J

Fig. 3.12. The Couder (aplanatic) anastigmatic telescope (1926) [3.25]

Fig. 3.12. The Couder (aplanatic) anastigmatic telescope (1926) [3.25]

from (3.177). The above data give

a higher value than for the Schwarzschild telescope because M2 is steeper although the Couder primary is less steep. The Couder system has the important advantage that the obstruction Ra is significantly less. Even with the field supplement for a field of ±1.5° (with the stop at the primary), the total linear obstruction is only 0.49.

In the Couder version with Di = 0.8 m, the above angular field gave a diameter of 126 mm for the field flattener. With a refractive index of 1.52, he gave a thickness supplement at the centre relative to the edge for a planoconvex lens of 5.07 mm. Of course, refractive correctors introduce chromatic aberrations, above all transverse aberration - see Chap. 4.

The Couder telescope, fulfilling the three conditions £ Si = £ Sjj = Y^Siii = 0, is the best 2-mirror solution in focal form from the point of view of imaging potential. But it has been hardly used because of its excessive length and poor baffling characteristics. It is highly significant that its optical length (without baffle extension) is 2f , exactly the same as that of the Schmidt telescope which also fulfils the above three aberration conditions. But the Schmidt has a spherical primary and field curvature less than half as great. Also the detector in the Schmidt is facing away from the incident light, a much more favourable baffling situation. On the other hand, the concave secondary of the Couder telescope, with its modest elliptical form, is far simpler to make than a large achromatic Schmidt plate.

Of course, the Couder telescope becomes more compact if its focal length is reduced by making it steeper, say f/1. But the solutions with more mirrors, discussed in § 3.6.5.3, are more compact and flexible in their designs. It seems, therefore, that the Couder telescope will remain an important theoretical limit case but will rarely be utilised in practice.

Figure 3.13 shows the spot-diagrams for the Couder telescope for an aperture of 1 m with f/3.0 and a field of ±2.0°, plotted for the optimum curved field of radius +1418 mm. The improvement in quality over the Schwarzschild

1 arcsec

Lange Space F/# Working F/# Obi - Space N.A. Stop Radius

Parax^Mag."9"' Entr. Pup. Dia. Entr. Pup. Pos. Exit Pupil Dia. Exit Pupil Pos.

Entrance Pupil ! 0.000000

Field Type: Angls # X-Value

2 (LOOOOOO

3 0.000000

Units^Microns

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