The telescope
 
Only a small-sized telescope is necessary to demonstrate the result of increasing the collecting aperture over the eye’s pupil and of utilizing the effect of magnifying power. Binoculars may also be used. For either kind of optics, the best appreciation is obtained if the instruments can be fixed to a device such as a tripod rather than just being handheld. Any telescope with a diameter of a few centimetres or more will reveal, with the aid of an eyepiece, craters on the Moon, the disks of planets, satellites of Jupiter and stars that cannot be seen by the unaided eye. Some stars which appear single to the unaided eye are seen as pairs when looked at with the aid of the telescope.
Objects of interest are usually listed with star maps and they will not be discussed here. Just to serve as an example, however, the improvement over the eye that the telescope affords can be appreciated by looking at the star  Lyrae. For an observer possessing exceptionally keen sight, this star may be seen with the unaided eye as two fifth magnitude stars, ∈¹ and ∈², separated by 208 arc sec. Not only are the stars seen more distinctly when viewed with the telescope, each may be seen as doubles. The area of the sky that can be investigated at any time depends on the field of view of the telescope. This may be determined by directing the telescope to the meridian, keeping it fixed and letting the stars drift across the field, timing them as they pass from one edge of the field to the other.
The recorded time must first be converted to angular measure and then multiplied by the secant of the declination of the star.
The timing of the drift of stars may also be used to determine the calibration of a micrometer eyepiece. Once this has been done, comparison of the separation of a range of double stars can be made with a star catalogue.
Some aspects of the visual use of a telescope may be demonstrated in the laboratory. For these experiments (the investigation of resolving power and measurements of magnifying power), a small telescope needs to be fitted with a large iris diaphragm or provided with a set of masks with circular holes to be fitted over its aperture. The required source consists of two closely separated, illuminated pinholes (see figure 1). The pinholes, or ‘artificial double stars’, should be set at a distance so that they cannot be resolved as a pair by eye or with the telescope when the aperture is reduced but readily

Figure1. An experiment to investigate the resolution of a telescope when viewing an artificial double star.
resolved at full aperture. If possible, provide means for the insertion of red, green and blue colour filters.