From the ‘Astronomische Nachrichten’, No. 75 (1826), pp 37-44
(translated by Chris Plicht, annotations in () by CP)
On the Reception and Setup of the Refractor by Fraunhofer
at the Observatory of the Imperial University at Dorpat.
On 10th November the great telescope arrived, packed in 22 crates, which
had a total weight of over 5000 lb Russian weight. On opening the crates
it was clear, that the 300 mile long transport on land did not harm the parts
in the slightest. But masterly was the packing of the individual parts. All
bearings for example, which are needed to mount the different parts, were
lined with velvet; the most valuable part, the objective glass, filled a
crate by its own, held in the middle by elastic supports, so that dropping
the crate from an impressive height would not have harmed the glass.
With the large number of individual parts the assembly of the complete (instrument)
seemed not to be an easy task. The high weight of the main parts increased
the difficulty, and unfortunately the artist missed to include the guide
to the installation, to which pointed a series of 50 small packets, which
were marked, with single small parts and screws. Following a thorough study
of the individual parts using a perspective drawing, which I had received
earlier, I begun assembling in good faith on the 11th and was most happy
to complete it on the 15th. On the 16th I was delighted to make first observations
in the clear morning hours of the Moon and some double stars.
[38]
I stood in awe in front of the beautiful work of art, uncertain what to admire
more, the beauty of the form of the whole and its completion down to the
finest detail, or the functionality of the mounting and the mechanism to
move it, or the incomparable optical power and precision of the view.
The instrument is now in the western room of the observatory at a temporary
place, from where it is possible to observe within 1 ½ hours of the
meridian and to 45° elevation through the high window facing south. In
the next summer it will be moved to its dedicated position atop a tower of
the observatory under a special furbished dome from where it may be used
in any direction.
When in vertical position the objective is 16 feet 4 inch (Parisien units)
above the ground, of which 13 feet 7 inch are the tube length, and the eyepiece
is then 2 feet 9 inch off the ground. The weight of the complete instrument
may well be over 3000 lb Russian weight, of which about 1000 lb are for the
pier and bearings, and about 2000 lb are being moved on the pier. – The mounting
is parallactic.
The stand (or pier) of the instrument is a pair of crossed beams of 9 feet
8 inch length, 7 inch
[39]
width and 7 ½ inch height. They are reinforced by 4 beams which form
a square. Through the beams 8 bolts pass vertically, 4 at the ends and 4
closer to the centre, on which the instrument rests. One of the beams is
laid in the direction of the meridian, and both are adjusted by means of
the bolts. In the centre of the beams is a vertical piece, 6 feet 1 inch
high and 7 inch in the other dimensions. Three elliptically formed supports
reinforce this beam in N., O. and W., a beam tilted under the angle of the
polar elevation of the same size rests on the slanting upper part of the
vertical (beam) and on the South pointing part of the meridian beam. – These
are the wooden parts of the pier, made of oak, but covered most elegantly
with Mahogany. The connection of those is done by 29 iron bolts, making the
whole free of vibration and sway.
The upper part of the instrument consists of the tube, the axes, around which
the movement takes place, two divided circles and a system of levers and
weights to achieve equilibrium in all positions, and to reduce friction.
To the beam of the pier, which is tilted to the polar elevation, the bearings
of the main axis are attached by 8 bolts passing through the full thickness
of the wood. This axis, parallel to the World axis, is of steel, 39 inch
long and of considerable diameter. It rests in two cylindrical bearings and
is supported at its lower, convex, highly polished end against a steel bearing,
that is touched only at a single point. On this axis, at the lower end, is
the 13 inch hour circle, divided into time minutes, with 2 Verniers giving
4 time seconds: but a ½ second may be read easily. On the upper end
of this axis is the brass bushing for the 2nd axis, mounted with 12 strong
steel bolts. Through this passes the 2nd axis at right angle to the 1st,
to which it is comparable in dimension. It therefore rests in the plane of
the Equator. At one end is the 19 inch declination circle, divided 10’ to
10’, its Vernier gives 10”: 5” may be estimated. To the other end is the
cartridge in which the tube rests, attached by 12 bolts. The tube is 13 feet
long, made of fir, and in a manner that ensures stiffness and rigidity,
[40]
then faced (covered) with Mahogany which is finished so that one believes
to see a tube of highly polished copper. The objectiv cell and eyepiece drawtube
are made of metal and equipped with adjustment screws, allowing to set the
axes of the glasses on one line. The aperture of the objective is 9 Parisien
inch. – The finder of the telescope is an achromat of 30 inch focal length
and 29 lines aperture, set in a metal cell, and not an insignificant telescope
in its own right. – Two counterweights, mounted on levers, correct for the
weight of the objective half of the tube and the bending (sag) of it, by
being mounted in the same manner as the weights, which, with the Reichenbach
meridian circles, compensate the influence of gravity to the tube, the only
difference with our giant being that the levers rotate around double axes,
because of the variation in the position of the tube. Two more weights, of
which one is attached to a strong iron arm, which rotates by means of a double
circle around the bushing of the axis lying in the plane of the Equator,
the second (weight) being attached to the end of this axis, bring the centre
of gravity of all parts that are attached to the World axis in the same line
of it and, at the same time, reduce the friction of the 2nd axis in it’s
bushing. Yet another counterweight supports the World axis, in the centre
of gravity of all moving parts of the instrument, by two friction rollers,
so that now the rotation of the whole around the World axis is done with
ease.
