Abstract:
An apparatus for enhanced viewing of objects in two separate simultaneous frequency bands of light that is constructed in accordance with the principles of the present invention includes a primary mirror and a moveable secondary mirror that is disposed above the primary mirror to provide a long focal length. A beam splitter splits the optical path into two paths, one for infrared and another path for visible. Various optical elements and devices are described along each path. The secondary mirror is displaced away from its concentric position over the primary mirror to provide shorter focal lengths in IR and visible. Focal Plane Arrays (FPA&#39;s) are disposed where desired along both optical paths for real-time viewing and data acquisition. The preferred FPA in visible is a CCD. The preferred FPA in IR is disposed in a Dewar.

Description:
[0001]     This invention is related to prior U.S. patent Ser. No. 10/763,297, entitled “Structure for Supporting an Optical Telescope”, that was filed on Jan. 21, 2004 and this application fully includes the content, specification, and drawings of the above application by reference herein and claims benefit of priority thereof. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention, in general, relates to telescopes and, more particularly, to a variable field of view telescope that is adapted for simultaneous use with visible and infrared wavelengths.  
         [0004]     Cassegrain types of telescopes are well known reflector-types of telescopes that are used in both the recreational arts for astronomy and ground observation and also for commercial purposes as well as for certain military applications. In general, they offer a longer focal length in a more compact package. It is desirable with all optical systems to obtain as much light energy as possible (i.e., to provide a large aperture) in as compact and rigid a structure as is possible.  
         [0005]     Variable field of view telescopes that include a plurality of field of view optical groups for insertion into the optical path are also known. The known prior art devices utilize a “C” structure that extends in an arc from the base of a primary mirror to a position over the primary mirror. A turret attached to the upper end of the C structure is used to suspend the plurality of optical groups a predetermined distance over the primary mirror.  
         [0006]     The plurality of optical groups that are disposed on the turret are arranged for sequential insertion of one group into the field of view following the simultaneous withdrawal of a preceding group from the field of view. Accordingly, one optical group is always disposed in the field of view.  
         [0007]     While providing the benefit of being able to withdraw and then insert one of several optical groups into the field of view (to affect the field of view), there are several disadvantages inherent with this type of design. An important disadvantage is that the C structure incurs considerable flexing that is an inherent characteristic due to the length of the arm and the fact that it is supported only at the base of the structure.  
         [0008]     This can cause misalignment of the optical groups and resultant distortion in the optical path. Also, because of the long length of the C structure, thermal expansion and contraction effects can further contribute to misalignment and error along the optical path or limit the temperature range in which the equipment can properly function.  
         [0009]     Also, the C structure obstructs a significant quantity of light energy that would otherwise impede upon the primary mirror. The C structure provides only one point of mechanical support. Therefore, it must be mechanically large and strong enough so as to sufficiently limit flexing and vibration. Because the structure is supported on the outside of the primary mirror, a long moment arm is also created from the base of the C structure to the turret. It is the long moment arm that is subject to flexing and the turret that is subject to vibration.  
         [0010]     Vibration can be in response to any mechanical energy that the C structure experiences. For example, vibration of the engine(s) that propel the vehicle or aircraft may be transmitted to the C structure. While there is always the possibility for vibration of the moment arm, there is also the possibility that the length of the arm can resonate in a harmonic frequency thereby increasing the amplitude of vibration.  
         [0011]     To minimize flexing and vibration of the C structure design, it is built as large and as strong as it needs to be in order to function in any given environment. However, this is not desirable because it is preferable that the C structure be as small as possible so as to minimize the amount of light energy that it obstructs. Accordingly, a tradeoff is made in the prior art design that sacrifices light gathering ability for necessary rigidity.  
         [0012]     Also, the larger the C structure is, the heavier it also becomes. This is another limitation that is an especially important consideration in various circumstances, for example, when the telescope is used in an aircraft or spacecraft. While it is desirable that a telescope be lightweight, this is crucial in certain applications.  
         [0013]     The prior art design also relies upon the use of moveable hard stops to repeat the position that the plurality of optical groups are maintained in. This limits the speed by which optical groups can be changed without causing damage.  
         [0014]     Furthermore, when any of the plurality of optical groups are moved into and out of the optical path, it is desirable that precise repeatability of their position occur, else the optical path is adversely affected. The prior art designs that require variability in the stops are unable to provide optimum repeatability in the positioning of the optical groups.  
