These instruments are always sent out in correct adjustment, and
with moderate care during transit, and afterwards, will remain so,
but as the performance of the instrument greatly depends on the
accuracy of the adjustments the following instructions will enable
the observer not only to ascertain whether they are perfect, but
also to render them so if found defective. These adjustments are by
no means difficult, and will be easily understood by attention to
the following remarks:—
Into the draw tube screw the “adjusting piece,” which is a small
brass circle with a hole in its centre about 1/20 of an inch in
diameter. (The draw tube should be in about the same position as
when at focus with an eyepiece.) Place the large mirror in its cell
in the tube or body of the telescope, taking care that the three
bayonet-joint pins are correctly placed, that is, with grooves
pointing downward. They will be found to drop easily into their
corresponding holes; care must however be taken that the grooves
have gone well home. Both the speculum and small mirror, or “flat,”
must be uncovered. On looking through the aperture of the adjusting
piece, if the mirrors are in correct adjustment their reflections
will be seen as follows:—the small oval mirror being placed at an
angle of 45° will appear circular, and reflected exactly in the
centre of this circle will be seen the bright image of the large
mirror with a dark round spot in its centre, as shown by Fig. 1.
This dark spot is the double reflection of the “flat,” and should be
concentric with both the bright reflection of the large mirror and
the circular outline of the “flat.” All these should also be
perfectly concentric with the circle given by the stop in the draw
tube. Should these circles not be all central the adjustments are
not perfect and must be rectified as follows.
To adjust the “Flat” or small diagonal Mirror.
If the bright reflection of the large mirror is seen as a perfect
circle, but not exactly in the centre of the “flat,” the latter
requires adjustment; for this purpose loosen the milled head screw
at the middle of the back of the “flat” which in large instruments
is made sufficiently heavy to act as a counterpoise to prevent
vibration. This will allow the “flat” to be rotated by the hand
vertically with respect to the tube of the telescope. Bring the
bright circular reflection of the mirror exactly central in this
direction, and fasten in position by screwing up the milled head
screw or counterpoise. If the circular reflection is quite central
no further adjustment is required, but if not, then, after
completing the vertical adjustment, as described, proceed to make
the horizontal adjustment by turning in one or other direction the
milled head of the horizontal adjustment screw, situated in front of
the vertical adjustment screw; this will bring the circle of light
exactly into its proper horizontal and central position, and the
adjustments are then completed.
If the bright reflection of the large mirror is _not_ seen as a
perfect circle, and the small dark spot not in the centre, the
speculum is out of adjustment, and consequently the adjustment of
the “flat” is best performed by removing the large mirror or
speculum with its cell and so arranging the body of the telescope
that on looking through the “adjusting piece” a large sheet of white
paper spread on the ground a short distance from the open end
appears as a white circle of light reflected in the “flat.” Now
bring this white circle exactly into the centre of the flat
precisely in the way described above, and on this being accomplished
replace the speculum uncovered with its cell, and proceed.
To adjust the large Mirror or Speculum.
[ Illustration: Fig. 1. Fig. 2. 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 _un_screwed 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.
A stop 1/4 inch less in diameter than the speculum, is often useful,
in order to cut off the internal reflections of the tube.
With mirrors 6-1/2 inches in diameter and under stops are seldom
required; with the 6-1/2 inch a 5 inch stop may however sometimes be
used with advantage on very bright objects. With an 8-1/2 inch
mirror a 7 inch stop often improves planetary definition.
With larger sizes than 8-1/2 inches several stops of different
diameter may be used, when experiment will determine what size is
best suited to the condition of the atmosphere, and the character
and brilliancy of the object to be observed. These stops can be
easily cut out of thin cardboard and afterwards blacked with
Indian-ink or lampblack.
On Observing the Sun.
No larger aperture than 4-1/2 inches should be employed for solar
observation, except with a specially constructed “Solar Eye-piece,”
as even a 4-1/2 inch speculum often concentrates sufficient heat to
crack the coloured glass of an ordinary sun-cap, if exposed for any
length of time.
If a “solar eye-piece” is employed, the whole aperture may be used.
In viewing the moon a slightly tinted glass is often most useful,
especially to persons with weak sight, as it takes off a great deal
of the glare.