M. A. AINSLEY
As it will be necessary, in order to finally test the performance of the mirror in the telescope, to get it into good adjustment, I will briefly describe how this is done.
It is necessary, in order that the definiteness of a star may be as good as possible, that the image should be axiel - i. e., should lie on a straight line joining the center of curvature to the center of the mirror. Of course, the image is actually formed at one side of the tube and outside it; but this image is merely there-flection of an image formed in the axis of the tube. The first thing is to get the center of the flat exactly
level with, and central in, the e. p. tube. The disc of brass before mentioned, with the 1-16 in. hole in it, is screwed into the eyepiece tube - or a fairly high-power eyepiece with lenses removed may be used - to keep the eye central. On looking through this at the flat, the latter will appear circular; the end of the eyepiece tube will appear as a larger circle; and the two must be made exactly concentric, the six screws on the flat mounting being loosened and the brass rod supporting the flat moved until this is so. (Fig. 1.)
The center of the flat being placed exactly opposite the center of the e. p. tube, the reflection of the speculum will be seen as a circle somewhat smaller than the circle presented by the flat itself. The flat is now gently moved by means of the adjusting screws until the image of the mirror is exactly central in the flat. This is shown in Fig. 2, where F is the flat, M the speculum, and Tthe end of the e. p. tube.
The adjustment of the speculum may now be considered. The eye being kept at the aperture in the brass cap, the flat will be seen as a very much smaller circle reflected in the speculum, as in Fig. 3, where it is shown out of adjustment. This small image has to be got exactly central with the other circles seen, and here a little method is of great assistance. If the screws at the back of the speculum are arranged as I advised in No. V, a good deal of time will be saved by always working as follows: The top screw is always turned by itself, turning it, or rather the nut, to the-right; move the image of the flat downwards, in a di rection at right angles to the axis of the tube. Turning to the left, of course has the opposite effect.
The two lower screws are always turned simulta-taneously, but in opposite directions. Turning them outward moves the image of the flat to the left, or up* the tube and sway from the speculum: turning them inwards moves the image to the right or down the tube. Thus in Fig. 3 the top screw would be turned to the right until the image k of the flat is level with the center of the circles k, and then the two lower screws would be turned inwards until theimage is central, k. Some such rule as this will be found to save much time, as one is always certain what the effect of moving any screw will be; but if the screws push instead of pulling, as in my tube, the rule will be exactly reversed.
When all the circles are perfectly concentric, the mirrors are in adjustment, and any false light at one side of the image of the star will, if not due to the speculum, be due to thee. p. not being properly made, or the end of the e. p. tube not being cut off at right angles, or to the screw-thread by which the e. p. is held not being properly cut. To see where the fault lies, the e. p. tube may be rotated. If the fault is in the e. p., the wing, or irregularity of whatever sort, will also rotate round the image; but if it does not the fault lies in the speculum - less probably in the flat.
To adjust the finder, direct the telescope to a distant object or, though less conveniently, to the moon, and apply a low power. A little patience and care in moving the tube will soon get some object in the center of the field of view. The Under is then adjusted by means of its supporting screws until the object in question comes on or, as I prefer, just above the intersection of the cross wires, these wires being placed so as to make an angle of 45° with the vertical. A higher power may be now applied, and the adjustment repeated; but even the preliminary adjustment will serve to get the Pole Star well into the field of the lowest power, and it can then be readily brought to the center and a higher power applied. As I said previously, it is well to use the eyepiece with which most work is likely to be done in the case of a 9 in., this will be 250 to 300. Once a star is in the centre of the field of view, the finder can be finally adjusted, and it must be remembered that finder must be readjusted every time the adjustments of the mirrors are altered in any way.
My own plan is to carry out the adjustment of the mirrors when required, which is seldom, by day, pointing the telescope to the sky; the finder is then adjusted by means of a house on a hill two miles off, and I am certain of being able to pick up a star with the lowest power; the final adjustment of the finder is then easy at the commencement of observations, the system I have adopted of mounting the finder greatly facilitating the work.
The appearance of a star in the telescope I have already described; but I may say a few more words on the subject. The image, when carefully focussed, should be a very minute, perfectly round disc, with one or two fine rings round it, and without any trace of wings or flare - except that when the mirror is silvered and a bright star observed, a faint band of light, due to the flat support, will be seen extending across the field of view in the direction of the axis of the telescope. This is a defraction effect and cannot be altogether avoided; but it is of very little importance, and practically not noticeable with any but bright stars, while, with planets, I have never seen it. In the ordinary way of supporting the flat, by three springs, six of these rays are produced.
If the flat justifies its name and really is a plane mirror, the image of a star should be perfectly round - that is, assuming the speculum to be a good surface of revolution - whatever its size may be. But, if the flat is defective, the disc of the star will be a cross in focus, and if the focus is very slightly disturbed one way or the other, it will become an ellipse, the major axis being directed either along or across the main tube. If the flat gives this appearance it should be rejected, as no critical definition can be looked for - and the defect is by no means uncommon. One fiat I tried, though the work of a professional optician, was utterly defective in this way, and the telescope was quite useless until I obtained another, which at once removed all trace of the defect. This defect, of an oval disc, may be due to the speculum, as in the case of Dr. Common's first 5 ft. mirror, where it was due to some molecular stain in the glass of the speculum ; but any such defect would be at once recognized in the testing at the center of the curvature.
It must be remembered that any and every night is not good enough for perfect definition; and it is only when the atmosphere is at rest and free from currents of irregularly heated air, that these very delicate tests can be applied; but if a good night can be got, the defects, if any, in the mirrors, will be obvious.