         [0015]     An especially significant limitation with the prior art designs is that the field of view changes in steps according to the number and optical characteristics of the various optical groups. It is desirable to be able to step in IR and also to provide a progressive zoom capability in visible as well, which the prior art fails to provide.  
         [0016]     The prior art designs are also limited in their ability to provide both IR (infrared) and visible light gathering ability. Separate optical paths are typically required. If the telescope is used in a gimbal ball, for example, this would result in having two apertures in the gimbal ball, one for visible and one for IR. This is undesirable for many reasons.  
         [0017]     There are many light frequencies that may be of interest. It is desirable to be able to view two or more channels simultaneously and also to change the field of view (FOV) from between wide angle FOV (lower magnification) and narrow FOV (higher magnification) capabilities. Prior art designs have been unable to effectively switch back and forth between infrared and visible light while also providing a zoom capability in visible.  
         [0018]     For many applications it is desirable to be able to zoom in the visible spectrum. For example, if after detecting an object of interest using a wide field of view (in either the visible spectrum or IR spectrum), it is desirable to then be able to narrow the field of view and zoom in to complete a closer examination of the object. If the object of interest was first discovered as a presence in the infrared spectrum, it may be desirable to switch to the visible spectrum and zoom in accordingly.  
         [0019]     It is also desirable to be able to gather data in both spectrums simultaneously, for example in a narrow FOV in IR and in visible. Ideally, the data in both wavelengths could be recorded for future comparison and subsequent analysis. It is desirable then that a recording camera or some other type of transducer (i.e., a focal plane array; FPA) be placed in the field of view, as desired.  
         [0020]     Prior art designs have provided limited zoom capability in the visible spectrum. This is because a rigid and relatively long physical path is required to house the optical elements that typically accompany an optical zoom system.  
         [0021]     Furthermore, space is often a commodity in short supply, especially if the telescope is to be housed in a gimbal ball.  
         [0022]     Accordingly, it is desirable to be able to provide a space to accommodate the optical elements of a visible zoom optical system and do so in such manner as to provide an optimally long linear path that is especially rigid. It would also be a significant and unexpected benefit if the structures used for the visible optical zoom system were able to provide increased rigidity to the overall structure for supporting an optical telescope.  
         [0023]     As mentioned briefly hereinabove, there are many possible frequencies of light (electromagnetic) energy that may of interest depending upon the application. It is desirable to provide versatility in such an instrument to change the transducers and analyze the relevant spectrums that may be of interest.  
         [0024]     The structural limitations that have, heretobefore, prevented the design of such an optical system have been satisfied by the disclosure of the related patent application, herein incorporated by reference. Even so, an optical system that provides such versatility has been previously unavailable and it is the subject of the instant disclosure to describe such capability.  
         [0025]     Accordingly, there exists today a need for an optical telescope that helps ameliorate the above-mentioned problems and limitations.  
         [0026]     Clearly, such an apparatus would be an especially useful and desirable device.  
         [0027]     2. Description of Prior Art  
         [0028]     Telescopes and zoom lenses are, in general, known. For example, the following patent describes various types of these devices:  
         [0029]     U.S. Pat. No. 5,907,433 to Voigt et al, May 25, 1999; and  
         [0030]     U.S. Pat. No. 5,940,222 to Sinclair et al, Aug. 17, 1999.  
         [0031]     While the structural arrangements of the above described devices, at first appearance, have similarities with the present invention, they differ in material respects. These differences, which will be described in more detail hereinafter, are essential for the effective use of the invention and which admit of the advantages that are not available with the prior devices.  
       OBJECTS AND SUMMARY OF THE INVENTION  
       [0032]     It is an object of the present invention to provide an optical telescope that is especially compact.  
         [0033]     It is also an important object of the invention to provide an optical telescope that provides simultaneous viewing in infrared and visible wavelengths.  
         [0034]     It is yet an important object of the invention to provide an optical telescope that includes a simultaneous dual optical path.  
         [0035]     It continues as an important object of the invention to provide an optical telescope that includes a simultaneous dual optical path wherein each path includes a different wavelength of light.  
         [0036]     Another object of the invention is to provide an optical telescope that includes a focal plane array (FPA) to capture an image.  
