GB2480735A

GB2480735A - Optical system for calibrating a light source

Info

Publication number: GB2480735A
Application number: GB1107676A
Authority: GB
Inventors: Rolf Sand; Martin Gerken
Original assignee: Carl Zeiss AG; Carl Zeiss Optronics GmbH
Current assignee: Carl Zeiss AG; Hensoldt Optronics GmbH
Priority date: 2010-05-18
Filing date: 2011-05-09
Publication date: 2011-11-30
Anticipated expiration: 2031-05-09
Also published as: DE102010029089B4; GB2480735B; DE102010029089A1; GB201107676D0

Abstract

An optical system 1 for calibrating a light source, such as a high-power laser arranged in a satellite, comprises an optical axis (10, Fig 2) and a light entrance side A, at which the light of a light source enters the optical system 1. Starting from the light entrance side A and pointing away from the light entrance side A, the optical system 1 comprises a first lens unit 4, a second lens unit 5, a diaphragm unit 6, a third lens unit 7, a fourth lens unit S and a reflection element 9. The system may also include a quarter-wave plate 11 and a filter element 3. The reflection element 9 may be a corner reflector. The diaphragm unit 6 may be interchangeable, may be arranged to be adjustable along the optical axis (10, Fig 2) and may be rotatable and/or tiltable. After leaving the optical system 1 the reflected light may be detected and evaluated by a detector (D, Fig 2).

Description

Optical System for Calibrating a Light Source

Description

The invention relates to an optical system for calibrating a light source, e.g. a high-power laser, which is arranged in a satellite.

Light sources, e.g. a high-power laser, are used for various purposes, e.g. for the transfer of data or for measurement, e.g. of distances. Moreover, such lasers are also used in material processing and in sensor technology. The aforementioned examples are not definitive. A light source used in such a manner should be calibrated from time to time to enable it to be used effectively. A calibration should be undertaken in particular when properties of the light source have been changed as a result of external influences. This applies, for example, to a laser that is arranged in a satellite. Such a laser is subjected to high forces during transport of the satellite from earth into space, and therefore it is absolutely possible that the positioning and alignment of the laser were changed as a result of these acting forces.

An interferometer is known from patent document DD 72 601. The known interferometer has a laser, an interference arrangement, a telescope and a movable reflector arrangement with a lens and a mirror. Lens units are arranged in the telescope. The known interferometer serves to measure small increments of relative long distances. Calibration of a light source, i.e. measurement of the light source per se, is not provided. The known interferometer is also unsuitable for this purpose, since it only serves to measure the distance between two points.

Therefore, the object forming the basis of the invention is to specify an optical system that is suitable for calibrating a light source, in particular a high-power laser arranged in a satellite, e.g. with a power in the range of 5 watts to 15 watts.

This object is achieved with an optical system with the features of claim 1. Further features of the invention are evident from the further claims, the following description and/or the attached drawings.

According to the invention, an optical system for calibrating a light source, has an optical axis and a light entrance side, at which the light of a light source enters the optical system. In

I

addition, the optical system has at least one first lens unit, at least one second lens unit, at least one third lens unit and also at least one reflection element. Moreover, at least one fourth lens unit and at least one diaphragm unit are additionally provided in the optical system according to the invention. Starting from the light entrance side and pointing away from the light entrance side, the arrangement is the first lens unit, the second lens unit, the diaphragm unit, the third lens unit, the fourth lens unit and the reflection element. In other words, the individual units of the optical system according to the invention are arranged in the following sequence starting from the light entrance side: the first lens unit -the second lens unit -the diaphragm unit -the third lens unit -the fourth lens unit -the reflection element.

In the above and following description a lens unit is understood to mean a lens unit that is formed from a single lens or that is formed from multiple lenses (or lens groups).

The diaphragm unit can be configured as a pinhole diaphragm, for example. Additionally or alternatively hereto, it can be provided that the diaphragm unit is formed as a diaphragm system with at least one first diaphragm and at least one second diaphragm.

