RU2785224C1 - Objective of a mirror-lens telescope - Google Patents

Abstract

FIELD: telescopic systems.

SUBSTANCE: invention can be used as a lens for telescopic systems for astronomical and other visual observations, photo and video recording. The lens is made of optically interconnected and located along the beam of the main parabolic mirror, a secondary flat mirror located at an angle of 45 degrees to the optical axis of the main parabolic mirror, and a lens field corrector located behind the focal plane of the main parabolic mirror. The lens field corrector contains a meniscus sequentially located and separated by air gaps, with its concave surface facing the focus of the main mirror, a biconvex lens, a biconcave lens, two biconvex lenses and a negative meniscus, with its convex surface facing the equivalent focus of the telescopic system objective.

EFFECT: increase in image contrast and relative aperture, decrease in overall length and increase in the angular field of view while maintaining high image quality and without additional shielding of the entrance pupil of the lens.

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Description

The invention relates to the field of optical instrumentation and can be used as a lens for telescopic systems for astronomical and other visual observations, photo and video recording.

Telescopes with a main parabolic mirror are widely used in astronomy. From the theory of aberrations of optical systems, it is known that a parabolic mirror is capable of constructing an aberration-free image of an infinitely distant point (star) in the focal plane, strictly on the axis. However, all other points in the image will be aberrated. The main aberrations of a parabolic mirror are coma and astigmatism, and it is coma that severely limits the field of view of telescopes with a moderate relative focus. So, for example, for a classic amateur Newtonian telescope with a main mirror diameter of 8 inches and a relative focal length of F / 6, the total field of view in image space, free from coma, is only a little more than 12 arc minutes. And if we talk about more modern, so-called "fast" telescopes with a main parabolic mirror with a relative focal length of F / 3, then the field free from coma will be less than 4 arc minutes, which is not enough for full-fledged observations. Therefore, in the objectives of Newtonian telescopes with a parabolic primary mirror, subaperture field correctors are used to increase the field of view.

Known is the lens of a Newtonian telescope with a main parabolic mirror with a Wynn corrector [C.G. Wynne, Apple. Opt. 6, 1227 (1967)]. The corrector consists of four lenses with spherical surfaces separated by air gaps. All lenses are made from the same material. The corrector is installed along the path of rays reflected by the main mirror, in front of the focus of the main mirror, at a distance from the focus point of about 10% of the focal length of the mirror.

The disadvantage of this technical solution is the fact that the corrector introduces additional central shielding of the telescope pupil, which leads to a decrease in image contrast.

Known lens of the Newtonian telescope with a meniscus corrector [US Patent 4718753, Jan. 12, 1988 Telescope with correcting lens], consisting of one meniscus lens with its concave side facing the main mirror, having the same radii of spherical surfaces or, as an embodiment, the corrector may consist of two lenses (plano-concave and plano-convex) with the same radii of spherical surfaces and made of the same material, separated by an air gap. The corrector is installed near the focus of the primary mirror, between the secondary mirror and the image plane.

The disadvantages of this technical solution are poor image quality and a small field of view, free from aberrations. In addition, to ensure acceptable image quality, it is necessary to install the meniscus as close as possible to the main mirror, which will lead to screening of the entrance pupil of the telescope with a corrector, which will partially block the rays coming from the object space to the main mirror.

All known types of telescope objectives with sub-aperture lens correctors have a common property: the correctors in them are installed in front of the focus of the primary mirror and, as a rule, introduce additional central shielding of the telescope's entrance pupil.

The closest in technical essence to the proposed invention is a lens with a corrector, which is installed behind the focal plane of its main parabolic mirror is a mirror corrector (secondary mirror), and such a lens is known as the Gregory system [Maksutov D.D. Astronomical optics. L., Nauka, 1979, p. 299]. The Gregory lens consists of optically coupled primary parabolic mirror located along the beam and a secondary concave elliptical mirror mounted behind the focus of the primary mirror. The lens provides a straight image.

The disadvantages of this technical solution are:

- small relative aperture (1:15-1:20) due to residual aberrations;

- the fundamental presence of a central screening of the entrance pupil of the telescope lens by a corrector (mirror);

- significant elongation of the focal length of the system and the dimensions of the tube in the axial direction due to the presence of magnification on the secondary mirror, which also leads to an increase in astigmatism and field curvature;

- aspherical surface of the secondary mirror (ellipsoid).

The technical objective of the present invention is to create a design of a mirror lens with a lens corrector with high technical characteristics.

The technical result consists in increasing the image contrast, increasing the relative aperture, reducing the overall length of the lens and increasing the angular field of view of the optical system while maintaining high image quality and the absence of additional shielding of the entrance pupil of the lens by corrector elements.

