Abstract:
A target equipped with a reflector can be tracked in a measuring system including a laser tracker. The reflector is tracked by means of a tracking unit in a normal tracking mode and by means of a surveying device in an extraordinary tracking mode. A capturing unit having a detection range lying between the detection ranges of the tracking unit and of the surveying device is also present. If the target cannot be detected by the tracking unit but can be detected by the capturing unit, the orientation of the tracking unit is controlled according to a measurement by the capturing unit. If the target can then be detected by the tracking unit, a transition to the normal tracking mode is initiated. If the target can be detected only by the surveying device, the orientation of the tracking unit is controlled according to a measurement of the surveying device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention lies in the field of measurement technology and relates to a tracking method and to a measurement system with a laser tracker, according to the preambles of the respective patent claims. The tracking method serves for the automatic tracking of a target point, in particular of a moving target point, with the measurement beam of a laser tracker. The measurement system with the laser tracker is equipped for carrying out the method. 
     2. Description of Related Art 
     So-called laser trackers are frequently applied for measurement of the position of moving target points. The term laser trackers is to be understood as devices which comprise at least one distance meter operating with a focussed laser beam (called measurement beam in the following description). For example, the direction of the measurement beam is set to the target point with the help of a mirror which is rotatable about two axes, and is detected with angle sensors assigned to the rotation axes. The target point to be measured is provided with a retro-reflector (in particular cube-corner prism or arrangement of three mirrors which are perpendicular to one another), wherein the retroreflector reflects the measurement beam of the laser tracker which is incident thereon, back to this laser tracker. Thereby, the reflected measurement beam runs coaxially to the emitted measurement beam when the measurement beam hits the reflector in an exactly central manner, and runs offset parallel thereto, when the measurement beam does not hit the reflector in a centric manner. An absolute distance between the laser tracker and the target point and/or a change of this distance is deduced from a comparison of the emitted and reflected laser light, depending on the embodiment of the tracker. The position of the reflector or of the target point relative to the tracker is computed from the angles detected by the angle sensors and the distance detected by the distance meter. 
     A part of the reflected measurement beam is usually led onto a PSD (position sensitive device). One can infer the parallel shift of the reflected beam relative to the emitted measurement beam, from the position, in which the reflected measurement beam is incident on the light-sensitive surface of the PSD. The measurement data which is determined by way of this defines the parallel offset of the reflected measurement beam and is used for a control of the measurement beam direction, in a manner such that the measurement beam follows the target point (tracking) when this moves. This means that by way of a suitable change of the measurement beam direction or the alignment of the mirror aligning the measurement beam, one ensures that the parallel offset between the emitted and reflected measurement beam is reduced or remains as small as possible. 
     It is evident that the control of the measurement beam direction by way of the parallel offset between the emitted and the reflected measurement beam, although having a small delay, however has a delay which is not negligible and limits the speed at which a target point may move and thereby be tracked. If the target point moves more rapidly, the measurement beam, before its direction can be suitably corrected, no longer hits the reflector, and the tracking as well as positioning, are interrupted by way of this. The same may happen if an obstacle gets between the tracker and the target point, so that the measurement beam is interrupted. If the laser tracker or the measurement beam of the laser tracker “loses” the reflector, the operating person is made aware of this and a search routine can be started given a suitable design of the tracker. 
     The measurement of the position of the target point and its tracking by the measurement beam can be assumed again as soon as the target point is “found” again, which is to say that the measurement beam is again incident on the reflector and is reflected by this, for which the distance measurement must be newly initiated as the case may be. The mentioned tracking interruptions become more frequent, the less controlled are movements of the target point and the smaller are the applied reflector and the diameter of the measurement beam. The same conditions as during the mentioned tracking interruptions usually also prevail at the beginning of a measurement process, when the tracker is not at all yet set onto the target point. 