Deliberations have determined that the aforementioned optical system is particularly well suited to the calibration of a light source, in particular a high-power laser. In this case, -depending on the quality of the reflection element -the reflection element can have a reflection accuracy of one arc-second. It is also possible to determine the visual field of the optical system by means of the diaphragm unit so that a calibration of the light source can proceed particularly well.

For calibration light of the light source is introduced into the optical system through the light entrance side of the optical system and directed through the first lens unit, the second lens unit, the diaphragm unit, the third lens unit, the fourth lens unit and the reflection element.

The reflection element reflects the incident light back so that the reflected light passes through the optical system again, but in the reverse sequence to the incident light of the light source. After leaving the optical system the reflected light is then detected and evaluated by means of a detector.

In addition, the optical system according to the invention has the advantage that it is compact in structure and can be configured in such a manner that it is insensitive to forces acting on the optical system. Moreover, the optical system can be formed from units that are insensitive to a temperature change. Consequently, the optical system is preferably athermal, i.e. insensitive to temperature. For example, the optical system is insensitive to a temperature change in the range of± 25°C, working from a starting temperature (e.g. 0°C). However, it is explicitly indicated that the invention is not restricted to this temperature range. Rather, the invention is also usable in other temperature ranges.

In addition, it is provided, for example, to use radiation-resistant glasses, which are usable in the composition of the first lens unit, the second lens unit, the third lens unit and/or the fourth lens unit. A radiation-resistant glass is understood here to mean a glass having a material resistance that is not damaged as a result of incident radiation and whose properties do not change, or change only slightly, as a result of incident radiation. The radiation-resistant glasses currently available on the market have been tested for resistance on the basis of incident electron radiation, proton radiation and gamma radiation in different radiation doses.

These have all exhibited a sufficiently good resistance. An example to be mentioned here is the glass BK7G18.

In a first embodiment of the optical system according to the invention, the first lens unit, the second lens unit, the third lens unit and the fourth lens unit respectively consist of a single lens. This is of particular advantage, since the weight of the optical system is low compared to numerous other optical systems from the prior art that always use multiple lenses for a single lens unit. However, as mentioned above, the individual lens units can also comprise multiple lenses, in particular composite lenses.

In a further configuration of the invention, at least one quarter-wave plate (i.e. a "?i4" plate) is arranged between the fourth lens unit and the reflection element. Moreover, it is also preferably provided here to coat at least one reflection surface of the reflection element with a metal. This embodiment enables the polarisation state (more precisely the linear polarisation) of the light incident into the optical system and the polarisation state of the light reflected by the reflection element to be maintained (i.e. to remain the same).

The optical system according to the invention preferably has at least one filter element arranged in the direction of the light entrance side viewed from the first lens unit that serves as entrance window of the optical system. The purpose of the filter element is to absorb energy. In some embodiments it is desirable that the power of the incident light is reduced.

In addition, it is preferably provided that the reflection element is configured as a retroreflector. It is particularly preferred if the reflection element is configured as a corner reflector. In this exemplary embodiment the light incident into the optical system is retroreflected by the reflection element substantially in the direction of a light source, from which the incident light originates.

Moreover, it is provided that the optical system according to the invention is preferably afocal in configuration. In addition, the optical system according to the invention is suitable in particular for calibrating a light source, which radiates a light beam with a wavelength of 1064 nm into the optical system.

In a further configuration of the optical system according to the invention, the first lens unit, the second lens unit, the third lens unit and the fourth lens unit are provided with a respective first face and a respective second face. Deliberations have determined that it is advantageous if the first face and/or the second face of each of the aforementioned lens units is/are spherical in configuration.

