Chapter 6:  Schupmann's Medial Telescopes.

Ludwig Schupmann's two "Medial Telescopes" occupy a unique position in the range of telescope designs.  They are both "catadioptric" (i.e. mirror plus lens) arrangements, and constitute the only major catadioptric designs for telescopes to use refraction, rather than reflection, as their principal source of optical power.  The catadioptrics with which amateur astronomers are most familiar, namely the Schmidt-Cassegrain and the various Maksutovs, all derive their optical power from mirrors, and use lenses or aspheric plates to correct aberrations.  But the Medials do the opposite, depending on a large objective lens initially to focus the light, and then a len/mirror combination--we may loosely call it a "Mangin" mirror--to correct aberrations and bring the light to a final focus.  It is this use of a large objective lens in combination with a smaller concave mirror to make a hybrid instrument which suggested to Schupmann the name "Medial," meaning "intermediate" between a pure refractor and a pure reflector.

Ludwig Schupmann was a professor at the Technical University in Aachen, Germany, and he developed his designs in the 1890s, publishing a detailed monograph about them in 1899, entitled The Medial-Telescopes:  A New Design for Large Astronomical Instruments [cf. L. Schupmann, Die Medial-Fernrohre.  Eine neue Konstruktion f ü r grosse astronomische Instrumente (B.G. Teubner, 1899)], principally to overcome the problem of secondary spectrum in large observatory refractors .  Whereas Schott and Abbe had tried to develop new types of glass to solve the problem, and Taylor developed his famous triplet, Schupmann struck out in novel design direction.  

As Schupmann explains in the preface to his book (pp. III-IV), all telescopes up till then had either been refractors or reflectors.  Reflectors have the advantage that they are color-free; possess relatively fast focal ratios; and are relatively cheap to build in moderate sizes.  Their disadvantages are the impermanence of their polished surfaces (i.e. metallic coatings); rather inconvenient mountings; need for very precise figuring; and flexures which are too great to allow instruments over 600mm in aperture to make the most precise observatiions [This last statement was largely true before G.W. Ritchey solved the problem of mounting large telescopes and produced the 60" reflector on Mt. Wilson in 1908:  cf. H. King, History of the Telescope (Dover reprint, 1979), pp.327-332; R. Riekher, Fernrohre und ihre Meister, 2nd ed. (Verlag Technik, 1990), pp. 269-276].  Refractors have the advantage that their mountings make them convenient to use; that they possess great image sharpness; and that they are dependable for measurements.  Their disadvantages are their secondary spectrum; very long tubes; and high cost to construct.

Attempts to ameliorate the situation, Schupmann suggests, have so far not succeeded:  the lack of homogeneity and instability of the many new Schott glasses, as well as the insufficiently different dispersions and strong resultant curves make large doublet apochromats unlikely to be built.  Similarly, triplet apochromats need strong curves and rather large airspaces, making them very sensitive to centration errors and thermal effects.  As for dialytes, Schupmann says that he designed a diverse number and got examples of them built.  Those that decreased secondary spectrum noticeably required many glass elements and the use of dense flints.  The yellowish coloration of these flints, as well as their instability, posed a stumbling block to building large systems.

Having thus summarized the state of telescope making in 1899, Schupmann then commenced the main body of his book and described in great detail his two new catadioptric systems, showing how to calculate them precisely and recounting their advantages and disadvantages.  He takes the reader, in the first chapter of his book (pp. 1-6), on a thought experiment leading from a conventional doublet achromat to his own prefered new designs.  Along the way, there are four stages, each with historical precedents.  Schupmann begins with the common achromatic doublet.  On the first stage, he tells of the advantages gained by dispensing with the large, heavy disk of flint glass, replacing it with Rogers' dialytic doublet, which we discussed in Chapter 5 above (cf. Layout 1).  Not only can we lighten the objective lens considerably in this way, we can also profoundly weaken its curves, thereby simultaneously weakening the monochromatic aberrations caused by the more highly curved surfaces necessary in an ordinary achromat, and making the wavefront passing through the lens less subject to errors arising through flexure of its surfaces.

