On the left we have the large crown singlet objective lens.
Light initially passes from left to right in the diagram, and then
encounters the much smaller flint element on the right. The rear surface
of the flint can be silvered or aluminized to return the rays (now moving from
right to left) back through the flint and on to focus near the objective.
An elliptical diagonal mirror (not pictured) would be needed--as in a
Newtonian reflector--to direct the final light beam out of the tube at right
angles, making it accessible to an eyepiece.
In order to understand what all this means, let us
go through the design steps more carefully, one by one using illustrations.
First, in regard to the objective Schupmann bent it into a shape which
would give exactly zero coma to the wavefront passing through it, as well as
minimal spherical aberration. Then he placed his "Mangin" corrector beyond
the prime focus. It can in principle go anywhere beyond focus, though
practical considerations impose limits. But by moving to beyond focus,
Schupmann could choose the curvature for his Mangin such that the light rays
would return back upon themselves and proceed back to the prime focus of the
objective. The key insight here is that any spherical mirror will image
point objects lying slightly to the side of its center of curvature (i.e.
off-axis) also as point images slightly to the other side of its center of
curvature. In other words, it will image coma-free off-axis objects as
coma-free off-axis images [cf. L. Schupmann, "Über Medial-Fernrohre von kurzer
Brennweite,"
33 (1933), pp. 308-312,
especially p. 310 for his explicit discussion of how this relates to the
Medial]. Now, since the objective also images without coma, the system
will be coma-free.
Second, by making the corrector of a very similar--or
even better, exactly identical--glass to the objective, it is possible to
correct perfectly for secondary spectrum. Schupmann originally choose a
light flint for his correctors because he wished to make the correctors very
small, from 1/8 to 1/12 this size of the objective. He appears to have
proposed that because he wished to retain the classic look of large observatory
refractors--something familiar and reassuring to astronomers--to which would be
added a modest appendage, namely his corrector, attached inconspicuously at a
right-angle to the bottom of the telescope tube in an "elbow" arrangement.
In a similar way, astronomers of the day attached micrometers or
plateholders or spectrographs. So, no radical threatening change in the
appearance or function of telescopes would occur by building Schupmann's
proposed design. But the disadvantage was that by making the corrector so
small, it needed great lens powers to do its job, which in turn made it
impossible to correct the singlet objective's spherical aberration,
if
the corrector were made of the same crown glass as the objective. By
choosing a light flint, and moreover by making the corrector a doublet,
Schupmann could retain his small appendage of a corrector, achromatizing the
objective and correcting for spherical aberration.
Unfortunately, this
version of the design did not work very well. Nor shall I illustrate it
for the moment. Let us instead first illustrate and discuss the better,
recent version of Schupmann's Medial called the "Super-Schupmann," and only then
talk about the history of the Medial from Schupmann's time until today,
illustrating that talk with a design approximating the earliest historically
important Medial: the 333mm f/15 Urania Observatory instrument completed
in Berlin in 1902.
For the present "Super Schupmann" design, I will
assume an objective and corrector both made of identical BK7 glass. Thus
the front surface of the Mangin mirror will be chosen--as in the Hamiltonian--so
that light traversing through it twice will acquire exactly enough negative
chromatic aberration to cancel that of the objective. The mirrored surface
of the Mangin will still cause the light rays to proceed back on themselves to
the original coma-free focus. And so, now we have a telescope which
focuses, and is free of coma and longitudinal chromatic aberration--more or
less. Unfortunately, in actual practice the system so far specified does
not quite work. But nevertheless, I will give a layout and analysis so
that the reader can see more clearly where we have come so far:
It is easy to see that on-axis the Medial does not give
excellent color correction, and off-axis it shows considerable lateral color, as
most dialytic systems do. Spherochromatism is not well corrected, and coma
is noticeable, varying by color. Schupmann escaped from this situation by
adding another lens element, which can either consist of a small convex singlet
lens or a concave mirror placed at the prime focus. What that element does
is ingenious.
As we saw in Chapter 5, the problem with dialytic systems
is that polychromatic chief rays entering the entrance pupil arrive at distant
correctors badly off-center, striking them obliquely and dispersing according to
their component colors. What needs to happen is somehow for the distance
between the separated elements to collapse to nothing, so that what are chief
rays for the objective still remain in essence chief rays for the corrector.
Unfortunately, in a dialyte that cannot easily happen as it can in an
ordinary doublet or triplet refractor, where the lens elements lie nearly on top
of one another. But Schupmann hit on a clever strategm, which cannot be
done in the Hamiltonian, the Brachymedial, or any of the designs shown in
Chapter 5.
What Schupmann did was to introduce a small relay lens
between the objective and the corrector, placed near their mutual focus.