     It is also known to provide laser trackers with an overview apparatus. This camera which has an, as large as possible, field of view (for example over ±20° in all directions), is arranged on the tracker and is aligned in a manner such that the measurement beam can be directed onto a target point recognised on the camera picture. The alignment of the measurement beam onto this target point is initiated by an operating person observing the camera picture, by way of this operating person suitably indicating the picture region in which the target point is imaged. 
     A tracking method and a measurement system with a laser tracker which has two tracking modes, and the measurement system switches from one of the tracking modes into the other when the measurement beam of the laser tracker “loses” the target point or “finds it again”, is described in WO 2007/079601 A1. The normal or ordinary tracking mode is the tracking which is known for laser trackers and which is based on the measurement beam, in which thus for example, the parallel offset between the emitted and reflected measurement beam is detected and one strives for a reduction of this offset by way of changing the measurement beam direction. In the normal tracking mode, the tracker detects the measurement beam which is reflected by the reflector, and a determining of the target point position is possible at any time. In the extraordinary tracking mode, in which the measurement system operates when the tracker cannot detect the reflected measurement beam, the change of the measurement beam direction is controlled by way of data which is recorded by an overview apparatus assigned to the laser tracker. The overview apparatus, for example, is a digital overview camera which provides picture data and has a light-sensitive surface (e.g. CCD) and optics which give the overview camera a viewing angle for example of ±20° in all directions, which is common for an overview apparatus. The overview apparatus can however for example also be a PSD (position sensitive device) which is equipped with the same or similar optics and which only provides position data with respect to the sensor, thus direction data with regard to the apparatus. A direction to the reflector is determined from the data registered by the overview apparatus and, with a suitable change in the measurement beam direction, one attempts to direct this onto the reflector. The extraordinary tracking mode thus operates without a detection of the reflected measurement beam, and an exact determining of the position of the target point with the help of the tracker is not possible in the extraordinary tracking mode. The extraordinary tracking mode is switched on as soon as no reflected measurement beam is detected in the tracker. In the extraordinary tracking mode however, one always checks again and again, as to whether a reflected measurement beam is detected or not, and as soon as this is the case, the system switches again into the ordinary tracking mode and the position measurement is released. 
     The described device and the corresponding method thus can localise a “lost” target point again and thereafter again determine the position of the target point. However, their capability of following rapid changes in the angular position of the target point with regard to the tracker is however limited. This is of particular relevance, if the target point is located close to the tracker, and a given absolute position change—compared to a target point distanced further away—corresponds to a greater change of the angle at which the target point is seen from the tracker. Furthermore, it is necessary for the target point to be able to be held in a relatively calm manner for localising, until the capture is concluded and the position measurement can be activated again. 
     Similar tracking methods using cameras or sensors with a narrow or wide viewing angle are also described in the following three publications: 
     EP 2 071 283 A2 describes the use of two separate cameras with a wide and narrow viewing angle, in each case with their own light source coupled into the camera optics. The cameras are arranged separately from one another, one of these with the viewing axis colinear to a distance meter, and operate with visible light. A target recognition is accomplished in each case by switching on/off the respective light source and a subsequent difference formation from the respective pictures. 
     WO 2009/046763 A1 shows two stages with the target tracking, wherein one switches over between a close range setting of optics, with a wide viewing angle, and a long range setting with a narrow viewing angle. 
     U.S. Pat. No. 7,292,788 B2 describes a laser-based communication with a satellite, wherein a received light beam is tracked with wide field sensors and narrow field sensors.  FIG. 4B  shows an apparatus for a two-stage measurement: either an intermediate/acquisition track sensor ( 660 ) or a fine track quad cell ( 650 ) is applied, in order to lead a laser beam into a fibre-optic ( 640 ). Other embodiments or experimental arrangements ( FIG. 4A ) likewise use two-stage methods. 
     BRIEF SUMMARY OF THE INVENTION 
     It is the object of the invention to provide a tracking method and a measurement system with laser trackers which is designed for the tracking method, which permit the tracking interruptions discussed above, to be bridged not only automatically, but also with high dynamics. A further object is to provide the possibility, given a tracking interruption, of capturing the target point again and starting the position measurement again, while the target point is moved. A further object is to provide a measurement system which permits a comparatively large opening angle (viewing angle) of the overview camera. 