Schupmann explicitly mentions Rogers as the source of this dialytic telescope.  In the second stage, Schupmann suggests that it would be even more advantageous to rid Rogers' dialytic corrector of its small crown element, as J.J. von Littrow had earlier tried to do (cf. Chapter 5 above).  By eliminating this small crown, we can considerably weaken the curves on the remaining flint element.  Unfortunately, eliminating the small crown also has the effect of making the instrument afocal, since the negative powers of the sub-aperture flint necessary to achromatize the full-sized crown objective must be stronger than those needed in a full-sized flint.  Thus in order to achromatize, the small flint will now cause light to diverge on its way out of the telescope.  

To overcome the problem of divergence, Schupmann takes us to his third stage.  If we now add a concave mirror behind the flint, we can reconverge the light and make the flint even weaker than in stage 2, since light will now pass through the flint twice on the way to focus.  Indeed, this arrangement has the valuable property that its residual secondary spectrum is only about 1/3 as bad as in an equivalent achromat, since the full-sized crown objective is now also so much weaker in power than the crown element of an equivalent doublet achromat.  The telescope's tube length can also be much shorter, since the mirror reflects light back up toward the objective.  As much as 1/2 the tube length can be saved.  Of course, one will need an elliptical flat or right-angle prism to redirect the light outside the tube, as in a Newtonian.  But the smaller mounting and dome size necessary to house the instrument could yield significant savings in construction costs, making the slight inconvenience of a secondary mirror worthwhile. 

This type of compact Medial Schupmann christened the "Brachymedial (p. 6)," brachys being the ancient Greek word for "short."  One must not confuse the Brachymedial with the "Brachyteleskop" (or "Brachyt" for short), as is sometimes done in the literature on Medials.  The Brachyt was a form of unobstructed Cassegrain, first produced around 1876 by the Viennese firm of Forster & Fritsch [cf. L. Ambronn, Handbuch der astronomischen Instrumentenkunde, vol. 1 (Springer, 1899), pp. 377-379; A. Danjon & A. Couder, Lunettes et Té lescopes (Paris, 1935), pp. 249-251; R. Riekher, Fernrohre und ihre Meister, 2nd ed. (Verlag Technik, 1990), pp. 235].  In the 20th century, the Brachyt led to the modified Neo-Brachyt and to Anton Kutter's Schiefspiegler, both meant as improvements of the original Brachyt.  But none of these reflectors has anything to do with Schupmann's obstructed, catadioptric Brachymedial.

A.  The Hamiltonian and the Brachymedial
.
Schupmann does not mention, and may not have known, that his Brachymedial had a predecessor.  A similiar form of telescope had been proposed in 1814 by W.Fr. Hamilton who may have been attempting to overcome the problem which Alexander Rogers also faced (cf. Chapter 5 above), namely how to build a large achromat when only small pieces of homogeneous flint glass were available.  Hamilton patented his catadioptric design, but apparently published nothing in detail about it [English Patent 3781 v.12.4.1814:  cf. S. Czapski & O. Eppenstein, Grundzü ge der Theorie der optischen Instrumente nach Abbe , 3rd ed. (Verlag J.A. Barth, 1924), p. 572; Riekher, pp. 232-233 & p. 236; and R.N. Wilson, Reflecting Telescope Optics , vol. 1 (Springer, 2000), p. 212-213; & p. 486, n. 3.68].

Hamilton's design looked approximately as follows:

Hamiltonian
Layout 1:  150mm f/15 Hamiltonian Catadioptric Refractor

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.  

The design of the above Hamiltonian is as follows:

Surface
Type
Radius
Thickness
Glass
Diameter
Conic
Object
Standard
Infinity
Infinity

0
0
Stop
Standard
1047.33
15
BK7
160
0
2
Standard
8610.24
1125

160
0
3
Standard
-782.489
10
F2
104
0
4
Standard
-2023.428
0
Mirror
104
0
5
Standard
-2023.428
-10
F2
104
0
6
Standard
-782.489
-1125

104
0
7
Standard
Infinity
0

39.987
0
Image
Standard
Infinity


39.987
0

Table 1:  150mm f/15 Hamiltonian Catadioptric Refractor

Notice that in this prescription for the first time we have large negative distances shown in the "Thickness" column.  This is because by convention in ray tracing, when light travels from right to left it moves in a negative direction as explained in Chapter 1.  The "MIRROR" element is shown as having zero thickness because light does not travel through it, and it is not separated from the F2 flint element.  It merely consists of a coating attached directly to the rear surface of the F2.  Since no refraction occurs at that surface (there being no change of medium), but only a reflection, the radius of curvature "-2023.428" attached to the rear surface of the F2 has no meaning.  Any number could be inserted there with no effect in ZEMAX.  I have elected to repeat the radiius of the mirror surface simply because it is a convenient place-filler in Table 1 above.

