Abstract

The optical elements of a cassegrainian telescope are commonly tested individually, with their axes in a horizontal position. When these optical elements are inserted in the telescope, the resulting imagery is often disappointing. The quality of the imagery in the telescope may be predicted more accurately if the primary and secondary mirrors, with their axes in the vertical position, are tested against each other with the aid of null compensating reimaging optics. An example is given to illustrate the application of the technique.

© 1970 Optical Society of America

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References

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  1. A. Offner, Appl. Opt. 2, 153 (1963).
    [CrossRef]
  2. J. H. Hindle, Monthly Notices Roy. Astron. Soc. 91, 592 (1931).

1963

1931

J. H. Hindle, Monthly Notices Roy. Astron. Soc. 91, 592 (1931).

Hindle, J. H.

J. H. Hindle, Monthly Notices Roy. Astron. Soc. 91, 592 (1931).

Offner, A.

Appl. Opt.

Monthly Notices Roy. Astron. Soc.

J. H. Hindle, Monthly Notices Roy. Astron. Soc. 91, 592 (1931).

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Figures (8)

Fig. 1
Fig. 1

Schematic diagram of the cassegrainian system with the fundamental parameters labeled.

Fig. 2
Fig. 2

Primary and secondary mirrors tested together. The optical system is illustrated here in the horizontal position, but it should be deployed vertically. The diagram is not to scale, the sizes of the folding flat and reimaging optical system being exaggerated for clarity.

Fig. 3
Fig. 3

An f/6 system. Obscuration ratio ρ is plotted as a function of secondary mirror amplification m for image clearance ratios of 0, 0.2, 0.5, and 1.0. Liberal clearance is obtained only when ρ or m is very high.

Fig. 4
Fig. 4

An f/16 system. Obscuration ratio ρ is plotted as a function of secondary mirror amplification m for image clearance ratios of 0.2, 0.5, and 1.0. In a slow system such as this, a wide range of image clearance is possible at little expense to obscuration or magnification.

Fig. 5
Fig. 5

Testing system length. L is plotted as a function of obscuration ratio ρ and secondary amplification m for several different values of L. In the gray area, S1 > L, yielding an impractical system. Note that very low amplification factors require unwieldy testing systems. Three sample curves similar to those of Fig. 3 and Fig. 4 are superimposed. Solid heavy line: N = 6.0, ω = 1.0; dashed heavy line: N = 10.0, ω = 1.0; dotted heavy line: N = 10.0, ω = 0.5. The test system length for a cassegrainian system is represented by the crossing of its heavy line with a line representing L = const. For example, an f/10.0 cassegrainian system having ρ ≃ 0.3, m ≃ 3.0, and ω = 0.5 requires a test system whose length is between 2.0 and 2.25 times the focal length of the primary.

Fig. 6
Fig. 6

The Offner corrector in use. The optical system is illustrated here in the horizontal position but it should be deployed vertically. The cell containing the folding flat and field lenses has been rotated so that the figure of the primary may be checked.

Fig. 7
Fig. 7

A possible test system for the 396-cm telescope. The system, illustrated here in the horizontal position, should be deployed vertically. For clarity, the dimensions of the primary and secondary mirrors are not to scale. Constructional parameters of the system (in centimeters) are tabulated below. Test wavelength is 0.6328 μm.

Fig. 8
Fig. 8

Wavefront spherical aberration residual. The aberration residual in micrometers is plotted as a function of the normalized aperture coordinate.

Tables (1)

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Table I Constructional Parameters of 396-cm Aperture Cassegrainian Telescope

Equations (4)

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m = N ( ρ - 1 ) / ( ω - N ρ ) .
S 1 = 2 m / ( m + 1 ) .
S 2 = - 2 m / ( 1 - m )
L = [ 2 m / ( 1 - m ) ] ( ρ - 1 ) .

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