     This object is achieved by the tracking method and the measurement system with a laser tracker, as defined in the patent claims. 
     Thus a target provided with a reflector is tracked by a measurement beam of a laser tracker in the tracking method. In a normal tracking mode, the measurement beam reflected by the reflector is detected in a tracking unit, and a variable for the control of the alignment of the measurement beam is computed from the detection. In an extraordinary tracking mode, in which the measurement beam reflected by the reflector is not detected in the tracking unit, variables for the control of the alignment of the measurement beam are computed from data which are acquired by at least one further apparatus. Thereby, the laser tracker comprises a capture unit and an overview apparatus, wherein the capture unit as well as the overview apparatus have a known position and orientation relative to the measurement beam. The capture unit comprises a detection region or detection angle which lies between the detection region of the tracking unit and the detection region of the overview apparatus. 
     The method in the extraordinary tracking mode carries out the following steps:
         in the case that the target is detectable by the capture unit, controlling the alignment of the measurement beam in accordance with an angle at which the target is visible to the capture unit, and checking as to whether the target can be detected by the tracking unit;   in the case that the target is then detectable by the tracking unit, changing over to the normal tracking mode;   in the case that the target is only detectable by the overview apparatus, controlling the alignment of the measurement beam in accordance with an angle at which the target is visible to the overview apparatus, and checking as to whether the target can be detected by the capture unit.       

     In other words, thus in the extraordinary tracking mode, the variables for the control of the alignment of the measurement beam is computed from data which is acquired selectively by the capture unit or by the overview apparatus, and, if necessary, one changes between the localisation of the target with the capture unit and with the overview apparatus, until the target is detected with the tracking unit. The described steps are thus carried out repeatedly until the transition to the normal tracking mode is effected. 
     By way of this, the opening angle or the field of view of the overview camera is no longer limited by way of the resolution of the overview camera having to be adequately high, so that the target can be reliably captured by the tracker. The intermediately arranged capture unit captures the target by way of the data of the overview camera, and refines the alignment of the tracker such that the target is captured by the tracker. 
     By way of this, it is possible to increase the viewing angle or detection range of the overview apparatus, such that it is also possible to follow the target even with rapid changes of the angle at which the measurement apparatus sees the target, thus above all when the target is moved close to the measurement apparatus. 
     Theoretically, in the case that the target can only be detected by the overview apparatus, then instead of the alignment of the measurement beam, one can firstly only carry out an alignment of the capture unit. Since, as a rule, the capture unit and the tracking unit are however moved with one another, this, as a rule, is also equivalent to the alignment of the measurement beam. 
     In a preferred embodiment of the invention, with the transition to the normal tracking mode, one carries out an absolute distance initialisation (on the fly), for determining an absolute distance between the laser tracker and the target. Such a method is described for example in the published patent applications EP 1 647 838 A1 and US 2009/0033945. With this, apart from the azimuth and elevation of the target (with respect to the laser tracker), its distance is also known. 
     In a further preferred embodiment of the invention, the overview apparatus comprises a zoom function and with this an adjustable detection angle, and, in the case that the target cannot be detected by the overview apparatus at a small detection angle, the following steps are carried out:
         increasing the detection angle of the overview apparatus, and checking as to whether the target can be detected by the overview apparatus;   in the case that the target can be detected by the overview apparatus, controlling the alignment of the measurement beam in accordance with the angle at which the target is visible to the overview apparatus; and   reducing the detection angle of the overview apparatus; or   optionally, in the case that the target cannot be detected by the overview apparatus, carrying out a search routine for localising the target by way of moving the overview apparatus.       