The Hamiltonian's performance is as follows:


Hamiltonian Ray Fan
Figure 1:  Ray Fan Plots for 150mm f/15 Hamiltonian Catadioptric Achromat


Spot for Hamiltonian
Figure 2:  Axial Spot Diagram for 150mm f/15 Hamiltonian Catadioptric Achromat

The axial chromatic correction is much better than in a conventional 150mm f/15 achromat.  Notice in Figure 2 the size of the red and blue color blurs in relation to the Airy disk.  By choosing the optical glass for the corrector carefully (for example, by using K10 crown with a BK7 objective), it is possible to obtain a superachromatic type of axial color correction.  But on the other hand, Figure 1 (right) shows the terrible primary lateral color for a star only 1/2 degree off-axis.  Just as in a Rogers or Plössl-type dialyte, all stars off-axis would be spread into obvious spectra.  Special eyepieces designed with countervailing lateral color would be needed to overcome the problem.

Schupmann's Brachymedial is similar, except that it makes the flint corrector a separate optic from the mirror, thus adding an another degree of freedom to the optical system.   Schupmann found that by spacing the flint corrector further away from the objective, he could obtain apochromatic correction with flint [cf, Schupmann, Die Medial-Fernrohre, p. 7].  But primary lateral color remains.  Schupmann accepted that and for the sake of the light compact optics, good axial color correction, and large potential gain in aperture, he proposed to design the special eyepieces.  In fact, even in the case of his Medial telescope, which when properly designed shows no lateral color, Schupmann conceived of the optical system from objective to eyepieces as one integral whole.  Thus, in his book he explicitly discussed how to design the eyepieces in order to optimize the visual imagery in both his Brachymedial and his Medial [cf. Schupmann, pp. 46-53].  He also discussed compensations inherent in his Medial to overcome production errors in the optics or flexures during use [pp. 54-65].  In addition, he analyzed the light through-put of his Medial [pp. 84-96]; its ghost images [pp. 106-109]; as well as mountings and questions of production [pp. 110-125].  In the case of the Brachymedial, he discussed several different possible configurations, showing how the light train can be sent down the declination axis (with the counterweights moved to the side) for convenient observation [cf. Schupmann, pp. 126-14]), thus antecipating Russell W. Porter's "Springfield" mounting by 20 years [cf. Riekher, p. 235 & 252].

It is not reasonable to complain that Schupmann should not have considered the Brachymedial design because of its lateral color problem.  The Brachymedial was not Schupmann's preference, yet he recognized that within the context of late 19th century scientific needs, the Brachymedial gave a possible optical system which would allow substantially larger refractors to be built than the 40" Yerkes, then brand new.  We must remember that Schupmann was not interested in small amateur instruments, but in objectives larger than 1 meter in diameter.  And the large observatory reflector was not yet a reality.  Thus, the Brachymedial seemed a viable option in 1899.  

It remains so even today for an amateur astronomer.  Although few amateur telescope makers would choose to construct a special set of eyepieces to be used with just one instrument, a simple modification of the Hamiltonian makes possible the building of a large apochromat from common crowns and flints, packaged in a relatively compact form which could easily be built by the dedicated ATM.  If we make our objective AND our corrector both achromatic doublets, we can radically reduce the lateral color, turning it into a small secondary lateral.  The following is a design which could be built by an ATM.  It is a 150mm f/17 achromatized Hamiltonian, which would fit into a tube only about 1200mm long (about 48").  One would view through the telescope as through a Newtonian, by means of a small diagonal mirror.  That diagonal could have a minor axis as small as 15mm, giving a mere 10% obstruction ratio, which would exert almost no effect on the diffraction image:

Achromatized Hamiltonian Layout 2:  150mm f/17 Hamiltonian with Doublet Objective and Corrector


The prescription is as follows:

Surface
Type
Radius
Thickness
Glass
Diameter
Conic
Object
Standard
Infinity
Infinity