An advantageous configuration of the optical system according to the invention is provided by the following properties: Surface Radii Curvature Aperture Material e [mm] Diameter ________ ______ _______ [mm] _____ ____ ___ Firstfaceof -18.3030 convex 13.6246 BK7G18 1.52170 63.44 first lens unit Secondfaceof -805.9700 concave 14.3400 first lens unit ___________ ___________ ________ ______ Firstfaceof -14.9616 convex 10.0525 BK7G18 1.52170 63.44 second lens unit __________ ___________ __________ ________ _______ _____ Second face of -8.6600 concave 12.9776 second lens unit __________ ___________ __________ ________ _______ _____ Firstfaceof 3.9243 concave 5.1417 BK7G18 1.52170 63.44 third lens unit __________ ___________ __________ ________ _______ _____ Secondfaceof 7.0790 convex 7.1341 third lens unit __________ ___________ __________ ________ _______ _____ First face of -34.2288 convex 7.6871 BK7G18 1.52170 63.44 fourth lens unit __________ ___________ __________ ________ _______ _____ Secondfaceof 14.1250 convex 8.3246 fourth lens unit __________ ___________ __________ ________ _______ _____ The individual surfaces of the first lens unit, the second lens unit, the third lens unit and also the fourth lens unit are indicated in the above table, and their radii, curvature and aperture diameters are specified. In addition the material of each lens is indicated. The refractive indexes ne and the Abbe numbers ye are also specified.

In a further embodiment of the optical system according to the invention a specific coefficient of expansion is provided to allow a particularly good calibration. It is preferably provided that the coefficient of expansion of the first lens unit, the second lens unit, the third lens unit and/or the fourth lens unit is selected such that in a temperature range of -3 0°C to 70°C it respectively lies in a range of 5.5 x 106/Kto 9.5 x 106/K, preferably 6.5 x 106/Kto 9.0 x 10 6/K, in particular 7.0 x 106/K to 8.5 x 106/K. In this case the aforementioned limit values of the ranges are also included in the preferred ranges. The limit values are to be understood as approximate values. They can readily vary slightly. A coefficient of expansion from the aforementioned ranges allows that the calibration of the light source is not deteriorated by temperature influences.

In an additional embodiment it is provided that the temperature coefficient dnidT of the first lens unit, the second lens unit, the third lens unit and/or the fourth lens unit is selected such that it lies in a range of 2.0 x 106/K to 2.7 x 106/K. Similarly in this case, the aforementioned limit values of the ranges are also included in the preferred ranges. The limit values are to be understood as approximate values. They can readily still vary slightly.

In a further configuration of the optical system according to the invention, the diaphragm unit is interchangeable. In other words, it is possible to insert different diaphragm units into the optical system. This ensures that a different visual field and therefore a defined field boundary can be selected, as required. It is preferably provided that the diaphragm unit has a diameter (i.e. an aperture diameter) in the range of 0.1 to 1 mm, preferably in the range of 0.2 mm to 0.5 mm. Particularly advantageous is the use of a diaphragm unit with a diameter of 0.2 mm, with which a visual field in the range of 1 mrad can be achieved. However, the invention is not restricted to the aforementioned visual field. Rather, special embodiments of the optical system have a visual field that lies in the range of 0.5 mrad to 8 mrad, preferably in the range of 1 mrad to 5 mrad. Interchange of the diaphragm unit in this case can preferably be achieved without replacement of one or more of the first lens unit, the second lens unit, the third lens unit and the fourth lens unit. In a further embodiment it is possible to use a diaphragm unit with adjustable diaphragm aperture, so that the diaphragm unit does not have to be replaced for adjustment of the diaphragm aperture.

In a further configuration of the optical system according to the invention, the first lens unit, the second lens unit, the third lens unit and/or the fourth lens unit are arranged to be adjustable along the optical axis. It is additionally provided that the diaphragm unit is preferably arranged to be adjustable along the optical axis of the optical system, wherein the diaphragm unit is preferably additionally configured to be rotatable and/or tiltable in relation to a plane arranged perpendicular to the optical axis. For example, the diaphragm unit is arranged to tilt by 1.50 to the aforementioned plane.

The invention is explained in more detail below on the basis of an exemplary embodiment represented in the following figures.