         [0037]     Still another object of the invention is to provide an optical telescope that provides a step field of view change in infrared.  
         [0038]     Still yet another object of the invention is to provide an optical telescope that provides zoom capability in visible.  
         [0039]     Yet another important object of the invention is to provide an optical telescope that provides approximately a five to one change in field of view per step change in the field of view in infrared.  
         [0040]     Still yet another important object of the invention is to provide an optical telescope that provides approximately a twenty-five to one change in focal length in infrared.  
         [0041]     A first continuing object of the invention is to provide an optical telescope that includes a CCD as the FPA in a visible wavelength beam path.  
         [0042]     A second continuing object of the invention is to provide an optical telescope that includes an FPA in a Dewar in an infrared beam path.  
         [0043]     A third continuing object of the invention to provide an optical telescope that includes a wide and mid FOV in infrared without the use of either a primary or a secondary mirror.  
         [0044]     A fourth continuing object of the invention to provide an optical telescope that includes a narrow FOV in infrared with the use of a primary and secondary mirror.  
         [0045]     A fifth continuing object of the invention to provide an optical telescope that includes a large aperture for narrow FOV focal lengths in either IR or visible.  
         [0046]     A sixth continuing object of the invention to provide an optical telescope that includes an IR turret carousel that is adapted to rotate between a plurality of positions, the carousel being disposed along an infrared path.  
         [0047]     A seventh continuing object of the invention to provide an optical telescope that includes an independent zoom lens group in visible.  
         [0048]     An eighth continuing object of the invention to provide an optical telescope that includes a zoom lens group in visible that has a front zoom group of lenses and a back zoom group of lenses that move toward each other or away from each other to increase or decrease focal length.  
         [0049]     A ninth continuing object of the invention to provide an optical telescope that includes an IR turret carousel that is adapted to rotate between three positions along an infrared path, and wherein the carousel includes two groups of lenses for use in mid and wide FOV and a clear optical path with no lenses for use in narrow FOV.  
         [0050]     A tenth continuing object of the invention to provide an optical telescope that includes an FPA in the infrared optical path.  
         [0051]     An eleventh continuing object of the invention to provide an optical telescope that includes an FPA in the visible optical path.  
         [0052]     A twelfth continuing object of the invention to provide an optical telescope that includes an FPA in the infrared optical path and an FPA in the visible path for simultaneous acquisition of data on two different wavelengths.  
         [0053]     A thirteenth continuing object of the invention to provide an optical telescope that includes an FPA in the infrared optical path and an FPA in the visible path for simultaneous acquisition of data on two different wavelengths and at two different focal lengths (i.e., FOV).  
         [0054]     A fourteenth continuing object of the invention to provide an optical telescope that includes an optimized diffraction-limited focal length in both IR and visible.  
         [0055]     Briefly, an optical telescope that is constructed in accordance with the principles of the present invention includes the support structures of the related patent, herein incorporated by reference. When a secondary mirror is in position, a dichroic beam splitter acts as a band pass filter to split a primary optical path into two paths, one for infrared and another path for visible. A narrow FOV is provided for infrared along one path and a narrow FOV is provided in visible along the other path when the secondary mirror is disposed over the primary mirror and both mirrors are included in the optical paths. When a secondary mirror is displaced away from its concentric position over the primary mirror, the primary mirror is not used and a shorter focal length (i.e., a wider FOV) is provided for both visible and infrared. An IR turret functions as a carousel to insert or remove groups of lenses from the IR path, thereby providing a step function to vary the IR focal length from wide to intermediate FOV. When the secondary and primary mirrors are used, the IR turret is moved to a position that does not insert any groups of lenses into the IR path. As desired, a zoom lens group is provided in visible that receives visible light that has been collimated after reflecting off the primary and secondary mirrors and the dichroic beam splitter. When the secondary mirror is pivoted into a second position (away from the primary), the dichroic splitter and certain of the visible lens groups look directly out into space and receives collimated ambient light (all frequencies). The IR and visible paths can then be used for calibration purposes or, if desired, for viewing at shorter focal lengths than when the primary and the secondary mirror are in use. Focal Plane Arrays (FPA&#39;s) are disposed where desired along both optical paths for simultaneous acquisition of real-time data. The output of the FPA&#39;s is saved on any preferred media, as desired, for later analysis. The preferred FPA in visible is a CCD. The preferred FPA in IR is disposed in a Dewar. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0056]      FIG. 1  is a cross sectional view of an optical telescope taken on the line B-B in  FIG. 2 .  