The relay lens served to focus an image of the objective itself onto the
corrector, relaying as it were the former to the latter as though no distance
lay between the two (i.e. objective and corrector lie at two finite conjugate
points for the relay). By so doing, the relay effectively abolished the
separation, and the system behaved as if it were not dialytic. The result
is that lateral color is radically diminished, and in principle can be totally
abolished [cf. J.G. Baker, "The Catadioptric Refractor,"
Astronomical
Journal 59 (1954), pp. 74-83, especially p. 79]. Moreover, red rays
and blue rays from the axial image are brought much closer together, almost as
they were when they departed from the objective originally, making the job of
the Mangin corrector much easier. The Mangin itself is now placed further
from the objective and its resultant curves become such as to correct the
objective's spherical aberration. Since all colors of light now enter the
Mangin much closer together, there is less variation of spherical aberration
with wavelength. And likewise with coma.
At present most Medials
are built with relay mirrors rather than lenses. Thus, I give the layout
of a complete Super-Schupmann with relay mirror below:
 Layout 6:
150mm f/10 Super-Schupmann with Relay
Mirror |
Light enters from left, is
focused by the objective on to the very small relay mirror at extreme right, is
then reflected back to the Mangin mirror in the middle and is then re-reflected
back to the position of the original focus at extreme right. The
performance is as follows:
Figure 7: Ray Fan Plots
for 150mm f/10 Axially Centered Super-Schupmann
Figure 8:
Spot Diagrams for 150mm f/10 Axially Centered Super-Schupmann
Clearly, the imagery is now vastly improved, by far the best
we have ever seen. Only a negligeable amount of spherochromatism, coma,
and astigmatism now remain. Therefore, the spots are tiny compared with
the Airy disk, and the color correction is incomparable. There is no
lateral color. But alas, the system is unusable!
Unfortunately, the focus falls on the relay mirror and the
Mangin corrector obstructs the beam coming from the objective. Every
possible Medial design suffers from one or both of these problems when it is
used in an axially centered arrangement. Thus, in order to gain access to
the focus, we must tilt one or more components. In the above design we
must tilt the relay mirror in order to displace the corrector to the side of the
beam incoming from the objective. This first tilt is practically harmless.
But then in every Medial system a second, quite harmful tilt must occur:
namely, the corrector must be tilted slightly to displace the final focus
into an accessible position. For our Medial design, the final
configuration will look as follows:
 Layout 7: 150mm
f/10 Super-Schupmann with Tilted
Elements |
The arrangement of
elements is exactly as in Layout 6 above, but with the minimum practical tilts
applied, so that the final focus (on extreme right above relay mirror) will be
accessible for an eyepiece. These tilts make the Medial a TCT, or "Tilted
Component Telescope." Such telescopes do not form perfectly symmetrical
and round blur spots and have other peculiar properties. Fortunately, most
of the Super-Schupmann's image oddities are comparatively small and need not
detain us. The only problem to note is that now, because of the tilted
optics, the images show pronounced astigmatism:
Figure 9: Spots for 150mm
f/10 Tilted Super-Schupmann Medial
In order to compensate this problem, Schupmann recommeded
tilting the objective lens in a plane perpendicular to that of the tilts of the
field mirror and corrector. In other words, while the field mirror and
corrector are tilted in the y/z-plane, the objective should be tilted in the
x/z-plane (i.e. toward or away from the viewer). The tilt angle is small
and because the objective is coma-free, a small tilt in its orientation produces
only astigmatism, which can compensate the astigmatism produced by the tilt of
the corrector. As an alternative, James Baker suggested polishing a
slightly torroidal figure into the corrector itself. Either method can
work and both largely do the same thing to the wavefront. Using a small
objective tilt, we obtain the following ray fan plots and spot
diagrams:
Figure 10:
Ray Fan Plots for 150mm f/10 Super-Schupmann with Tilted
Objective
Figure 11: Spot
Diagrams for 150mm f/10 Super-Schupmann with Tilted Objective
An explanation of these peculiar diagrams is in order.
In both Figures 10 and 11, we have in the upper left corner the ray fan
plot and spot diagram for the field center. Here the Medial forms a very
good image for all colors. The other plots and diagrams show field
positions 0.1 degree off-axis in the four cardinal directions, along the x- and
y- coordinate axes specified in Chapter 1. Moreover, the image plane has
now been tilted about 2 degrees along the x-axis and 1 degree along the y-.
Image plane tilts are a common feature of TCTs. The plane of the
image is thus not quite square with the optical axis. Fortunately, the
tilts seen in the Medial are very small and of no practical importance.
Other TCTs can involve image plane tilts of 10 degrees or more. But,
it is necessary to include the tilts in our ray trace in order to establish the
Medial's ultimate imaging capabilities.
