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Since the effective injurious coma is only about 0,32 of the calculated theoretical value, i.e.¹, 4,8″.0,32 = 1,54″, the anastigmatic device of the Schiefspiegler is capable for instruments up to 2 1/2″ aperture. By slightly diminishing of φ₂, relatively Δ′ ( about 15%), the anastigmatic device with spherical secondary becomes likewise capable for an aperture of 4". By this step the inclination of the primary φ₁ will be likewise diminished and a slight silhouetting on the edge of the primary will occur ( about 2-3% of its surface ). That e d g e -silhouetting will, contrary to a central silhouetting, do no harm. Fig. 3 shows a 4″ (110 mm) Schiefspiegler of anastigmatic device with spherical primary and secondary². Its definition being magni- ficent, this reflector is specific for the juniors under the ATM's. For all reflectors of greater aperture the residual coma must be elimi- nated by deforming the curve of the spherical secondary, along its meridional diameter ( in fig.2 the meridional diameter coincides with the paper plane ). While the center of the secondary keeps its r.o.c., calculated for the fundamental system (formula v.fig.1 ), shortens the r.o.c. in direction to the upper edge of the secondary continuously till the minimum value and lengthens the r.o.c. in direction to the lower edge of the second- dary till the maximum value

This strong deformation requires special tools and grinding methods, but is a wonderfull hobby for old hands. The Schiefspiegler shown in fig. 4 of 6 1/2″(168 mm) aperture and 132″ (3360 mm) equivalent focal length is of that anastigmatic device. Its definition equals that of a first class apochromatic refractor.

The adjustment of the anastigmatic device is very simple, supposed that one of the three adjustment-screws of primary and secondary lies in the meridional plane ( i.e. the paper plane of fig. 2 ) of the system. This screw will be called ‘meridional adjustment-screw’. By looking in full daylight through the eyepiece-mounting, closed by a diaphragm with a hole of about l/8″, one turns the secondary's adjustment-screws till the full surface of the primary is seen exactly concentrically with the edge of the secondary. Generally the observer sees then the reflected image of the secondary's tube-end somewhere in the reflected image of the primary. Funny to describe these there-and-back-reflections, but easy to discern in practice. By operating the two adjustment-screws

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1. The phrase 4,8″.0,32 = 1,54″ should be read: 48/10 seconds of arc multiplied by 32/100 equals 154/100 seconds of arc. Elsewhere on this page, the double prime symbol ″ is used as an abbreviation for inch or inches.

2.The design mentioned may be similar to Oscar Knab's 4.25 inch. Knab's design may have a slightly longer focal length in order to eliminate the secondary shadowing of the primary. (The longer focal length would also reduce remaining aberrations somwhat.) There is a link to a drawing of this design in the bibliography. It is also the one described in Knab's 1961 Sky & Telescope article.