From the 'Astronomische Nachrichten', No. 74 (1826), pp 17-24
(translated by Chris Plicht, annotations in () by CP)


On the Construction of the just finished Great Refractor,
by Dr. Jos. Fraunhofer.

(with one Copperplate)

Read at the public meeting of the Royal Bavarian Academy of Sciences, 10th July 1824.

[17]
The instrument, about which I have the honour to speak, is for the Imperial Observatory at Dorpat. It is the largest of its kind and new in respect to the important parts of the mounting.

The largest viewing tools existing so far, are the telescopes with metal mirrors. Since even the most perfect metal mirror reflects only a small part of the incoming light, the larger part being absorbed, reflectors have to be very large to have a positive result, thus the intensity of the light reaching the eye of the observer will remain low. - In addition, with reflectors, the aberration of the light rays due to the spherical form of the reflecting surfaces, which is very prominent, can not be corrected. (Original: kann ... nicht gehoben werden.)  For this and several other reasons the reflectors could not be used to advance of the mathematical-astronomical observations, and the reflector was never used as a meridian instrument
 etc.

Since almost all light is passed through the glass, and with a telescope constructed from Crown- and Flint not only the aberration by chromatic dispersion is compensated, but also that from the spherical glass surfaces, the effect of an achromatic telescope, compared to that of a reflector, is unequally larger. Partly to this reason, in part because their construction makes them suitable for all kinds of observations, almost all astronomical observations are made with achromatic telescopes.

[18]
Although the achromatic telescopes used so far, being small compared to the reflectors, the first have achieved more in several fields than the latter. The most rigorous test of a telescope is, as is common knowledge, the observation of double stars, and here the impact of the newer achromatic telescopes is much greater than from the reflectors. Discovered by Bessel in Koenigsberg, for example, with an achromatic telescope from here (Fraunhofer's workshop), with an aperture of 48 lines, that the double star 4th class Zeta Bootis, discovered by Herschel with his telescope, is also one of the 1st class, i.e. he saw that there is another star close to the main star, which was not seen by Herschel. Likewise several other fixed stars, which were observed often with telescopes in the past, were recognized as double stars by the use of achromatic telescopes.

As it is known, the effect of a telescope is not dependent on its length, but on the aperture of its object glass, so that with equal perfection that telescope, which has double the size of a comparable one, has twice the effect. The difficulties which are to be faced when making larger, equally good telescopes as smaller ones, do not grow by the relation of the diameter, but even more, with the relation of the cubes of those (the diameter). Since it has not been possible to overcome those difficulties until now, those larger achromatic telescopes, with objective apertures of over 48 lines, which were tried to be made, were not of equal perfection as smaller ones, and

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with even larger (apertures) it (perfection) was reduced. One of the difficulties was, that the glass, which was to be used to manufacture the objectives, could not be made in that perfect way that is necessary for larger telescopes. Namely the English Flint glass has wave-like streaks which disperse the passing light in an irregular way. Since there are more streaks in a larger and thicker glass than in a smaller one, which must, if an increase in impact is desired, be the other way round, the effect was reduced with objectives of larger diameter. In addition, the English Crown glass, as any other glass which was used so far, has these wave-like streaks, which, although not always visible to the naked eye, giving the light rays, by uneven refraction, a wrong direction. The Bavarian Flint as the Crown glass is free of these streaks and of equal density (within the glass). Since the Flint glass differs from regular glass only by the stronger colour dispersion, and this dispersion of English Flint relates to normal glass as 3 : 2, but with Bavarian Flint as 4 : 2, therefore the latter is also better in this regard.

Until now the achromatic objectives were not completely made following certain theoretical principles, one had to rely on good luck within reasonable limits, therefore a great number of glasses were ground and mated as pairs (by trial and error) until the errors almost compensated. Since the probability of such a chance is much smaller with large objectives than with smaller ones, also objectives of medium size are rarely perfect, and even with good Flint glass no thought would be spent on large achromatic objectives. The main causes for this procedure were: in part, the theory of achromatic objectives is not fully understood; in part, that the refractive and colour dispersing characteristic of the glass used, which must be known exactly when calculating, was determined not exactly enough by the means employed earlier; in part, finally, that the methods, which were used to grind and polish the glass, did not follow the theory as predicted, if no observable deterioration should be observed.

[20]
The mentioned obstacles, plus some more, were here, in part by invention, in part by discoveries, which were made in the course of working on this objective, happily removed. I will probably have the opportunity to talk about these in depth at another date.

The objective of the here discussed great refractor has 108 Parisien Lines aperture and 160 inch focal length. - The effect of a telescope is best shown when comparing with another one, pointed at the same object. While observing with large telescopes, the largest obstacle is the imperfect air, and here mainly the apparent undulating of it. These disadvantages grow with larger telescopes with the square of their diameter, but the effect grows only with the (linear) relation of the diameter; therefore, even when the sky seems to be clear, and the air is only a bit imperfect in the mentioned sense, observations are not possible with larger telescopes. Since the air, everywhere in space, is perfect in this sense only on a few days per year, to learn about the relative effect of this large telescope, a special object for this purpose was placed (on the earth); because in this case a shorter layer of air has to be passed, and its imperfections would be less harmful. The experiments, executed as described, showed, that the effect of the large refractor increases in relation to its diameter, fulfilling the maximum expectations. - It would be too much to mention the means which were employed to, for example, adjust the centres of the lenses onto a common line, remove the effects of expansion and contraction of the metal lens cell at different temperatures etc., what had to be considered to make sure that the best effect is achieved with this instrument.