Because of the diverse
tilts in the design, we cannot expect symmetrical spots from a Medial and
clearly we do not get them. Even so, over a field of about 12 minutes of
arc we find essentially diffraction limited performance in a well-made and
collimated 150mm f/10 Super-Schupmann. Beyond that, astigmatism limits the
image quality. Yet, very likely a user of the above system would perceive
nothing but pin-point images over a 1 1/4" diameter field, since the astigmatism
in not large. So the above image errors should not be condemned. A
Medial is intended as a narrow-field high resolution instrument, not an
astrograph. It is far cheaper to build than a triplet apochromat, has
almost perfect color correction, and can eliminate atmospheric dispersion from
images of planets or stars lying at a low altitude by tilting the field mirror.
It can even act as a stellar spectroscope, since its field lens or mirror
can not only remove lateral color, but also
induce it! Schupmann
was well aware of that property in Medials and discussed how to optimize it [cf.
L. Schupmann, "Mechanische Einrichtung und Gebrauch der Medial-Fernrohre mit
einfachem Spiegel,"
Zeitschrift für Instrumentenkunde 41 (1921), pp.
253-258, especially, p. 255]. Thus, the Medial is a versatile instrument
indeed!
C. History of the Medial and the Brachymedial.It
will be appropriate now briefly to sketch the history of the Medial and
Brachymedial designs and their reception. As noted above, Schupmann began
his research into dialytic refractors by the early 1890s. In 1892, he
indicates, he had formed the plan of getting a Medial built [Schupmann,
Die
Medial-Fernrohre , p. 9]. Clearly, then by the same year he had
already developed the concept of the Medial. But the difficulties
involved, he notes, seemed worse than they actually were [p. 9]. So he
made other attempts with dialytes, calculating a diverse number and getting them
built [p. IV & 9]. Certainly by 1893 he had constructed a primitive
Brachymedial and observed through it in the fashion of a Herschelian [p. 4].
Yet in the end he came back to the idea of the Medial [p. 9]. By the
summer of 1897, he says, he had gotten built a Medial of 120mm aperture, 1500mm
focal length [f/12.5], and employing a corrector of 14mm diameter [p.
9].
At some point in the 1890s, he decided to write a book about the
design. Possibly this was after an incident in which he presented the idea
to Ernst Abbe, who remarked that actually, he himself ought to have come up with
it! [cf. C. Wolter & R. Merz, "The Neglected Schupmann Refractor,"
Sky
and Telescope (March, 1983), pp. 273-278, epecially p. 273.] At any
rate, write a book he did--an extremely thoughtful and thorough book of 20
chapters and 146 pages--getting it published in 1899 by the well-known scholarly
book publisher, B.G. Teubner, in Germany. The date coincided, of course,
with the many attempts in Germany, England, and America, to break through the
technological impasse at which professional astronomy had arrived in the
construction of "great telescopes" [cf. Chapters 4a and 4b above]. And
Schupmann's efforts must be seen in that context.
Despite Alvan Graham
Clark's confidence in 1893 that "great telescopes of the future" would be
achromats, the limitations of normal doublets were increasingly prohibitive [cf.
A.G. Clark, "Great Telescopes of the Future,"
Astronomy and Astro-Physics
12.8 (1893), pp. 673-678]. Already in 1879, C.S. Hastings had declared
that secondary spectrum "is positively obnoxious in the large instruments and
will speedily put an end to farther [sic] increase in dimensions" [cf. C.S.
Hastings, "On Triple Objectives with complete Color Correction,"
The American
Journal of Science and Arts, 3rd series, vol. 18 (1879), pp. 429-435,
especially p. 429]. Yet the increase continued until 1897 when the
Clarks--or rather their employees, the Lundins--finished the Yerkes 40".
At almost the same time, Max Pauly was working on his experimental
200mm doublet for Zeiss (cf. Chapter 4a), Albert König was working on the design
of his triplet (cf. Chapter 4b), Taylor was busily making large triplets up to
315mm (cf. Chapter 4b), and Ritchey was working on the improvement of
reflectors. No one could say for certain which way telescopes would go in
1899.
With hindsight it seems glaringly obvious that the reflector
would win the competition and become the standard large instrument. But
when Yerkes was new, the reflector was still bulky, awkward to use, and in need
of fresh silver coatings several times per year. Good reflectors
were small, large reflectors were useless for precision work. They seemed
impermanent, clumsy instruments, the "poor man's telescope" when he could not
afford a sleek achromatic model "from one of the finest makers."
This was the technological setting in which Schupmann
proposed his "New Construction for Large Astronomical Instruments." There
was no large, convenient, apochromatic telescope; every design contained major
flaws. But somehow everyone hoped to break the 1-meter barrier for
professional telescopes. Thus, Schupmann could seriously propose the
Brachymedial and Medial designs, each of which offered something new and useful:
the former relative compactness and large a aperture; the latter better
image correction at the expense of a standard tube length. Both offered
apochromatic axial color correction and apertures potentially in excess of 1
meter. Both were meant to be far more immune to image-damaging flexures
than the achromat or especially the reflector.