0
0
Stop
Standard
654.788
15
BK7
160
0
2
Standard
-2463.763
0.238

160
0
3
Standard
-2227.495
10
F2
160
0
4
Standard
2205.443
1125

160
0
5
Standard
-331.410
10
F2
100
0
6
Standard
-210.301
10
BK7
100
0
7
Standard
-1170.952
0
MIRROR
100
0
8
Standard
-1170.952
-10
BK7
100
0
9
Standard
-210.301
-10
F2
100
0
10
Standard
-331.410
-1215

100
0
Image
Standard
Infinity


22.246
0

Table 2:  150mm f/17 Hamiltonian with Doublet Objective and Corrector

The performance is as follows:

Achromatized Hamilton Ray Fan
Figure 3:  Ray Fan Plots for 150mm f/17 Hamiltonian
with Doublet Objective and Corrector


Achromatized Hamilton Spots
Figure 4:  Spot Diagrams for 150mm f/17 Hamiltonian
with Doublet Objective and Corrector

The performance is extremely good over a 1/2 degree field.  On-axis, as the spot diagram in Figure 4 shows, there is essentially superachromatic performance--using only common glass types.  Off-axis at 1/4 degree, the lateral color is still small enough to fit into the Airy disk.  The field is fully flat.  Construction would not be too difficult:  the first surface of the objective is admittedly rather strong as is the first surface of the corrector; but they are less strongly curved then a Maksutov shell, putting them within reach of amateurs.  The rest of the objective's surfaces are weak, occasioning no problem; the two elements of the corrector are oil-spaced; and a silver or aluminum coating can be applied directly to the back of the BK7 element.  Colllimation will pose some challenges, but not as much as in a Schiefspiegler.

Other advantages are that the diagonal mirror can be mounted directly to the back of the objective, avoiding diffraction spikes.  The tube will be sealed, avoiding many thermal problems associated with open-tubed telescopes.  The focal ratio is high, resulting in a comfortable depth of focus and good eyepiece performance.  And the light cone quickly draws away from the tube walls which promotes image stability.  Thus, while the design is not as trouble-free as a long-focus oil-spaced triplet apochromat, it gives excellent axial performance using much cheaper glasses and is very compact for its focal ratio.  It can easily be scaled up to larger versions.  I have designed a 250mm f/12 version, which employs a 150mm fused silica/F4 corrector, and gives just as excellent on-axis performance as the above design.  Off-axis at 1/4 degree, the F-, e-, and C-lines still focus within the Airy disk, though the barely visible g- and r- fall just beyond it.  Such a system could give a careful user access in effect to a 250mm superachromat in a 1500mm long tube at a comparatively modest price.

B.  The "Super-Schupmann."
But to return to the Medial telescopes, the fourth stage in Schupmann's thought experiment was to move the singlet corrector of his Brachymedial to beyond the focus of his singlet objective.  This placed the corrector in the objective's light cone where it was again expanding.  Then Schupmann brought on his stroke of genius:  he placed a field lens at the focus of the objective.  This field lens acted as a relay projecting an image of the objective on to the corrector, allowing for lateral color to be removed.

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," Zeitschrift für Instrumentenkunde 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:


Layout 1
 
Layout 3:   Basic Design of a 150mm f/10 Medial Telescope
 
The light enters from the left in a collimated beam, encountering the objective and converging to a focus.  It then diverges to the right until encountering the Mangin mirror, where it passes through the front transparent surface and is reflected by the rear silvered surface.  Finally it travels back from right to left, passing again through the front Mangin surface and proceeding to the final focus, which coincides in position with the objective's original focus.

To give a better idea of the shapes of the lenses, the following are closeups:

Layout 2
Layout 4:  Biconvex
Objective Lens
Layout 3
Layout 5: Meniscus
Corrector Lens
 

The performance of the system as so far constructed is the following:

Ray Fan 1
Figure 5:  Ran Fan Plots for Partial 150mm f/10 Super-Schupmann


Spot Diagram 1
Figure 6:  Spot Diagrams for Partial 150mm f10 Super-Schupmann

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 4

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:

Ray Fan 2
Figure 7:  Ray Fan Plots for 150mm f/10 Axially Centered Super-Schupmann


Spot Diagram 2
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 with Tilted Elements
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:

Spot Diagram 3
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:

Ray Fan with Tilted Objective
 
Figure 10:  Ray Fan Plots for 150mm f/10 Super-Schupmann with Tilted Objective


Spot Diagrams 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 of Uraniasternwarte Medial
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:

Elbow Arrangement of Uraniasternwarte Medial
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:

Corrector Doublet for a Medial
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.

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