     In another preferred embodiment of the invention, by way of a deflection device, selectively
         either, in a first operating mode of the deflection device, the tracking unit and the capture unit;   or, in a second operating mode of the deflection device, the overview apparatus
 
can be aligned or guided with their beam path onto the target. Thereby, the method comprises the following further steps:
   operating in the first operating mode, or changing to the first operating mode, when the tracking unit or the capture unit detect or search the target,   operating in the second operating mode, or changing to the second operating mode, when the overview apparatus detects or searches the target.       

     In these preferred embodiments of the invention therefore, the target is not simultaneously visible to the tracking unit and the overview apparatus, for example because the beam path either of the tracking unit or of the overview apparatus can be selectively directed to the target by way of a tracking mirror. With this embodiment, although it would be possible—without the capture unit—for a moved target to be localised by the overview apparatus and for the alignment of the measurement beam to be corrected accordingly, after turning the tracking mirror however, this alignment would no longer be correct due to the time delay. Here, the capture unit permits the capture of the target even with an imprecise alignment after turning the mirror. 
     A target provided with a reflector can be tracked by a measurement beam of a laser tracker, in the measurement system with laser tracker. The laser tracker, in a normal tracking mode, is designed to detect the measurement beam reflected by the reflector, with a tracking unit, and to compute a variable for the control of the alignment of the measurement beam from the detection, wherein additionally the laser tracker is designed, in an extraordinary tracking mode, in which the measurement beam reflected by the reflector cannot be detected in the tracking unit, to compute variables for the control of the alignment of the measurement beam from data which is acquired by at least one further apparatus. Thereby, the laser tracker comprises a capture unit and an overview apparatus, wherein the capture unit as well as the overview apparatus have a known position and orientation relative to the measurement beam. The capture unit comprises a detection region which lies between the detection region of the tracking unit and the detection region of the overview apparatus. The laser tracker is set up, in the extraordinary tracking mode, to carry out the above described method steps.
         in the case that the target can be detected by the capture unit, controlling the alignment of the measurement beam in accordance with an angle at which the target is visible to the capture unit, and checking as to whether the target can be detected by the tracking unit;   in the case that the target can be detected by the tracking unit, initiating a change-over to the normal tracking mode;
           in the case that the target can only be detected by the overview apparatus, controlling the alignment of the measurement beam in accordance with an angle at which the target is visible to the overview apparatus, and checking as to whether the target can be detected by the capture unit.   
               

     In a preferred embodiment of the invention, the capture unit uses the same measurement beam as the tracking unit, for determining the position of the reflector in its field of view. Alternatively, the capture unit itself can emit measurement light which (outside the laser tracker) runs coaxially to the measurement beam of the tracking unit and enters with this through common exit optics. 
     In a further preferred embodiment of the invention, the beam path of the measurement beam—and of the measurement light of the capture unit, in the case that this is not identical to the measurement beam—can be guided by the deflection device onto the target. Then selectively, by way of the deflection device,
         in a first operating mode of the defection device, either the tracking unit and the capture unit,   or, in a second operating mode of the deflection device, the overview apparatus,
 
with their beam path, can be aligned onto the target.
       

     In other preferred embodiments of the invention, the capture unit emits measurement light which is parallel and not coaxial to the measurement beam of the tracking unit and enters through separate exit optics. Thereby, the capture unit preferably has its own illumination means. Moreover, the capture unit and the tracking unit can both operate with infrared light (i.e. be sensitive to infrared), wherein preferably spectral sensitivity regions of the capture unit and of the tracking unit with regard to incident light are different from one another, and in particular do not overlap one another. The two units, thus, do not react to the light of the respective other unit. 
     In a further preferred embodiment of the invention, the capture unit comprises a picture sensor for detecting a picture of the target. Thus not only is a PSD present, which only provides X and Y position signals of a light point, but a complete picture, by way of which on the one hand, as with PSD, readings for tracking the target, but on the other hand also further functions of the laser tracker can be realised. Such further functions are, for example, the determining of the orientation of the target, the identification of an object as the target, the tracking of an object by way of optical features (“feature detection and object tracking”). With this therefore, one can also identify and track objects which are not point-like, or entire light point arrangements. 