Schupmann got his first
chance to demonstrate a large Medial in 1901, when the Urania Observatory in
Berlin offered to let him convert their traditional 12" achromat to a Medial
[cf. K. Graff, "Ueber das auf der Uraniasternwarte in Berlin ausgeprobte
Medialfernrohr," Astronomische Nachrichten 158 (1902), pp. 279-282].
Schupmann calculated the design and the Munich firm of Reinfelder and
Hertel built the instrument. Dr. K. Graff reported on the results in the
widely read astronomy journal Astronomische Nachrichten. Graff's
report was honest, noting that the axial images were very sharp (chi Aquilae was
well split in rather poor seeing), the color correction was nearly ideal, and
that Mars gave "blameless images" with a surprising fullness of detail.
Yet the light-loss from so many extra surfaces (the Urania Observatory
Medial had 10 uncoated refractive surfaces and 2 reflective) noticeably dimmed
the image relative to the standard 12" achromat; and the usable field of view
was quite limited. There were also important difficulties in applying a
micrometer to the instrument. Yet, Graff hastens to add that his remarks
in no way constituted a detraction to the invention, since the deficiencies were
partly due to the inconvenience of having to adapt a new optical system to usual
tube of a refractor.
Alas, this was not an auspicious beginning to Schupmann's
hope of building large new instruments. Yet, the positive report
concerning axial image sharpness and excellent planetary detail was encouraging.
The problem which Schupmann was facing was that his design--quite
different from the later "Super-Schupmann" illustrated above--involved a very
small doublet corrector of 41.6mm clear aperture with highly curved surfaces to
correct an objective of 335mm diameter. Thus, the ratio in size of
corrector (or "compensator") to objective was 1:8. The design was more or
less identical to what Schupmann had proposed in his book. Schematically
it looked approximately as follows:
 Layout 8:
Urania Observatory 335mm f/15 Medial with "Elbow"
Corrector |
Clearly the corrector,
seen on the right above the prime focus for the objective, is far smaller in
relation to its objective than the corrector shown above in Layout 7 for the
Super-Schupmann. The right-angle deviation of the beam in Layout 8 is
accomplished by means of a right-angle prism cemented to the back of the field
lens. The next layout shows the prism-corrector end of the system in
greater detail:
 Layout 9:
Closeup of Elbow Arrangement in the Urania Observatory Medial of
1902, Showing the Field Lens plus the Right-Angle Prism,
Corrector, and Focal Plane |
Ray
bundles for three field positions are shown, giving three foci to define the
focal plane. In the case of this Medial, the tilt of the corrector has
been made very small, so that the ray bundles just barely pass the hypoteneuse
of the right-angle prism. That is precisely how Schupmann designed his
systems, and he makes a point of emphasizing it several times. Indeed, in
his final published article, he gives a marvelously drawn perspective
illustration showing the view into the back of the focuser tube of a Medial, and
it is plain to see that the prism extends into the upper 1/5 of the field of
view [cf. L. Schupmann, "Mechanische Einrichtung und Gebrauch der
Medial-Fernrohre mit einfachem Spiegel,"
Zeitschrift für
Instrumentenkunde 41 (1921), pp. 253-258, especially, p. 256, Figure 7; also
cf. pp. 254-255, where he discusses and illustrates the splitting in half of the
focuser tube to allow the eyepiece's axis and the prism's axis to coincide as
closely as possible; and cf. L. Schupmann, "Über Medial-Fernrohre von kurzer
Brennweite,"
Zeitschrift für Instrumentenkunde 33 (1913), pp. 308-312,
especially, p. 309-310].
A further closeup of the corrector doublet gives
an idea of what all the correctors in Schupmann's telescopes looked like during
his lifetime. Only in his later work did he explore the possibility of a
single-lens corrector and only after his death was the switch actuallly
made:
 Layout 10:
Corrector Doublet for Urania Observatory
Medial |
I will not attempt to simulate the spot diagrams and ray fan
plots for this instrument since my ZEMAX results are not fully convincing.
Certainly, the instrument had worse performance than that seen above in
the Super-Schupmann, showing some secondary spectrum (due to the light-flint
corrector) and enough off-axis image aberration to limit the field. K.
Graff's report, cited above, is sufficient testamony to the telescope's
advantages and limitations. Whether Schupmann calculated a special set of
eyepieces to be used with this telescope, I do not know.
