to bottom and left to right)

 The magnification of a telescope refers to its angular magnification.  As an example, two stars 1 degree apart will appear 20 degrees apart with 20X (20 times) magnification.  20X magnification has the same effect as making an object 20X bigger or moving it 20X closer.  Note that the telescope’s magnification depends ultimately on its eyepiece.

 Each exposure (1/16, 1/4, and 1 second) is scaled so that it is presented at the same brightness level.  Ask visitors to describe the difference they see in the different exposures.  Longer exposures increase the Signal-to-Noise ratio. For example exposing for 16X longer increases the signal 16X but the noise only increases 4X. We see this as better quality image.

 Refracting telescopes bend light to a focus using curved lens. Reflecting telescopes reflect light to a focus using curved mirrors.  Today’s research telescopes are reflecting telescopes for a number of reasons. 1) They are more compact. 2) Large mirrors are much easier to produce than large lens. 3) Mirrors reflect all colors of light at the same angle whereas lens refract different colors through different angles.  Image device such as a CCD camera are placed at the focal point and do not require an eyepiece.