     Further preferred embodiments are to be deduced from the dependent patent claims. Thereby, features of the method claims, in an analogous fashion, can be combined with the device claims and vice versa. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Hereinafter, the subject matter of the invention is explained in more detail by way of preferred embodiments which are represented in the accompanying drawings. In each case there are shown schematically in: 
         FIGS. 1 to 3  different embodiments of the invention; and 
         FIGS. 4 and 5  flow diagrams according to exemplary realisations of the inventive method. 
     
    
    
     The reference numerals used in the drawings and their significance are listed in a conclusive manner in the list of reference numerals. Basically, the same parts have been provided with the same reference numerals in the figures. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a structure of a laser tracker  10  in a first preferred embodiment of the invention. The laser tracker  10  comprises a tracking unit  11  with a measurement beam M, a capture unit  12  with a capture region E, and an overview apparatus  13  for localisation, with a localisation region L. The laser tracker  10  determines azimuth and elevation as well as the distance of a target  9 , preferably of a retroreflector  17  on the target  9 , with respect to the laser tracker  10 . An absolute distance meter (ADM) and/or an interferometric distance meter (IFM) are present in a distance measurement unit  22 , for determining and tracking the target distance. The overview apparatus  13  can have a zoom function. 
     The tracking unit  11  and the capture unit  12  use common output optics  8 , i.e. the light is coupled in from and to the two units onto a common beam path. This contains the measurement beam and is aligned by way of a motorically driven tracking mirror  21  onto a reflector  17 , for example a retro-reflector such as a corner reflector prism or a triple mirror. 
     The tracking unit  11  comprises a picture sensor or however a PSD  14  (position sensitive device) which produces signals corresponding to the location of a light point on a surface of the PSD. The tracking unit  11  operates in the known manner by way of determining the position of the reflected measurement beam M on the PSD  14 , for the correction of the alignment of the measurement beam by way of the computation and control unit  19  and actuators for moving the tracking mirror  21 . The tracking unit  11  is thus responsible for the highly precise tracking of the measurement beam, and for this has a detection region with collimated (parallel) measurement light with a width of the measurement light beam of e.g. 1 mm to 2 mm. 
     The capture unit  12  comprises a camera or a two-dimensional picture sensor  15 . A light beam produces a light point on the picture sensor  15 . This light beam can be the reflected measurement beam (i.e. a part of the light coupled out from the measurement beam), or a second beam which preferably runs coaxially to the measurement beam M and is coupled into its beam path, but has a different wavelength. The capture unit  12  permits the capture of a moved target and the transition for the target tracking by way of the tracking unit  11  also during the movement of the target. An opening angle detected by the picture sensor  15  is preferably about ±5°, thus in total of 10°. 
     The overview apparatus  13  can be rotatable at least about the vertical axis, optionally also about the elevation axis, together with the alignment of the measurement beam. The overview apparatus  13  optionally has illumination means  16 , by way of which reflecting elements fastened on the target (not drawn), can be illuminated and thus can be better visible to the overview apparatus  13 . It is also possible for the illumination means  16  to be designed to communicate with the target. The reflector  17  is preferably provided with illumination means  18  in order to be recognised by the overview apparatus  13 . The reflector  17  and illumination means  18  are arranged on a target  9  which can, for example, be provided with a scanning tip. The illumination means  18  can also be used, in order to determine the orientation of the target  9  by way of the overview apparatus or a further camera arranged on the laser tracker  10 , so that all six degrees of freedom of the target  9  can be determined. The overview apparatus  13  is preferably a camera which is sensitive to light in the visible region. An opening angle which is detected by the picture sensor  15  is preferably about ±5 to ±15°. The overview apparatus  13  supplies picture data to the computation and control unit  19 , for evaluation. 
     The data of the capture unit  12  and of the tracking unit  11  is likewise processed by the computation and control unit  19  and is used with the control of the alignment of the measurement beam M for tracking the reflector  17 . The computation and control unit  19  is set up, in particular programmed, for carrying out the method according to the invention. 