The so assembled instrument is, after proper adjustment of the counterweights,
in absolute equilibrium in any position. It is possible to turn the instrument
with one finger around the axis which lies in the plane of the equator, with
even smaller force around the world axis since a weight of 3 lb attached
at the distance of the eyepiece overcomes the friction. So this huge tube
may be moved much faster and safer to any position in relation to the horizon
than any earlier telescope. – But also the finer movement is taken care of.
A clamp blocks the declination circle and is moved by means of a micrometer
screw against a strong arm attached to the bushing. This screw is turned
by a key, which the observer holds with the hand while the eye is at the
eyepiece. The adjustment in declination leaves nothing to be asked for: it
is so good as with the most perfect
[41]
meridian instrument. The fine adjustment of the world axis is done by an
‘endless’ screw, which’s thread is set against the accordingly cut hour circle.
A spring is pressing the screw with constant force, a lever helps to lift
it off the circle. Attached to the head of the screw is a 2nd long key with
which the movement is done during observations. But the most perfect motion
around the world axis is done by a clockwork, and this is the highest triumph
of the instrument. The mechanism is as simple as ingenious. A weight, which
is hung from a gerabox which connects to the toothed head of the ‘endless’
screw, overcomes the friction of the machine. The clock with the circular
movement adjusts only the motion with an ‘endless’ screw which is set into
the gearbox. The weight of the clock and the friction weight may be set without
interrupting the motion. – As soon as the telescope is set into motion a
star stays calmly in the field of view even at a magnification of 700 times.
No vibrating and no touch is noticed; one has the impression to look at an
immovable sky with the telescope. – But the genius artist did even more.
By adjusting a pointer on a divided disk on the clock the speed of the clock
may be changed immediately, and so a star, that is not in the middle of the
field of view, may be brought to any desired location in the field of view,
depending if the instrument is advancing or receding against the motion of
the sky, and then be kept in this position by resetting the pointer (of the
clock) to its normal position. The same provision is also used to set the
motion to that of the Sun and the Moon.
Four eyepieces are for this telescope. The weakest magnifies 175 times, the
strongest 700 times; in fact I have not measured those values with highest
accuracy by now. With the highest magnification the view is of the most perfect
accuracy when the air is calm.
It is very difficult to give a value for the optical performance of this
masterpiece. So much is clear, the 25 feet reflector by Schröter falls
way back behind our achromat when observing faint objects. Schröter
has, after finishing the reflector in 1794, published his observations of
s Orionis with this instrument, and supplied a small chart
[42]
of the stars forming this multiple star (see Bode’s Jahrbuch 1797). He saw
s Orionis as 12-fold, probably 13-fold. Although Orion is closer to the horizon
from here than from Lilienthal, I did not only see all 13 stars, including
the uncertain one mentioned by Schröter, but 3 more; so that, while
the 25 feet reflector shows it only 12-fold with certainty, the refractor
showed it to be at least 16-fold. – The high altitude of Saturn has not allowed
to test the effect of the telescope on this celestial body and its satellites,
so I will be able to make observations on the visibility of the Saturn satellites,
namely the 6th and 7th, only when the instrument is at its final position.
If one compares how smaller achromats by Fraunhofer compare to reflectors
of 13 and 15 feet; so one may be quite sure that our refractor will boldly
stand beside the best known of all reflector telescopes, the 40 feet by Herschel,
while leaving the same far behind in regard to the ease of use, the multitude
of applications. So I like to rate our achromat as the most perfect work
of art in optics that ever existed.
In regard of the use of this instrument for observations, it is a most useful
equatorial due to the divided circles and the very stable base, with
which the positions of the faintest objects, nebulae and comets may be determined
with ease. Trials have convinced me, that the position of an object may be
determined, by setting the object in the middle of the field of view and
reading the circles, with such an accuracy that is not reached with faint
comets and using a circular micrometer. It will be easy to increase the accuracy
of this method by using a ring of small diameter in the focal plane, or by
use of crosswires which are visible in an un-illuminated field. Most exquisite
will be the use of this instrument to measure small angles by using the complete
micrometric apparatus, which will be finished soon, and consists of the following
parts: 4 ring micrometer, of which 2 are double, an illuminated circle micrometer
with 4 eyepieces, one net
[43]
micrometer with 4 eyepieces, a repetitive illuminated filar micrometer with
position circle and 4 eyepieces. The position circle has divisions on silver,
and 2 Verniers, which give a minute, and a clamp and screw to turn the micrometer.
– To get practice in micrometric observations with this instrument, and to
make observations of double stars with it,
[44]
I mounted the beautiful filar micrometer, which I have from the same artistic
hand, and which was used for observations until now with a 5 feet achromat
by Troughton, with a new drawtube to the Fraunhofer refractor and can already
report on some samples of what may be achieved in respect to measuring small
angles.
(to
be finished.)