This is achieved by the fact that the lens of a mirror-lens telescope is made of optically interconnected and located along the beam of the main parabolic mirror, a secondary flat mirror located at an angle of 45 degrees to the optical axis of the main parabolic mirror and a lens field corrector located behind the focal plane of the main parabolic mirror, according to the invention, the lens field corrector contains a meniscus sequentially located and separated by air gaps, the concave surface facing the focus of the main mirror, a biconvex lens, a biconcave lens, two biconvex lenses and a negative meniscus, the convex surface facing the equivalent focus of the objective of the telescopic system.

The essence of the invention is illustrated by drawings, where in Fig. 1 shows the optical scheme of the lens of a reflex-lens telescope with a primary mirror field corrector; Fig. 2 - optical diagram of the lens field corrector of the main mirror, in Fig. 3 is a graph of the frequency-contrast response, in FIG. 4 is a graph of the change in the polychromatic Strehl number over the field of view, in FIG. 5 - a graph of changes in the contrast of the image across the field.

The optical scheme of the lens of a reflex-lens telescope with a field corrector of the main mirror (Fig. 1) contains the main parabolic mirror 1, the secondary flat mirror 2, located at an angle of 45 degrees to the optical axis of the main parabolic mirror 1, and the lens field corrector 3 of the main parabolic mirror 1, located behind the focal plane _Fgz of the main parabolic mirror 1.

The optical scheme of the lens corrector 3 of the field of the main mirror 1 (Fig. 2) contains a meniscus 4 sequentially located and separated by air gaps, facing the concave surface to the focus F _{rz of} the main parabolic mirror 1, a biconvex lens 5, a biconcave lens 6, two biconvex lenses 7 and 8 and negative meniscus 9, facing the convex surface to the equivalent focus F _eq of the lens of the telescopic system.

The lens of a mirror-lens telescope works as follows.

The rays coming from a distant object are collected by the main parabolic mirror 1, reflected from it and hit the secondary flat mirror 2, reflected by it at an angle of 90 degrees to the optical axis of the main parabolic mirror 1 and collected outside the telescope lens tube, in the focal plane F _z the main parabolic mirror 1. Next, the rays fall on the first lens of the lens corrector 3 and, being successively refracted on the surfaces of six lenses of the corrector 3, are focused in the equivalent focus F _eq of the telescopic system. Aberrations are corrected along the path of the rays. The angular magnification of the corrector 3 of the lens is close to unity, so the equivalent focal length of the system remains equal to the focal length of the main parabolic mirror 1, which does not lead to a decrease in the relative aperture of the entire optical system. All refractive surfaces of lenses are spherical. The image built by the lens is straight.

As an example, the objective of a Newtonian telescope with a main parabolic mirror with a diameter of 203 mm, a relative aperture of 1:6 and a corrector with the following technical characteristics is calculated:

- Working spectral range, nm 470-670 - Increase corrector, fold one - Field of view in the space of objects, 2ω, deg 0.5

The high quality of the image created by the proposed lens with a corrector is confirmed by the graphic materials shown in Fig. 3, Fig. 4 and FIG. 5.

On FIG. Figure 3 shows a graph of the frequency-contrast characteristic of a telescope lens with a corrector. The abscissa shows the spatial frequency in millimeters, referred to the lens image plane, and the ordinate shows the contrast transfer coefficient in relative units.

On FIG. 4 shows a graph of the change in the polychromatic Strehl number over the field. The abscissa shows the coordinate of the field point in degrees, and the ordinate shows the modulus of the transfer function. The graph also shows the diffraction limit according to the Rayleigh criterion.

On FIG. Figure 5 shows graphs of changes in contrast across the field in the sagittal and meridional planes for a spatial frequency of 60 lines/mm.

Thus, a telescope lens with a parabolic main mirror field corrector provides an increase in the working field of view of the telescope by correcting the main aberrations of the main mirror - coma and astigmatism at a high relative aperture (1:6 and higher) and reducing the dimensions of the telescope tube, while the lens corrector does not introduce additional shielding, which favorably affects the contrast of the image.

The use of the invention makes it possible to increase the aberration-free angular field of view of the telescope lens, increase the relative aperture, increase the image contrast, reduce the overall length of the lens due to the absence of a corrector increase, and completely eliminate the central screening of the entrance pupil of the telescope lens by the optical elements of the corrector.

Claims (1)

The lens of a mirror-lens telescope, made of optically interconnected and located along the beam of the main parabolic mirror, a secondary flat mirror located at an angle of 45 degrees to the optical axis of the main parabolic mirror, and a lens field corrector located behind the focal plane of the main parabolic mirror, characterized in that the lens field corrector contains a meniscus sequentially arranged and separated by air gaps, with its concave surface facing the focus of the main mirror, a biconvex lens, a biconcave lens, two biconvex lenses and a negative meniscus, with its convex surface facing the equivalent focus of the objective of the telescopic system.

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