Figure 1 is a schematic representation of an optical system for calibrating a light source; and Figure 2 is a simplified representation of the optical system according to Figure 1.

Figure 1 shows an optical system 1 for calibrating a light source in the form of a laser L. The optical system 1 is arranged in a housing 2 of relatively rigid configuration. The rigid housing 2 prevents units arranged in the housing 2 (which will be explained further below) from being damaged or displaced upon action of substantial forces, such as can occur during transport from earth into space, so that a calibration of the light source L, e.g. a high-power laser (e.g. in the range of 5 watts to 15 watts, in particular 10 watts) arranged in a satellite, is constantly guaranteed.

The optical system 1 has a light entrance side A, wherein light Li, L2 of the laser L enters the optical system 1 in the direction of arrow A. Viewed from the light entrance side A, a filter element 3, a first lens unit 4, a second lens unit 5, a diaphragm unit 6, a third lens unit 7, a fourth lens unit 8, a X14 plate ii and a corner reflector 9 are arranged along the optical axis of the optical system 1. Reference is made to the description above with respect to the definition of the terms "lens unit" and "diaphragm unit".

Figure 2 shows this arrangement in a simplified representation, wherein in Figure 2 the optical axis is provided with the reference sign 10 and the X14 plate ii is not shown for reasons of clarity.

The first lens unit 4, the second lens unit 5, the third lens unit 7 and also the fourth lens unit 8 are respectively formed from a single lens. The weight of the optical system 1 is thus kept low. However, as mentioned above, the invention is not restricted to the aforementioned lens units respectively being configured with only one lens. Rather, the lens units can also comprise multiple lenses, lens groups and/or composite lenses (e.g. lens components).

The lenses of the aforementioned lens units are formed from a radiation-resistant glass, which is particularly well suited for use in space. In addition, the lens of each of the aforementioned lens units has a first face and a second face. More precisely, the first lens unit 4 has a first face 4a and a second face 4b. The second lens unit 5 has a first face 5a and a second face Sb.

The third lens unit 7 in turn has a first face 7a and a second face 7b. Finally, the fourth lens unit 8 has a first face 8a and a second face 8b. Further possible properties are evident from

the following table:

Surface Radii Curvature Aperture Material lie ye [mm] Diameter _______ _______ ______ [mm] ______ ______ ______ 4a -18.3030 convex 13.6246 BK7G18 1.52170 63.44 4b -805.9700 concave 14.3400 __________ __________ _________ 5a -14.9616 convex 10.0525 BK7G18 1.52170 63.44 5b -8.6600 concave 12.9776 __________ __________ _________ 7a 3.9243 concave 5.1417 BK7G18 1.52170 63.44 7b 7.0790 convex 7.1341 __________ __________ _________ 8a -34.2288 convex 7.6871 BK7G18 1.52170 63.44 8b 14.1250 convex 8.3246 __________ __________ _________ The individual surfaces 4a, 4b, 5a, 5b, 7a, 7b, 8a and also 8b of the first lens unit 4, the second lens unit 5, the third lens unit 7 and also the fourth lens unit 8 are indicated in the above table, and their radii, curvature and aperture diameters are specified. In addition the material of each lens of the individual lens units is indicated. The refractive indexes ne and the Abbe numbers ye are also specified.

In this exemplary embodiment all lens units are formed from the same material, e.g. glass BK7G 18 from Schott. However, it is expressly indicated that the invention is not restricted to this material. Rather, individual, multiple or also all of the aforementioned lens units can be formed from a different material, e.g. from glass K5G20 or SK4GO6.

Moreover, the first lens unit 4, the second lens unit 5, the third lens unit 7 and also the fourth lens unit 8 have a specific coefficient of expansion. In a temperature range of -3 0°C to 70°C this coefficient lies at approximately 6.7 x 106i'K to 9.0 x 106/K for each of the aforementioned lens units. For example, the coefficient of expansion for BK7G18 lies at 7.0 x 106/K, for glass K5G20 at 9.0 x 106/K and for glass SK4GO6 at 6.7 x 106/K. In principle, it is preferably provided that the aforementioned lens units are formed from a material having a coefficient of expansion in the range of 5.5 x 106/K to 9.5 x 106/K, e.g. in the range of 6.5 x 106/K to 9.0 x 106/K. A coefficient of expansion from this range allows that the calibration of the light source is not deteriorated by temperature influences.

