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.

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

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,

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

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

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

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,

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.)