         [0057]      FIG. 2  is a top view of the telescope of  FIG. 1  with a secondary mirror disposed over and in concentric alignment with a primary mirror.  
         [0058]      FIG. 3  is a cross sectional view of an optical telescope taken on the line A-A in  FIG. 2 .  
         [0059]      FIG. 4  is a cross sectional view of an optical telescope taken on the line C-C in  FIG. 2 .  
         [0060]      FIG. 5  is a top view of the telescope of  FIG. 1  looking down along a center longitudinal axis of the primary mirror with a turret and the secondary mirror disposed in a second position, out of the optical path. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0061]     Whenever possible, the reference numerals used in patent application Ser. No. 10/763,297, entitled “Structure for Supporting an Optical Telescope”, that was filed on Jan. 21, 2004, and which is herein incorporated by reference, are used in this disclosure for the purpose of maintaining consistency. Certain portions of the specification of the prior patent application are essential to an understanding of the instant invention and these portions have been edited and included herein to improve clarity of the instant disclosure.  
         [0062]     Referring on occasion to all of the drawing figures and now in particular to  FIG. 1 , is shown in cross-section, a structure for supporting an optical telescope, identified in general by the reference numeral  10 .  
         [0063]     The structure  10  is used to support a variety of optical components, as are described in greater detail herein to provide an optical telescope  100 .  
         [0064]     A base assembly, identified in general by the reference numeral  12 , supports the essential component parts of the telescope  100 .  
         [0065]     A large primary mirror  14  gathers substantial incident radiation thereby providing a large aperture for the optical telescope  100  in a compact structure. The primary mirror  14  includes a center aperture  15  that includes a diameter sufficient to clear an upper portion  15   a  of the base  12 . The primary mirror  14  rests directly on the base assembly  12  on a support ring  13  that provides a circle of support that is proximate the center aperture  15 .  
         [0066]     A gap  13   a  exists intermediate the base assembly  12  and the primary mirror  14 . Therefore, the only support for the primary mirror is provided by the ring  13  that extends around the center aperture  15 .  
         [0067]     The primary mirror  14  is attached to the base assembly  12  by any method that is desired, typically by the use of an adhesive. Accordingly, the primary mirror  14  is adhered to the ring  13 .  
         [0068]     The center aperture  15  includes a center longitudinal axis  16  (shown in dashed line) that passes vertically through the center of the primary mirror&#39;s  14  aperture  15 .  
         [0069]     An upright member  18  includes a first end  18   a  and an opposite second end  18   b.  The first end  18   a  is attached to the base assembly  12  at a first location  20  and at a second location  22 . The first location  20  is located inside the aperture  15 . The second location  22  is located outside (i.e., beyond) the circumference of the primary mirror  14 . Attachment of the upright member  18  to the base  12  at the first and second locations  20 ,  22  is by a plurality of threaded screws (not shown) or any other preferred method.  
         [0070]     Accordingly, the upright member  18  is supported on two opposite sides (i.e., the first and second sides  20 ,  22 ) at the first end  18   a.  This provides maximum support and rigidity. The upright member  18 , because it is supported both inside the aperture  15  and beyond the circumference of the primary mirror  14 , is optimally thin (which blocks a minimum of incident radiation on the primary mirror  14 ) while also being maximally rigid. See  FIG. 2 .  
         [0071]     A secondary mirror  24  is attached to a turret assembly, identified in general by the reference numeral  26 . The turret  26  is attached to the second end  18   b  of the upright member  18 . The turret  26  is adapted to pivot about an axis  27  from a first position (as shown in  FIG. 1 ) in which a center of the secondary mirror  24  aligns with the center longitudinal axis  16  into a second position (as shown in  FIG. 5 ) in which the center of the secondary mirror  24  does not align with the center longitudinal axis  16 .  
         [0072]     In the first position, the turret  26  and the upright member  18  (disposed under the turret  26 ) obstruct a minimum of light energy. In the second position (as is described in greater detail hereinafter, the primary mirror  14  is not utilized and, accordingly, any obstruction of light energy by the turret  26  or secondary mirror  24  on the primary mirror  14  is, therefore, of no consequence.  