As mentioned above, the optical system 1 has a diaphragm unit 6. In this exemplary embodiment this is interchangeable. In other words, it is possible to insert different diaphragm units 6 into the optical system 1. This ensures that a different visual field and therefore a defined field boundary can be selected, as required. In the exemplary embodiment represented in the figures the diaphragm unit 6 has a diameter (i.e. an aperture diameter) in the range of 0.1 to 1 mm, preferably in the range of 0.2 mm to 0.5 mm. It is particularly advantageous if a diaphragm unit 6 with a diameter of 0.2 mm is provided, with which a visual field in the range of 1 mrad can be achieved. However, the invention is not restricted to the aforementioned visual field. Rather, special embodiments of the optical system 1 have a visual field that lies in the range of 0.5 mrad to 8 mrad, preferably in the range of 1 mrad to 5 mrad. It is possible to use a diaphragm unit with adjustable diaphragm aperture as an alternative to the aforementioned diaphragm unit 6. In a special embodiment this has the same properties with respect to the aperture diameter as the diaphragm unit 6.

The corner reflector 9 is a retroreflector and is made from a quartz. The corner reflector 9 has a light entrance side and at least two reflective surfaces, namely a first reflective surface 9a and a second reflective surface 9b. The light entrance side is simultaneously the light exit side for light reflected in the corner reflector 9. The first reflective surface 9a and the second reflective surface 9b are coated with a metal. The reflective surfaces coated with metal together with the X/4 plate 11 already mentioned above ensure that the polarisation state (more precisely the linear polarisation) of the light incident into the optical system 1 and the polarisation state of the light reflected by the corner reflector 9 is maintained (i.e. remains the same).

In the exemplary embodiment of the optical system 1 represented in the two figures, the first lens unit 4, the second lens unit 5, the third lens unit 7 and the fourth lens unit 8 are arranged to be adjustable along the optical axis 10. In addition, the diaphragm unit 6 is arranged to be adjustable along the optical axis 10 of the optical system 1. The adjustable arrangement assures adequate focussing of the light beam running in the optical system 1 to enable good calibration.

The diaphragm unit 6 is arranged in a moving device (not shown), with which the diaphragm unit 6 is displaceable along the optical axis 10. This enables the diaphragm unit 6 to be adjusted at the location of an intermediate image. The moving device is additionally provided with rotation units, so that the diaphragm unit 6 is configured to be rotatable (e.g. around the x axis) and/or tiltable (e.g. around the y axis) with respect to a plane which is arranged perpendicular to the optical axis 10 and which is fixed by the x-axis and the y-axis. A corresponding coordinate system is indicated in Figure 2, wherein the y axis is arranged perpendicular to the x axis and perpendicular to the z axis (and therefore stands perpendicular on the plane of the drawing in Figure 2). A rotation and/or tilting movement is advantageous in order to possibly filter reflexes present at the diaphragm unit 6 out of the visual field.

The diaphragm unit 6 is preferably arranged so that it is tilted 1.50 in relation to the aforementioned plane. However, the invention is not restricted to this value, but the aforementioned value is to be understood as a lower limit. Other angles larger than 1.50 can also be chosen for tilting, e.g. from a range of 1.50 to 100.

The X14 plate 11 preferably made from a quartz is arranged in such a manner that the fast axis of the X14 plate 11 is arranged so that it is tilted 45° in relation to the plane fixed by the x axis and the z axis.

