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
[19]
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
[23]
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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