     The detection region or the field of view or the opening angle of the capture unit  12  is, thus, greater than that of the tracking unit  11 , and the detection region of the overview apparatus  13  is larger than that of the capture unit  12 . As a rule, the maximum opening angle in the horizontal direction is approximately equal to that in the vertical direction. The opening angles then in both directions are in each case smaller or larger than the opening angles of the other units. 
       FIG. 2  shows a structure of a tracker  20  with a connectable overview apparatus  13  according to a second preferred embodiment of the invention. Thereby, hereinafter, only features which differ from the first embodiment are described. The overview apparatus  13  here is not aligned directly onto the reflector  17 . Instead, exit optics of the overview apparatus  13  are directed onto the tracking mirror  21 . The tracking mirror  21  is tilted, in order to operate the overview apparatus  13 , so that the overview apparatus  13  looks through the tracking mirror  21  to the reflector  17 . No individual mechanical drive is necessary for aligning the overview apparatus  13  by way of this. The overview apparatus  13  cannot be operated simultaneously with the tracking unit  11  or the capture unit  12  by way of this. 
       FIG. 3  shows a structure of a compact apparatus  39  according to a third preferred embodiment of the invention. Thereby, hereinafter only features which differ from the first embodiment are described. The tracking unit  11 , the capture unit  12  and the overview apparatus  13  are arranged on a carrier  31  in a commonly moved manner. They are, thus, arranged to one another in a fixed relation, and together are aligned onto the reflector  17  by way of a motorically driven movement of the carrier  31  with respect to a base  32 . The capture unit  12  and the tracking unit  11  here in each case have their own exit optics, but can also have single, common exit optics. Apart from the illumination means  16  for the overview apparatus  13 , further illumination means  33  for the capture unit  12  are also present. Preferably, these further illumination means  33  emit light in the infrared region, and the capture unit  12  is only sensitive in the infrared region. The tracking unit  11  preferably comprises a picture detection sensor, in order to detect the deviation of the detected measurement beam from the desired position. 
       FIG. 4  shows a preferred variant of the course of the method according to the invention. In the ordinary tracking mode, one checks whether the measurement beam M of the laser tracker is reflected by the target  9  or the reflector  17  and is visible to the tracking unit  11  (first decision  202  “LOCKED?” with respect to the detection of the reflected measurement beam). 
     If this is the case, then in a first following operation  203  “MEAS/ADJ”, the position of the reflected measurement beam in the tracking unit  11  (thus for example on a PSD  14 ) is determined, a corrective movement computed therefrom, and the measurement beam M moved (adjusted) accordingly. Subsequently, one continues further with the step of the first decision  202 . 
     If this is not the case, then one checks as to whether the target  9  is visible to the capture unit  12 . Preferably, this is likewise effected by way of the measurement beam M, but by way of its projection onto the picture sensor  15  (second decision  204  “CATCH?” with respect to the detection of the target). 
     If this is the case, then in a second following operation  205  “MEAS/ADJ”, the position of the reflected measurement beam is determined in the capture unit  12 , a corrective movement computed therefrom, and the measurement beam M is moved (adjusted) accordingly. Subsequently, in a third decision  206  “LOCKING?”, with respect to the detection of the reflected measurement beam, one checks whether the measurement beam M is visible to the tracking unit  11 .
         If this is the case, then preferably an absolute distance measurement is updated or is carried out afresh (absolute distance initialisation  207  “ADMinit”). Subsequently, one continues further with the step of the first decision  202 .   If this is not the case, then one continues further with the second decision  204 .       

     If the target  9  is not visible to the capture unit  12 , then one checks as to whether the target  9  is visible to the overview apparatus  13  (fourth decision “OVC?” with respect to the visibility of the target). This is preferably effected by way of the reflection of light of the illumination means  16  at the target, and/or by way of the illumination means  18  at the target  9 . Preferably, these illumination means and the overview apparatus  13  function with light in the visible region. 