The described exemplary optical system 1 is particularly well suited for calibrating a light source, in particular a high-power laser. Namely, the optical system 1 according to the invention ensures that the power density on the individual units of the optical system 1 remains so low that these are not damaged. The optical system 1 according to the invention ensures that the high-power laser beam does not strike a small surface in the optical system 1 in a focussed manner (e.g. with a beam diameter of 1 mm), but that the laser beam strikes as a bundle of rays with a diameter of approximately 8 mm to 12 mm, e.g. 10 mm. In this way, the power density is in an order of magnitude (e.g. in the range of approximately 0.015 W/mm2) that prevents damage during impact on the individual units of the optical system 1.

In this case, the quality of the corner reflector 9 is selected in such a manner that it has a reflection precision (angular error) of one arc-second. Thus, an angular error for the reflected radiation of less than one arc-second is generated as a result. This is of particular advantage for calibration.

It is also possible to determine the visual field of the optical system 1 by means of the diaphragm unit 6 so that a calibration of the light source can be conducted particularly favourably.

As already mentioned above, the properties of the light source, in particular a laser, are not always constant. This is explained in more detail below using a laser as light source. The properties of the laser are subject to certain fluctuations because of mechanical and/or electronic factors. For example, mirror units arranged in a laser can shift. In particular, the intensity, direction and/or polarisation of the laser beam can shift as a result of these factors, in particular when the laser has been exposed to high acceleration forces (e.g. during transport by launcher into space). The optical system 1 is used to establish the properties of the laser and measure possible deviations from a standard value (i.e. to calibrate the laser).

For calibration, light of the laser is passed into the optical system 1 through the light entrance side A of the optical system 1 and directed through the filter element 3, the first lens unit 4, the second lens unit 5, the diaphragm unit 6, the third lens unit 7, the fourth lens unit 8 and the corner reflector 9. The corner reflector 9 reflects the incident light back again so that the further units of the optical system 1 are passed through again, but in reverse sequence to the incident light of the laser. After leaving the optical system 1 the reflected light is then detected and evaluated by means of a detector D. After the evaluation, possible deviations from the corresponding standard values of the individual properties (e.g. intensity, direction and/or polarisation of the laser) can be corrected again, if necessary, by adjustment of units of the laser.

The optical system 1 according to the invention is compact in structure and is configured to be insensitive to forces acting on the optical system 1. Moreover, the optical system 1 can be formed from units that are insensitive to a temperature change. Consequently, the optical system 1 is athermal in configuration. Reference is made to the above statements in this regard.

List of Reference Numerals 1 optical system 2 housing 3 filter element 4 first lens unit 4a first face of the first lens unit 4b second face of the first lens unit second lens unit 5a first face of the second lens unit Sb second face of the second lens unit 6 diaphragm unit 7 third lens unit 7a first face of the third lens unit 7b second face of the third lens unit 8 fourth lens unit 8a first face of the fourth lens unit 8b second face of the fourth lens unit 9 corner reflector 9a first reflective surface 9b second reflective surface optical axis 11 214 plate (quarter-wave plate) A light entrance side L light source D detector