In
his later instruments, Schupmann eased the ratio in diameter of the compensator
to the objective down to about 1:5 or 1:4. The present-day Super-Schupmann
further eases this to about 1:1.8. As James Baker noted in 1954, perhaps
to some extent Schupmann's Medial never really took hold because of "Schupmann's
zeal to overdo the duties of his compensator" [J.G. Baker, "The Catadioptric
Refractor,"
Astronomical Journal 59 (1954), pp. 74-83, especially p. 78].
As we have already seen many times on this web site, optics with highly
curved surfaces often bring in their train large residual aberrations,
difficulties of centration, and thermal problems as the glass cools down during
the night. Larger Medial compensators, involving as they do weaker curves
and simpler constructions, definitely tend to bring better results--so long as
the compensators are not made positively huge [cf. J.A. Daley,
Amateur
Construction of Schupmann Medial Telescopes (Privately Printed, 1984), pp.
14 & 24].
Schupmann had to wait a decade, it seems, for another
attempt at a large instrument. In the meantime, Hale and Ritchey had
perfected the large observatory reflector by building the 60" on Mt. Wilson.
This was deemed to be so good and so much in advance of conventional
refractors for astrophysical research and photography that it was now improbable
that any professional astronomer would want to construct a large experimental
catadioptric refractor for a new observatory. Nevertheless, for smaller
observatories and for amateurs desiring a substantial refractor for visual
studies of the planets and moon, a Medial might still possess
attractions.
Thus, around 1912 Philipp Fauth, the well-known German
amateur selenographer, asked Schupmann to design a 385mm diameter Medial at a
focal ratio of f/10 giving good definition over a comparatively large field for
his lunar studies. Schupmann took up the case with thanks, and sometime in
1913 [perhaps February or May, the chronology is confused] the instrument was
readied by well-known Munich firm of G. & S. Merz for Fauth's
observatory near Landstuhl, Germany [cf. L. Schupmann, "Das Medial-Fernrohr zu
Landstuhl," Astronomische Nachrichten 196 (1913), pp. 101-106; and "Über
Medial-Fernrohre von kurzer Brennweite," Zeitschrift fü r
Instrumentenkunde 33 (1913), pp. 308-312; also H. Fauth, "Philipp Fauth and
the Moon," Sky and Telescope 19 (November, 1959), pp. 20-24, where
Fauth's son Hermann gives a date of 1911 for the Medial].
In order to
achieve Fauth's goal of f/10, Schupmann abandoned his tiny corrector design and
settled on a corrector/objective ratio of 1:5.4. Still using a doublet
corrector, but this time made of normal crown glass instead of the light flint
he had been forced to adopt for the Urania Observatory Medial, he could correct
both spherical aberration and coma, and achieve complete color correction.
Schupmann mentions in his 1913 Astronomische Nachrichten article
cited above that if an ordinary flint glass were used for Fauth's instrument
(say, F2 or F4), and a corrector/objective ratio of 1:4.2 were selected, it
would be possible to make a single-element corrector. Unfortunately,
however, the instrument would again have residual secondary spectrum equivalent
to that seen in a 385mm achromat operating at f/25.4. On the other hand,
by suitably increasing the radius of curvature of the field lens and designing
special eyepieces, it would be possible, he notes, to decrease that secondary
spectrum by 1/2 [cf. L. Schupman, Astronomische Nachrichten (1913), p.
104].
Thus was Schupmann constantly thinking and striving for
improvements in his Medial, and willing to consider even how the eyepieces might
be pressed into service to correct the whole optical system. Alas, that
also tended to condemn his instruments as specialized designs ill-suited to the
diverse needs of professional astronomers. Ritchey's reflectors were
constantly used at several focal ratios by inserting of various Cassegrainian
secondary mirrors, could take wide-angle prime-focus astrophotos or high
dispersion spectra, had convenient, stable mountings, and could be scaled up to
at least 5 meters diameter. It was hopeless to think that Schupmann's
Medials could compete with that.
In any case, Fauth's 385mm f/10
instrument (the geometry was similar to Layout 8 above) was completed and used
with great success for many years [cf. H. Fauth, "Philipp Fauth and the Moon,"
Sky and Telescope 19 (November, 1959), p. 22 for a photograph of the
instrument]. It was the largest Medial in the world until it was destroyed
at the end of the Second World War [cf. C. Wolter & R. Merz, "The
Neglected Schupmann Refractor," Sky and Telescope (March, 1983), pp.
273-278, epecially p. 273].
One feature of his Medial designs
which especially pleased Schupmann was their lightness. Fauth's Medial of
approximately 385mm aperture was carried on the mounting of a 175mm ordinary
refractor. In a report to Schupmann, Fauth states that the smaller dome
needed for his compact instrument produced noteworthy cost savings in its
construction and faster equilibration of the air, in addition to making the dome
easier to handle with no complexities in its motions. A simple handgear,
it seems, was enough to turn the dome. Fauth claims that over 75% of the
total construction cost was spent on the optical apparatus, a notable saving in
comparison with large 19th century achromats [cf. L. Schupmann, "Das
Medial-Fernrohr zu Landstuhl," Astronomische Nachrichten 196 (1913), pp.