     If the target  9  is visible to the overview apparatus  13 , then in a third following operation  209  “MEAS/ADJ”, the position of the reflected measurement beam is determined in the overview apparatus  13 , a corrective movement computed therefrom and the measurement beam M moved accordingly. Subsequently, one continues further with the step of the second decision  204 . 
     If the target  9  is not visible to the overview apparatus  13 , then for example a search routine  210  “SRCH” is carried out. Such search routines are known per se. For example, the alignment of the tracker or at least of the optical viewing axis of the overview apparatus  13  is changed according to a predefined pattern and thereby one constantly checks as to whether the target  9  can be found on the picture of the overview apparatus  13  which corresponds to the respective alignment, or cannot be found (fourth decision  208 ). If, for example, the search remains without success during a given time or after the completion of a complete routine, the system, as the case can be, can stop with a corresponding communication to the operating person. 
     With a start  201  of the measurement system, one preferably begins with the target search with the largest viewing angle, thus with the fourth decision  208  with respect to the visibility of the target. In another preferred embodiment of the invention (not shown in the figure), the method begins by way of the target being manually moved with the reflector  17  into the detection region of the tracking unit  11  and being automatically detected by the tracking unit  11  and then tracked. Thereupon, the absolute distance measurement is carried out for the first time (analogously to “ADMinit”). 
     In one variant of the invention, if according to the first decision  202 , the reflected measurement beam is not detected in the tracking unit  11 , the method is not continued with the second decision  204 , but with the fourth decision  208  with respect to the visibility of the target (dashed arrow in  FIG. 4 ). 
       FIG. 5  shows a further variant of the course of the method according to the invention for the case that the overview apparatus  13  has a zoom function. The dashed edged part of  FIG. 4  can then be replaced by elements of  FIG. 5 . The method then runs as follows: 
     If in the fourth decision  208  “OVC?” the target  9  is not visible to the overview apparatus  13 , then firstly in a step of the viewing angle opening  311  “ZOOMOUT”, the zoom objective  7  is set to a larger viewing angle or detection angle, and in a fifth decision  312  “VIS?”, one checks with regard to the visibility of the target as to whether the target  9  is visible to the overview apparatus  13 .
         If the target  9  is not visible to the overview apparatus  13 , then for example a search routine  210  “SRCH” is carried out as described above.   If the target  9  is visible to the overview apparatus  13 , then in a fourth following operation  313  “MES/ADJ”, the position of the reflected measurement beam in the overview apparatus  13  is determined, a corrective movement computed therefrom and the measurement beam is moved accordingly. In a step of viewing angle reduction  314  “ZOOMIN”, the zoom objective  7  is set again to a smaller viewing angle and one continues further with the step of the fourth decision  208 .       

     Basically of course, also further variants are possible in the sequence of the mentioned steps, which lead to the same result. 
     In a preferred embodiment of the invention, the position of the illumination means  18  at the target  9  is known and the overview apparatus  13  already during the capture, thus still during the extraordinary tracking mode, determines at least an estimation of the orientation of the target  9 , by way of the imaging of the illumination means in the overview apparatus  13 . Such methods for determining the orientation of a target  9  are known, but only in the context of a normal tracking mode, in which the distance between the tracker and the target is precisely known. 
     LIST OF REFERENCE NUMERALS 
     
         
           7  zoom optics 
           8  common exit optics 
           9  target 
           10  laser tracker 
           11  tracking unit 
           12  capture unit 
           13  overview apparatus for localisation 
           14  PSD 
           15  picture sensor 
           16  illumination means 
           17  reflector 
           18  illumination means on the target 
           19  computation and control unit 
           20  trackers with connectable overview apparatus 
           21  tracking mirror 
           22  absolute distance meter ADM and interferometer IFM 
           30  compact apparatus 
           31  carrier 
           32  base 
           33  illumination means for capture unit 
         M measurement beam 
         E capture region 
         L localisation region