Claims

Patent Claims Optical system (1) for calibrating a light source, wherein * the optical system (1) has an optical axis (10) and a light entrance side (A), at which the light of a light source enters the optical system (1), and wherein * the optical system (1) has at least one first lens unit (4), at least one second lens unit (5), at least one third lens unit (7) and also at least one reflection element (9), characterised in that * at least one fourth lens unit (8) and at least one diaphragm unit (6) are additionally provided in the optical system (1), and that * starting from the light entrance side (A) and pointing away from the light entrance side (A), the arrangement is the first lens unit (4), the second lens unit (5), the diaphragm unit (6), the third lens unit (7), the fourth lens unit (8) and the reflection element (9).
2. Optical system (1) according to claim 1, characterised in that the first lens unit (4), the second lens unit (5), the third lens unit (7) and the fourth lens unit (8) respectively consist of a single lens.
3. Optical system (1) according to claim 1 or 2, characterised in that at least one quarter-wave plate (11) is arranged between the fourth lens unit (8) and the reflection element (9).
4. Optical system (1) according to one of the preceding claims, characterised in that at least one filter element (3) is arranged in the direction of the light entrance side (A) viewed from the first lens unit (4).
5. Optical system (1) according to one of the preceding claims, characterised in that the reflection element (9) is configured as a corner reflector.
6. Optical system (1) according to one of the preceding claims, characterised in that the optical system (1) is afocal in configuration.
7. Optical system (1) according to one of the preceding claims, characterised in that the optical system (1) serves to calibrate a light source, which radiates a light beam with a wavelength of 1064 nm into the optical system (1).
8. Optical system (1) according to one of the preceding claims, characterised in that the first lens unit (4), the second lens unit (5), the third lens unit (7) and the fourth lens unit (8) have a respective first face (4a, 5a, 7a, 8a) and a respective second face (4b, 5b, 7b, 8b), wherein the first face (4a, 5a, 7a, 8a) and/or the second face (4b, Sb, 7b, 8b) of each of the aforementioned lens units (4, 5, 7, 8) is/are spherical in configuration.
9. Optical system (1) according to claim 8, characterised in that the optical system (1) has the following properties: Surface Radii Curvature Aperture Material lie ye [mm] Diameter ________ ______ _______ [mm] _____ ____ ___ First face(4a) -18.3030 convex 13.6246 BK7G18 1.52170 63.44 of first lens unit (4) __________ ___________ __________ _________ _______ _____ Second face -805.9700 concave 14.3400 (4b) of first lens unit (4) ___________ _____________ ___________ __________ ________ ______ First face(5a) -14.9616 convex
10.0525 BK7G18 1.52170 63.44 of second lens unit (5) ___________ _____________ ___________ __________ ________ ______ Second face -8.6600 concave 12.9776 (Sb) of second lens unit (5) ___________ _____________ ___________ __________ ________ ______ First face(7a) 3.9243 concave 5.1417 BK7G18 1.52170 63.44 of third lens unit (7) __________ ____________ __________ _________ ________ ______ Second face 7.0790 convex 7.1341 (7b) of third lens unit (7) ___________ _____________ ___________ __________ ________ ______ First face(8a) -34.2288 convex 7.6871 BK7G18 1.52170 63.44 of fourth lens unit (8) ___________ _____________ ___________ __________ ________ ______ Second face 14.1250 convex 8.3246 _______ ______ _____ (8b) of fourth lens unit (8) 10. Optical system (1) according to one of the preceding claims, characterised in that the coefficient of expansion of the first lens unit (4), the second lens unit (5), the third lens unit (7) and/or the fourth lens unit (8) in a temperature range of -30°C to 70°C respectively lies in a range of 5.5 x 106/K to 9.5 x 106/K, preferably 6.5 x 106/K to 9.0 x 10/K, in particular 7.0 x 106/K to 8.5 x 106/K.
11. Optical system (1) according to one of the preceding claims, characterised in that the diaphragm unit (6) is interchangeable.
12. Optical system (1) according to one of the preceding claims, characterised in that the diaphragm unit (6) has an aperture diameter in the range of 0.1 to 1 mm, preferably in the range of 0.2 mm to 0.5 mm.
13. Optical system (1) according to one of the preceding claims, characterised in that the visual field of the optical system (1) lies in the range of 0.5 mrad to 8 mrad, preferably in the range of 1 mrad to 5 mrad.
14. Optical system (1) according to one of the preceding claims, characterised in that the first lens unit (4), the second lens unit (5), the third lens unit (7) and/or the fourth lens unit (8) are arranged to be adjustable along the optical axis (10).
15. Optical system (1) according to one of the preceding claims, characterised in that the diaphragm unit (6) is arranged to be adjustable along the optical axis (10) of the optical system (1), and that the diaphragm unit (6) is configured to be rotatable and/or tiltable in relation to a plane arranged perpendicular to the optical axis (10).
16. An optical system substantially as hereinbefore described with reference to or as shown in the accompanying drawings.