C is now producing parallel light from each point of the slit, the width of which is now adjusted to a convenient size. N.B.: the focusing adjustment of the telescope must not be touched during this operation.
An image of the slit is obtained on the cross-wire and is brought into sharp focus by the focusing adjustment on the collimator. with an ordinary filament lamp placed quite close to the slit. It is then brought into line with the collimator and the slit of the latter is illuminated, e.g. T has now been adjusted to receive parallel light. An object at about 200 m or so away may be regarded as sufficiently distant for this purpose. The eyepiece should first be focused on the cross-wires and then, with the telescope turned out of line with the collimator, a distant object is brought into focus through the telescope so that there appears to be no parallax between it and the cross-wires, by adjusting the focusing screw of the telescope. The focal length f of the lens is then given by Thus O is a virtual image point corresponding to the real object point O′, or alternatively (considering the reflected light from the mirror) O is a virtual object point for which O′ is the corresponding real image point. as if from O, which is the centre of curvature of the mirror. Then O and O′ are conjugate points for the lens since in order for the light to return along its own path after reflection from the mirror it must strike the mirror normally, i.e. The diverging lens is now placed between the mirror and object and, provided O is nearer to the lens than its principal focus, a new position O′ can be found in which there is again coincidence of the object and its image. This point is the centre of curvature of the mirror. If O represents an illuminated cross-wire or a pin then, in the absence of the diverging lens the position of the concave mirror is adjusted relative to O so that the image by reflection is also at O. Instead of using a converging lens, a concave mirror may be used as an auxiliary element as illustrated in Fig.
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