101-106, especially p. 106].
The disruptions of the First World
War seem to have abstructed further building of Medials and ended Schupmann's
publications about them, despite Fauth's very positive report concerning his
385mm instrument. Near the end of the war in 1917, Professor Anton Staus
did order another Medial of 325mm aperture, and Schupmann appears to refer to
this instrument in his last article published after his death in 1920.
Schupmann speaks of it as half finished and hindered from completion first
by the war, and then by the post-war inflation which ruined Germany's economy.
G. & S. Merz, however, somehow managed to deliver the instrument and
in the hyperinflationary year of 1923, Max Mündler compared its performance to
the conventional 300mm Steinheil achromat of the Heidelberg Obervatory. He
seems to have been convinced that the Medial was of very high quality. In
recent years this Medial has been moved to Stuttgart and is apparently now the
oldest Medial in existence [cf. Wolter and Mertz, pp. 273-276 ; &
http://home.arcor.de/maranelli/astro/teleskope.html].
When Schupmann died
in 1920, he left behind two papers which were published a year later in the
journal Zeitschrift für Instrumentenkunde. The first paper dealt
with the design and calculation of Medials employing a single-element flint
corrector [cf. L. Schupmann, "Berechnung der Medial-Fernrohre mit einfacher
Spiegellinse," Zeitschrift für Instrumentenkunde 41 (1921), pp.
212-219]. As noted above, the idea of shifting from two-elements to one
had occurred to Schupmann while working on Fauth's Medial, since as Schupmann
gradually let go the idea of making tiny unobtrusive correctors, he realized
that he could also answer the complaint that too much light was lost by passage
through so many surfaces. At a stroke, he could save about 16-20% of the
light by dropping the first refractive element of his uncoated doublet
corrector. Unfortunately, this first paper published in 1921 was obviously
never finished and is missing an illustration directly referred to in its
opening paragraph. It also apparently contains several substantial typos,
and the whole is somewhat difficult to follow since it largely consists of
equations and calculations. Nevertheless, the idea contained within it is
clear and nowadays, it is not hard to model a similar type of instrument using
modern ray-tracing software.
Schupmann's second 1921 paper is complete
and deals admirably and in detail with the mechanical construction of a Medial
employing a single-lens corrector, as well as with questions of collimation, and
with what had become a rather anxious problem for Schupmann since the Urania
Observatory Medial, namely how best to use a filar micrometer with his
instruments [cf. L. Schupmann, "Mechanische Einrichtung und Gebrauch der
Medial-Fernrohre mit einfachem Spiegel," Zeitschrift für
Instrumentenkunde 41 (1921), pp. 253-258]. Schupmann had from the time
of his first book honestly tried to evaluate and improve the deficiencies of his
designs, and gradually he had come to realize that making accurate micrometric
measurements in a Medial was not a simple matter. One problem was the
image plane tilt which could cause a parallax error if the user were not
careful, since the micrometer webs would not lie parallel to the image plane.
And another problem was image distortion over the field, which precluded
use of the Medial to measure stellar parallaxes.
The solutions he
proposed may be valid, but they involved extra inconvenience and sources of
error when astronomers want just the opposite. One radical solution
involved making micrometric measurements at the uncorrected prime focus.
But that would be hard to achieve in practice because of the difficulties
of inserting (and repairing) the spider webs and mechanism to move them inside
the main tube of the telescope. Moreover, chromatic effects at the prime
focus would be severe, as Schupmann realized. Nevertheless, he apparently
never gave up hope that he could still make his clever Medial system a
scientific as well as an optical success.
Once Schupmann was
gone, the next steps forward were taken by others. Anton Kutter, who later
devised the Schiefspiegler and Tri-Schiefspiegler TCTs, used Fauth's Medial from
1932 onward, according to James A. Daley. Daley indicates on an internet
web site that he corresponded with Kutter, who in 1971 gave him unique
information about Schupmann's life [cf.
http://www.stellafane.com/schupmann/ludwig_schupmann.html]. In 1984, Daley
also privately published a highly useful booklet entitled, Amateur
Construction of Schupmann Medial Telescopes , which contains much excellent
information, complete instructions for designing and building the so-called
"Super-Schupmann" form of Medial, and a useful historical summary of the
development of Medials in recent decades. I owe key details in the
following paragraphs to Daley's two accounts.
