The adjustments being as near correctness as can be seen by the
usual method of procedure—of which the fullest details are given in
my little book, the telescope is ready to try on a star. If all does
not appear satisfactory on first observing the star, do not disturb
the adjustments for a while, but wait until the air is steadier,
when perhaps it will be quite satisfactory. If not, turn it on a
bright star; if the adjustments are at fault, the black shadow of
the flat, when the star is out of focus, will not be central in the
slightly expanded image, but will be on one side of the expanded
disk. Use a power of 200 to 300.
If the black spot is near one of the sides corresponding with major
axis of flat, the side screw will put it right. We will assume that
the black spot is nearer the edge of flat nearest the mirror, and
that the eyepiece is beyond the focus proper for the star; then
_unscrew_ the side screw. If the spot is nearer the other edge of
flat, _screw it up_ little by little, taking the hand out of the
telescope each time, and see what the effect is. The movement and
alteration can be watched while the hand is in the tube. If the spot
is seen either the top or bottom of the flat that is in the
direction of the diameter of tube, the middle screw must be slightly
loosened and the flat revolved. Let the screw just bite so as to
hold, and tighten it when correct. The definition of a star should
now be perfect, and by daylight observe the adjustments; it may
appear that the reflected circles are not concentric, but if the
definition on a star is good in still air, consider the appearances
correct adjustments, except that the reflection of the black spot on
mirror may be made concentric. It sometimes happens that a
persistent and rigid centreing of the reflections on _face of flat_
are not the positions for best definition, perhaps from a slight
optical eccentricity somewhere in the instrument.
The adjustments of a reflector are soon familiar and easy, and there
is this to be said, the adjustments can be manipulated upon to any
amount without the slightest injury to the instrument; there is no
danger of any sort, and the instrument can be perfected in its
adjustments by daylight, and this makes it both pleasant and
convenient to leisurely work at, for experience sake. The instrument
need not be out of doors either, unless an artificial star from a
black pin’s head—which is better in _strong_ sunlight than a
thermometer bulb—is to be used instead of a star at night, if so let
it be as high as possible.
Be careful not to screw up the small screws too tight, as the fine
threads are liable to be spoiled and the screw made useless.