One of the largest obstacles that was encountered when using large telescopes on celestial objects, is the apparent daily movement of the stars, which is magnified in the same relation as the telescope magnifies; so that stars close to the equator remain only a short time within the field of view of a high magnifying telescope, and pass quickly through the same. Even with the smoothest movement of the telescope

[22]
by hand with screws, small oscillations will be introduced, which will be magnified in relation to the magnification of the telescope. Before the telescopes comes to rest, the star will have passed the field of view, making it possible almost only by pure chance to see it for a short moment under good conditions. These favourable conditions are even rarer since a star can only be seen with the highest precision when being located in the middle of the field of view. The mentioned difficulties can only be met if the telescope would move like the observed star without the interference of a human hand. This had to be with the same precision for all stars, regardless of the ones apparently slower close to the pole, or those located near the equator moving very fast.

For this reason the large telescope was mounted paralactic in its own way, that is, one of the main axes around which it rotates, is elevated against the horizon in such a way, that its angle equals the polar altitude and is pointing at the pole. The second axis, named the declination axis, is mounted on the hour axis exactly vertical (at right angle). If the telescope, mounted in this way,  is pointed at any star, then only the hour axis needs to be turned with such a speed that it would turn once in 24 hours, like the Earth's axis, in which case the star, whichever it might be, remains within the field of view of the telescope as long as it is above the horizon. - This movement is applied to the hour axis by an apparatus similar to a clock, consisting of two works. The weight (force?) of one work overcomes the friction and the weight of the moving mass of several Zentner (a unit of mass, 50 kg, about 110 pounds); the other mechanism regulates the movement. To regulate the movement neither a pendulum nor a balance spring (like in a clock) could be used; because in this case the telescope would not move uninterrupted, but only in steps.

The regulator in this work is a centrifugal mechanism, which rotates uninterrupted in a conic housing in one direction. Also, when cranking up the weights the telescope moves with the same speed. The telescope may, while the clock moves on, be stopped at any time and set to move again equally fast. In addition, it may be moved by hand or

[21]
by means of a bolt in any direction. - The clock may be, at any time, adjusted to faster or slower, only by setting a spiral formed disk to another degree of its division. This is to advantage, because, if a star is not at the desired location in the field of view, it may be set to that place by using the clock, which is of great use with micrometer observations, and not advisable by other means, because of the dead travel etc. This spiral disk has also the advantage, that the telescope can be set to a movement equalling that of the Moon.

To assure this uniform movement of the large telescope, it must be in all positions, as different as they may be, balanced in relation to both main axes, and these balances may not disturb the telescope from being pointed to any part of the sky. In relation to the declination axis, the eccentric mounted telescope is balanced by two weights close to the eyepiece, mounted on individual conical brass tubes, each of which has two axes in the centre of gravity, at right angles, so that in this relation the telescope is equally balanced in any position. In relation to the hour axis this telescope is balanced by two weights, of which one is mounted directly to the declination axis. The second weight is attached to a rod of an individual (special) form, which is bent in the direction of the hour axis to form a ring; this ring touches, by two opposing axes, a second smaller (one); this second ring is attached to two axes, which are perpendicular to the first, a third even smaller (ring); and this finally turns at a bushing in which the declination axis is located, so that in relation to the hour axis the telescope is balanced in all positions. To eliminate the friction of the hour axis, and remove any pressure down or up, another special weight is attached which exerts a force on the bearing of two friction rollers. It is due to this installation that the telescope, regardless of the extraordinary weight, may be moved with one finger.

The pier of the instrument has a form that, although its position may never be altered, will never hinder the telescope from

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being pointed to any location in the sky. Although it seems that there are positions of the telescope in which the pier may prevent following a star; this instrument is constructed in such a manner that the telescope may be directed to an object in two ways, only by turning the hour axis by 180°; if the pier is hindering in one way, it can not in the other, and the telescope is free in this case.

As it is, with a telescope of high magnification, very difficult to point to an object and bring it into the field of view, usually a second smaller one is mounted with the axis exactly parallel to the larger one. The finder of the large refractor has 29 lines aperture and 30 inch focal length.

Each of the two main axes has an individual divided circle, named the hour and declination circle. These are fixed to their axes and turn with these. The division of the hour circle is 4 time seconds, the division of the declination circle 10 arc

[24]
seconds. With these one can (adjust) the telescope to stars which are off the meridian, and find and observe them by day, which, especially with stars of the 1st magnitude, may not be observed with  advantage by night.

The purpose of a large number of parts which were not mentioned here, could only be explained in an extended description.

                                Fraunhofer.


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1. 'Parisien Lines': 1 Line = 2.25 mm, 1 inch = 25 lines
2. inch = 25,4 mm (King in his 'History of the telescope', gives 14 feet for the focal length of the Dorpat refractor, which would equal 168 inches)

I like to thank Peter Abrahams for his help in translating this paper, his comments were very useful.
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