DE976880C - Three-lens apochromatic objective - Google Patents

Description

ISSUED JULY 2, 1964

Z 4183 IXa / 42h

The invention relates to an improvement of three-lens apochromatic objectives, consisting composed of two converging lenses enclosing a diffusing one. Apochromatic objectives from three lenses have been known per se for a long time. This is understood to mean lenses that are for the following image errors have been corrected: spherical longitudinal deviation, sine condition, chromatic Longitudinal deviation for two colors of a finite spectral range and also for one third color, the so-called secondary spectrum. If the thicknesses are neglected, the Theory as the conditions for obtaining the chromatic correction are the following known Relationships for eliminating longitudinal chromatic aberration for two colors:

- H H = 0, (i)

V ₁ V ₂ V ₃

therein φ ₁ ... φ ₃ denote the refractive powers of the individual lenses and V ₁ - ^ v ₃ the so-called Abbe numbers, ie the relative reciprocal values of the chromatic longitudinal deviation for the individual lenses I, II and III in relation to two colors.

Denotes the index ι and 3 values for the converging lenses I and III, 2 those for the Zer-

409 631/6

Diffusing lens II, N the resulting Abbe number of the two converging lenses I and III,

(2)

and

⁼ Ψι +

(3)

ίο the resulting refractive power of these two lenses, then (i) can be written

(4)

For the correction of the secondary spectrum, i.e. the elimination of the longitudinal color deviation for a third color, apart from equations (i) and (4), must also be fulfilled:

JXV ₁

(5)

Here ϋ · _ν # ₂ and # _s mean the relative partial dispersions of lenses I, II and III. If one relates these relative partial dispersions to the color G ' as the third color, then the following is defined:

—Nc

nc

(6)

if % q, %, Uq are the refractive indices for the Fraunhofer lines C ₁ F, G ' . The resulting relative partial dispersion of the two converging lenses I and III is denoted by Θ in (5). A prerequisite for the fulfillment of equation (5) is the requirement that the relative partial dispersions of all three lenses do not have the same linear relationship of the shape

& = A + Bv (7)

should suffice, ie for one of the three lenses the relative partial dispersion # as a function of ν must be represented with a different constant than it applies to the other two lenses.

In order to meet this requirement, so-called “short shotguns” have been used in combination with crown glasses. The refractive indices of all these glasses so far usual for apochromats were below 1.600 and, according to equation (2), resulted in a y-value N that differs only slightly from v ₂ . Since, in addition to equation (4), the »refractive power equation«

Ψι, + ψ2 = Φ (8)

(Φ = total refractive power required by the lens) must be fulfilled, the use of the previously common types of glass for apochromats means that the absolute values of the refractive power Cp ₁ ... φ _{Ά would have to be} very large compared to Φ . As a result, these apochromatic objectives exhibited a strong Gaussian error (chromatic difference in the spherical aberration) and a strong zone error in the spherical longitudinal aberration. Usable apochromats of this type have therefore only become known for aperture ratios less than 1:10. The invention is based on the discovery made in the systematic investigation of the optical glasses melted in recent years that especially the newer "heavy shotguns" with a refractive index greater than 1.61 and a j> value less than 35 also have $ values The course deviates from the normal linear function A + B · ν of the other optical glasses.

The invention consists in the fact that with a three-lens apochromatic objective from two converging lenses, which include a diffusing one, which is calculated in the direction of the light in the last place the converging lens cemented with the diverging lens and this cemented element is separated from the other converging lens by an air gap, both for one of the converging lenses as well as for the diverging lens glasses are used whose Abbe numbers are smaller than

35 and their relative partial distributions ■ & = - -

tip - n _c

are greater than 1.61, and that the diverging lens against one of the two converging lenses has a difference in refractive indices of less than 0.05 and against the other converging lens of more than 0.1 for the yellow helium line d .

For reasons of correcting the image errors, namely the spherical longitudinal deviation, the sine condition and the above-mentioned chromatic errors, as well as for reasons of the difficulties in manufacturing when cementing three lenses, it has proven particularly expedient according to the invention to use the divergent lens only with one of the converging lenses to cement and to arrange an air gap between the other convergent lens and the cemented member, such that the free-standing convergent lens is in the first position of the system with respect to the light direction. A particularly good correction of the chromatic aberrations results from the fact that in the further embodiment of the inventive concept the free-standing converging lens is the one with the lower refractive index and that n ₀ this refractive index is greater than 1.6 and the Abbe number is greater than 55.

According to the invention, it is sufficient if the air gap to be introduced is greater than 2% of the focal length of the lens. It should preferably be about 6% of the n ₅ focal length. At the same time, the measures according to the invention result in a shortening of the overall length of a telescope in the event that the objective is used as a telescope objective. The installation of the objectives according to the invention in telescopes is, however, only one of the possible types of use.

The numerical values given in the table below relate to an exemplary embodiment of an objective according to the present invention Invention, which is shown in the figure.

In addition to this example, however, embodiments are also possible in which, for example, the free The standing converging lens has a higher refractive index than the converging lens cemented with the diffusing lens Has lens.

In the following exemplary embodiment, r is the radii of the refractive surfaces, d is the thickness of the individual lenses L ₁ , L ₂ and L ₃ , / is the air gap between lenses L ₁ and L ₂ , and s is the back focal length from the image-side vertex of the last lens to the focal plane, with n _d the refractive indices and with ν the Abbe numbers. The numerical values are related to the focal length / = 1.

example Lens II Lens III Lens I. 1.7473 1.7760 n _c 1.6173 i, 7552 1.7847 % 1.6204 i, 7747 1.8065 n _P 1.6276 1.7920 1.8261 n _G 1.6334 27.5 25.7 Vd 60.3 1.631 1.644 # 1,566 ₂ = 0.024 d _s = 0.047 d _t = 0.057 d I ₁ = 0.061 = -o, 5543i r _± = +0.48001 r ₃ r ₄ = +0.29427 r ₂ = -0.80584 S. = +0.795

Claims (4)

PATENT CLAIMS: I. Three-lens apochromatic objective, consisting of two converging lenses, which include a diffusing one, in which the converging lens, calculated in the direction of light, is cemented to the diverging lens and this cemented element is separated from the other converging lens by an air gap, characterized in that both for which a converging lens and for the diverging lens glasses are used, whose Abbe numbers are less than 35 and whose relative partial dispersions $ = - larger than 1.61, and that also the diverging lens against one of the two converging lenses = -33.497 Difference in the refractive indices of less than 0.05 and against the other converging lens of more than 0.1 for the yellow helium line d . 2. Lens according to claim 1, characterized in that that the free-standing converging lens is the one with the lower refractive index and that this refractive index is greater than 1.6 and the Abbe number is greater than 55. 3. Lens according to claim 1, characterized in that that the air gap is greater than 2% of the focal length, preferably about 6% of the focal length. 4. Lens according to Claims 2 and 3, characterized in that the surface powers (An / r) differ by a maximum of ± 0.2 // from the values shown in the following numerical example: Radii T ₁ = + 0.48001 · /
r ₂ = -0.80584 / r ₃ = -o, 5543i /
r ₄ = + 0.29427 · / r ₅ = -33.497-f Thick d _L = 0.057 / / = 0.061 / d ₂ - 0.024 / d ₃ = 0.047 / % vd 1.6204 60.3 i, 7552 27.5 1.7847 25.7 An / r + 1,2925 // + 0.7699 // -1.3624 // + 0.1002 // + 0.0234 // Publications considered: German Patent No. 189255. For this purpose ι sheet of drawings © 509 656/225 1.56 (409 631/6 6.64)