According to Daley, Kutter
constructed two replicas of Fauth's Medial, the first of 122mm aperture in 1936,
and the second of 270mm aperture in 1938. The latter instrument was
destroyed during the Second World War. Then in 1946, Kutter built a third
instrument of 150mm and novel design arrangement meant to enclose the light path
fully inside a single telescope tube [cf. J. A. Daley, Amateur Construction
of Schupmann Medial Telescopes (Privately Printed, 1984), pp. 4-5; &
http://www.stellafane.com/schupmann/ludwig_schupmann.html for a photograph of
Kutter's instrument]. This instrument was the first Medial to use a
single-element corrector of flint.
Until 1941 there was apparently
no knowledge of Schupmann and his designs in the United States, when Carl A.
Hellman published a short article in the popular astronomy magazine, The
Sky , about the Brachymedial. Hellman termed the design, "the
Schupmann Telescope," apparently in ignorance of Schupmann's book and extensive
publications about the Medial proper. It seems that Hellman was working
from the US Patent [no. 620,978] which Schupmann had taken out in 1899 regarding
both of his designs. Hellman considered the Brachymedial as a
possibly interesting project for an ATM to build and constructed a 108mm
specimen for himself [cf. C.A. Hellman, "The Schupmann Telescope," The
Sky 5.2 (September, 1941), pp. 14-15]. Thus began the US amateur interest in
making Medial telescopes.
One and one-half years later in 1943, Albert G.
Ingalls, the well-known proponent of amateur telescope making in America took an
interest in Hellman's article and wrote about the Brachymedial himself in his
long-running column "Telescoptics," published in Scientific American
magazine. Not only did Ingalls obtain a copy of the US Patent and publish
some of its details, but he discussed the amateur construction of a 200mm
Brachymedial by Joseph Dwight in Massachusetts during 1942 [A.G. Ingalls,
"Telescoptics," Scientific American (April, 1942), pp. 191-192].
After the Second World War, American interest began to pick up.
A.G. Ingalls again reported on the Brachymedial in 1947, quoting
extensively from a letter by Dwight about his completed instrument [A.G.
Ingalls, "Telescoptics," Scientific Amercian (August, 1947), pp. 93-96].
Up to this time, the American ATM community seems still to have been
ignorant of Schupmann's publications and the past existence of three large
Medials and several smaller ones in Europe. Then in 1954 the situation
changed completely, when James G. Baker published an extensive review article in
The Astronomical Journal about Schupmann's dialytic method of correcting
aberrations. Baker discovered and read Schupmann's publications and
naturally understood the intricasies of his optical work. He again brought
that work to the attention of professional astronomers and optical designers,
giving a correct account of Schupmann's telescopes [cf. J.G. Baker, "The
Catadioptric Refractor," The Astronomical Journal 59 (1954), pp.
74-83].
Baker tried to interest professional
astronomers with the possibility of turning their large old achromats into far
more efficient apochromats by means of a modified crown-flint corrector.
He threw out Schupmann's conservative notion that the corrector should be
mounted in an elbow arrangement at the bottom of a standard refractor tube, and
therefore also the necessity of a small corrector. He advocated large
correctors having ratios from 1:3 even up to even 1:1 with their objectives.
The field lens plus right-angle prism could be replaced with a field
mirror when convenient or with a lens and specially shaped prism to send the
light back up the tube but displaced to the side of the main beam [cf. Baker,
Astronomical Journal 59 (1954), pp. 75-78].
Baker proposed and
began the building of a 735mm Medial to be sited at a high-elevation site in the
American Southwest [cf. Baker, Astronomical Journal 59 (1954), pp.
81-83]. And he seems to have persuaded Joseph H. Rush of the Medial's
capability as a coronagraph. In 1964, the latter reported the completion
of a new coronograph and spectrograph employing Schupmann's dialytic principle,
sited at the High Altitude Observatory in Climax, Colorado [cf. J.H. Rush &
G.K.Schnable, "High Altitude Observatory's New Coronograph and Spectrograph,"
Applied Optics 3.12 (1964), pp. 1347-1352]. Rush also published for
the first time a correct description of the Brachymedial (mistakenly called by
him the "Brachyt") as well as the Medial in Scientific American in 1958
[cf. J.H. Rush in C.L. Strong, "The Amateur Scientist," Scientific
American (May, 1958), pp. 130-138].
Baker also interested
telescope makers in the Boston area with the building of Medials. He
reported in 1954 that Chester Cook had completed a 200mm telescope, and James
Gagan and Richard Dunn were nearing completion with a 400mm [cf. Astronomical
Journal 59 (1954), pp. 78-79]. Four years later Rush reported that the
latter had been completed and had given fine results [cf. J.H. Rush in C.L.
Strong, "The Amateur Scientist," Scientific American (May, 1958), p.
138].
Then in November 1959, Hermann Fauth published a short article in
the popular astronomy magazine, Sky and Telescope , about his father's
lunar work, illustrated with a picture of the old 385mm Medial and with stunning
examples of his father's moon drawings [cf. H. Fauth, "Philipp Fauth and the
Moon," Sky and Telescope 19.1 (1959), pp. 20-24].
