Care must be taken not to leave too much aberration, as then the central disc is formed too positively outside the focus, and the rest of the light from the object appears as obtrusive rings and false light. The over-correction is bad, and acting as a negative lens the disc is formed too near the mirror. Such a correction, besides being objectionable on almost all classes of objects, prevents the use of the “Barlow” lens, and acts badly with all kinds of positive or Ramsden eye-pieces, which improve under-corrected, but “make bad worse” with over-corrected surfaces.
If these few and brief hints should stimulate the industrious and persevering student to make his own telescope, and thus enjoy the fruits of his own labours (which may add a relish and a pleasure to his astronomical work), they will have served the purpose for which they are written.
After a “Hint” on the choice of focus for the mirror, it only remains to say a few words about the plane, as this, with the large speculum, are the only parts that need be home-made as far as the optical work is concerned. Never adopt a “dumpy” for general use where high powers are sometimes wanted, for small and moderate sizes, say 8 to 12 diameters, and for large sizes not less than about 6 diameters of the mirror, as the larger ones practically admit of a shorter focus. The short ones can be mounted somewhat cheaper, but never choose them on this account, they will not make so satisfactory an instrument, and no adaptation of “Barlow’s” will make it so.
To make the plane, provide three well-turned metal discs of 7 inches or 9 inches diameter. Iron is the best, as the work will go down closer, and the plates or discs may be very thick—say an inch—and not so liable to “spring.” These turned discs must be scraped and ground perfectly flat on each other, until they are in good contact all over, so that there is nothing between the faces when testing them. In the earlier stage of “truing,” oil and colour can be used. When these discs are proved true, a disc of plate glass, same size, is cemented with pitch on one of them, or on a thick disc of glass, and care must be taken that it is not strained while on the block during working. Truly grind this plate on one of the tools to a fine semi-polish. Polish on the same tool with a piece of thick silk or alpaca, laid over and cemented down by a solution of resin dissolved in turpentine (as much resin as the turpentine will dissolve), then, with the tip of the finger, thinly smear over the tool and bind round the edge with cord, and the silk will keep in place. Fill up the texture of the silk or alpaca evenly with damp rouge, keep it uniformly damp, and never let the rouge and water work about. It may be polished on very hard strained pitch, but pitch for the amateur is not so safe, as it is liable in his hands to alter its form and destroy the truth of the plane, but it is the quickest and handiest if it can be managed.
After the polish is perfect remove the plate from the support and cut into squares a little larger than the size of the intended flat. Test these in the telescope on a star. If one plane turns out bad, the whole will most likely be so, and another plate must be worked with renewed energy and care, for a bad plane will spoil the action of the speculum however good, for there is no way of counteracting the curved surface of the flat or second reflector. The edge of the plate for about an inch should not be used.
When a good plane is found it can be edged by turning a piece of wood a little less in diameter than the minor axis of the plane required, turn the end square, and mark a line around it distant from the end equal to the diameter, and cut through to the opposite edge, and it will give the oval and will appear round at 45°, this will mount nicely in brass tubes of the proper size, and a cover should be made to fit easy.
“THE EQUATORIAL.”
Fig. 1 is a modification of the German principle, and it should be a sufficient recommendation to remark that it is the principle used by a maker of such experience as Mr. John Browning, and is, without doubt, the very best style of mounting for a fixed equatorial, especially when clockwork is employed, because clock power is applied to the polar axis itself direct from the driving worm.
The driving part, viz., the worm and wheel, which is out of the observer’s way (being between the standards), is not liable to meet with accident, and the driving wheel being near the lower end of the axis, is at the most rigid part, viz., the foundation of the instrument. As it has a long and very firm polar axis, and is connected with the foundation plate, it secures the utmost steadiness and freedom from strain.
An equatorial mounting, with two long and stiff shafts for the axes, has always the advantage of firmness; the holding portions of the instrument being in masses, are not liable to receive injury from blows, and thus be put radically out of order. No means should be resorted to to make an equatorial of light weight; an instrument cannot be light and slim and at the same time firm and steady—no amount of steadying rods, splines or strings, will make a slender tripod for a refractor so steady as a firmly made one, with proper size and weight in the parts.