Fig. 3.
Fig. 4.
When in perfect adjustment the large mirror viewed through the “adjusting piece” should appear as before stated, as a complete bright circle, with the image of the “flat” as a smaller dark circular spot exactly in the centre (Fig. 1). Should this not be the case it must be rectified by means of the three adjusting screws at the back of the cell. Proceed as follows:—First unloose (by a few turns) the small clamping screws which pass through the larger hollow adjusting ones; then, on looking through the adjusting piece the relative position of the image of the “flat” should be carefully noted. If the dark spot is nearer the bottom, and consequently more of the top part of the bright circle is seen (Fig. 2), the mirror reflects too much of the upper part of the tube, and therefore the top of the mirror leans too far back and must be pushed more forward by screwing in the top adjusting screw (a Fig. 1) a little at a time till the dark spot is central. Should the spot be towards the top (Fig. 2 inverted), the reverse holds good and the top adjusting screw (a) must be unscrewed or the two other adjusting screws turned in. Should the spot be towards the left (Fig. 3), screw up the right adjusting screw (c Fig. 1). If towards the right (Fig. 4), screw up the left adjusting screw (b Fig. 1). The dark spot is always furthest away from that part of the tube which is too much reflected, and from the adjusting screw that must be turned in to correct it.
When all the adjustments are considered perfect, as in Fig. 1, the small screws are to be clamped up to keep the adjusting screws in position. If, after the greatest care has been taken in adjustment, a flare should appear on looking at a star (say of the second magnitude) with an eye-piece of a high power, and the diffraction rings are not quite concentric, it can generally be rectified by turning the large screws of the mirror round in cell a little at a time. If this does not remove the flare the adjustment of the “flat”. is not sufficiently correct and must be altered by means of the screws at its back. If the flare is at the top or bottom of the star the “flat” must be very slightly revolved by the hand, after unloosing the middle screw, and when correct reclamping it. If at either side, namely, in the direction of the major axis of the “flat” and in a line with the tube, the “flat” must be altered by the long screw. It is always advisable to leave the telescope for a short time undisturbed, especially if, on first looking at a star, a flare should appear, as these appendages often vanish when the instrument has been for a short time in the air. These adjustments may at first appear somewhat difficult, but are rendered remarkably easy by observing how the different screw movements alter the positions of the reflections. A useful test as to the correctness of the adjustments may be obtained by viewing a star with a high power eye-piece out of focus. When in the centre of the field it should appear as a bright luminous circle with a circular dark spot in the centre, the size of the bright circle diminishing as the focus is approached and the dark spot remaining central.
The cell mount of the large mirror can be removed from the tube and replaced without disturbing its adjustments, but it is very advantageous if the entire instrument can always be left undisturbed when not in use in an observatory of light construction, having a skeleton revolving dome, covered with well oiled canvas or calico, and made with a wide opening and large shutters. The Rev. E. L. Berthon has described in the “English Mechanic,” October 13th, 1871, an observatory of this kind most admirably suited to shelter a reflector, as the temperature inside would be as nearly as possible that of the external air, and no annoyance from damp would be experienced. Where an observatory is not practicable the telescope might be protected by a close fitting covering of like make. Both the large and small mirrors should be protected by their covers (with which they are provided), when not in use, especially if left in the open air. The larger sized tubes have a door large enough to admit the cover and so allow of its being put on the large mirror without the necessity of the speculum being removed from the body of the telescope.
To Adjust the “Finder.”
Direct the telescope to any bright star (the Pole-star being by far the best, as it has very little apparent motion), and bring this star into the centre of the field of a low-power eye-piece. Now adjust the “finder” by means of the three screws bearing on it, till the star is bisected by the cross wires seen in the focus of the eye-lens of the “finder.” Change the low eye-piece to a high one, and perfect the adjustment as before described. Any well-defined terrestrial object at a distance can be employed in the day-time to roughly adjust the “finder,” leaving the final adjustments to be made by a star.
The use of Stops.