According to
James Daley, the ATM Edwin Olson became interested in Medials after reading
James G. Baker's Astronomical Journal article [cf. J.A. Daley, Amateur
Construction of Schupmann Medial Telescopes (Privately Printed, 1984), p.
7]. Olson through the offices of Baker contacted Cook, who supplied plans
for his own 200mm Medial. Olson then modified Cook's prescription and
constructed a 150mm f/15 Medial which was the first to employ a field mirror and
a full-sized corrector [cf. Daley, p. 7; &
http://www.stellafane.com/schupmann/ludwig_schupmann.html for a photograph].
This instrument led to the formation of a Boston, Massachusetts area
amateur club devoted to the development and popularization of a Medial design
suitable for amateur construction.
The single most important
achievement of this "Schupmann Club" occurred in 1969, when Cliff Ashcraft
discovered that it was possible to build an aplanatic Medial employing a singlet
corrector of the same type of glass as the objective, and utilizing only
spherical optical surfaces. As we have seen, Schupmann himself in his
later articles had demonstrated that an all-spherical aplanatic design was
possible, using a singlet of ordinary flint for the corrector [cf. L. Schupmann,
"Über Medial-Fernrohre von kurzer Brennweite," Zeitschrift für
Instrumentenkunde 33 (1913), pp. 308-312, especially, p. 309; &
"Berechnung der Medial-Fernrohre mit einfacher Spiegellinse," Zeitschrift
für Instrumentenkunde 41 (1921), pp. 212-219]. But Ashcraft, by
exploring the spherical aberration residuals which occurred when the size of a
crown corrector was allowed to vary, discovered that residual spherical goes to
zero when the ratio of corrector to objective size is about 1:1.86 [cf. Daley,
Amateur Construction of Schupmann Medial Telescopes (Privately Printed,
1984), p. 27-32]. Very likely if Schupmann himself had lived long enough,
he would have come to the same conclusion. In honor of Schupmann, Ashcraft
dubbed this version of the Medial, the "Super-Schupmann." It is the design
which I discussed and analyzed first in this chapter.
Many
successful examples of Super-Schupmanns have been built in the last 30 years.
Currently the largest amateur version exists in Vermont, USA at Stellafane
[cf. http://www.stellafane.com/schupmann/schupmann.html]. This has an
aperture of 330mm, making it marginally larger than the 1917 Heidelberg Medial
[cf. http://home.arcor.de/maranelli/astro/teleskope.html]. Among ecclectic
Medial designs, James Daley himself has recently completed a instrument with a
split-field arrangement, requiring no element tilts. This is a design type
originally suggested by James Baker in his 1954 Astronomcial Journal
article [cf. http://www.stellafane.com/images/scope_gallery/daley.html].
But the largest Medial system ever built is apparently the solar
telescope of the Royal Swedish Academy of Sciences in La Palma. This has
an aperture of 970mm [cf. http://www.astro.su.se/groups/solar/NSST/], making it
comparable in size with the instruments which Schupmann had hoped to build 100
years ago. And the glorious solar images it gives would make Schupmann cry
with joy!
That completes our brief tour of Medial history. By
contast with all this activity building Medials, the Brachymedial has remained
practically stillborn. No large example was built during Schupmann's life,
and I have only seen a handfull of reports of Brachymedials ever being built.
The first was Schupmann's own 1893 model, used in a Herschelian
arrangement; the second was Hellman's experimental 108mm model in 1941; and the
third was Dwight's 200mm model in 1942. And Rolf Riekher reported that
about 1960 the engineer Edwin Rolf, supported by the German Academy of Sciences
in Berlin, had built a 700mm Brachymedial in Rathenow, Germany [cf. R. Riekher,
Fernrohre und ihre Meister, 2nd ed. (Verlag Technik, 1990), p. 235].
This last was obviously the largest Brachymedial ever built. What
may have happened to it, I do not know. I myself hope to build a 250mm
f/12 Brachymedial with doublet objective and corrector.
And thus, we have
covered nearly every significant type of refractive system for visual
observation, from the simple cemented doublet achromats, to their air-spaced
cousins, to the oil-spaced triplet apochromats with their superb color and field
corrections, to the Petzval and sub-aperture color correctors, and finally to
the Medial systems. It is a vast world of possibilities, many of which are
indeed open to amateur telescope makers to build. They have only to
acquire the knowledge of how these designs work and what can reasonably be
expected from each one. The grinding and polishing are obstacles, but not
insurmountable. Many sources of help, guidance, and encouragement now
exist with the worldwide dissemination of the internet, email, and telescope
making lists.
Layout 2:
150mm f/17 Hamiltonian with Doublet Objective and
Corrector
