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We see that the SP form is afflicted not only with much worse coma than the DK form, but also with far more astigmatism. The field curvature is the same as for the other solutions. It is therefore rarely a useful practical form as such, but has great interest as a feeder telescope for more complex systems (see §

Finally, Fig. 3.7 shows the spot-diagrams for the SP solution with the same geometry (NTT) as in Fig. 3.6. Again with a circle of 1 arcsec, the field is limited by the coma to only about ± 20 arcsec. As with the DK solution, the spot-diagrams show pure coma only, since both astigmatism and field curvature are negligible at this field. General conclusions on 2-mirror telescopes from Tables 3.2

and 3.3. The aberration theory above, combined with the geometrical advantages of the telephoto effect treated in Chap. 2, reveal why the Cassegrain geometry, above all in RC or classical form, has become the standard solution for modern telescopes. The advantage of the RC form increases with angular field and inversely with m2. The DK solution is of interest for amateurs, and also for low-cost professional telescopes if a limited angular field is acceptable. The SP solution will rarely have practical interest as such, but the theory has great interest for its use as a feeder telescope for other solutions (see §

The equivalent Gregory forms are longer and will only be interesting because of three properties: the real primary image for solar telescopes, the





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