5. Adjust declination axis perpendicular to polar axis.
6. Adjust index of right ascension circle, and
7. Adjust optical axis of finder parallel to that of telescope.
Now let us take the simplest and commonest case, the adjustment of a portable equatorial on a tripod mount, when the instrument has a finder but neither circles nor driving clock. Adjustments 2 and 6 automatically drop out of sight, 5 vanishes for lack of any means to make the adjustment, and on a mount made with high precision, like the one before us, 4 is negligible for any purpose to which our instrument is applicable.
Adjustments 1, 3 and 7 are left and these should be performed in the order 7, 1, 3, for sake of simplicity. To begin with the finder has cross-wires in the focus of its eyepiece, and the next step is to provide the telescope itself with similar cross-wires.
These can readily be made, if not provided, by cutting out a disc of cardboard to fit snugly either the spring collar just in front of a positive eyepiece or the eyepiece itself at the diaphragm, if an ordinary Huygenian. Rule two diametral lines on the circle struck for cutting the cardboard, crossing at the center, cut out the central aperture, and then very carefully stretch over it, guided by the diametral lines, two very fine threads or wires made fast with wax or shellac.
Fig. 168.—Clark 5-inch with Tripod and Pier.
Now pointing the telescope at the most distant well defined object in view, rotate the spring collar or ocular, when, if the crossing of the threads is central, their intersection should stay on the object. If not shift a thread cautiously until the error is corrected.
Keeping the intersection set on the object by clamping the tube, one turns attention to the finder. Either the whole tube is adjustable in its supports or the cross-wires are capable of adjustment by screws just in front of the eyepiece. In either case finder tube or cross-wires should be shifted until the latter bear squarely upon the object which is in line with the cross threads of the main telescope. Then the adjusting screws should be tightened, and the finder is in correct alignment.