         [0073]     As is further shown in  FIG. 1 , while in the first position, a first optical path (shown in general by arrow  28 ) is provided in which incident light  28   a  that strikes the primary mirror  14 , is reflected to the secondary mirror  24  and down to a dichroic beam splitter  30  that is disposed on the upper portion  15   a  and elevated over the center aperture  15  of the primary mirror  14 .  
         [0074]     The dichroic beam splitter  30  divides (or splits) the first optical path  28  into a first secondary optical path (shown by arrow  32 ) that passes through the beam splitter  30  and through the center aperture  15  and into a second secondary optical path (shown by arrow  34 ) that is reflected up off of the beam splitter  30  and through a second center aperture  36  of the secondary mirror  24 .  
         [0075]     The dichroic beam splitter  30  is a band pass filter that allows longer wavelengths of light in the infrared spectrum to pass through the beam splitter  30  along the first secondary optical path  32 . The beam splitter  30  appears, essentially, as a mirror to the shorter wavelengths of light. Therefore, the shorter wavelengths of light comprising the visible spectrum and beyond are reflected off of the beam splitter  30  along the second secondary optical path  34 .  
         [0076]     Additional optical groups and elements are disposed along the first secondary optical path  32 , as shown. In particular, an IR turret  32   a  includes a plurality of movable optical groups that are described in greater detail hereinafter.  
         [0077]     The IR turret  32   a  is urged to rotate by a drive shaft  32   b  (see  FIG. 3 ) that passes through an opening provided in the upright member  18  proximate the first end  18   a  thereof. The drive shaft  32   b  is powered by an electrical motor that is disposed outside the primary mirror  14 .  
         [0078]     The IR turret  32   a  rotates about a turret axis  32   c  that passes through a center of the drive shaft  32   b  and is adapted to move at least one optical element  32   d  into a position that is below the surface of the primary mirror  14 . Placement of the optical element  32   d  below the surface of the primary mirror  14  provides a converging optical bundle having a low f number below the primary mirror  14 .  
         [0079]     Accordingly, the structure for supporting an optical telescope  10  allows for the placement of the optical element  32   d  below the primary mirror  14  in the optical field resulting in exceptional optical performance in a compact overall structure  10 . As the IR turret  32   a  is rotated, additional optical elements (that are included as a part of the IR turret  32   a ) are also adapted to be rotated and disposed in the optical field below the primary mirror  14 .  
         [0080]     The IR turret  32   a  is a type of a carousel that includes, preferably, three different optical configurations that can be rotated into position along the first secondary optical path  32 .  
         [0081]     According to a first optical configuration for the IR turret  32   a,  as shown in  FIGS. 1 and 3 , an upper group of lenses  33   a  is disposed proximate the dichroic beam splitter  30  and an opposite lower group of lenses  33   b  (including the one optical element  32   d ) are disposed at an opposite end of the IR turret  32   a.    
         [0082]     The upper and lower group of lenses  33   a,    33   b  direct the IR optical bundle onto an IR fold mirror  33   c  which folds (i.e., reflects) the optical bundle at approximately a ninety degree angle to a direction that is substantially perpendicular with respect to the center longitudinal axis  16 . This allows the placement of other optical groups, as are described in greater detail hereinafter, under the primary mirror  14 , thereby preserving the overall compact configuration of the optical telescope  100 .  
         [0083]     The IR turret  32   a  includes a second optical configuration (that it is rotated into) that includes a second upper group of lenses that, when rotated into position, are also disposed proximate the dichroic beam splitter  30  and an opposite second lower group of lenses that are also disposed distally away from the dichroic beam splitter  30 .  
         [0084]     The first and second optical configurations are used when the secondary mirror  24  is not in use, when the turret  26  is rotated into the second position. Accordingly, the first and second optical configurations of the IR turret  32   a  provide a wide FOV and mid-range FOV in infrared, along the first secondary optical path  32 .  
         [0085]     It is to be understood that the terms infrared and visible encompass a range of frequencies and wavelengths and that this range is varied as desired for either the first secondary optical path  32  or the second secondary optical path  34 .  
         [0086]     If desired, a single frequency (i.e., monochromatic) of light can also be used along either optical path, if that is of interest. Filtering techniques for selecting any band or frequency are known to those possessing ordinary skill and these techniques can be incorporated after having had benefit of the instant disclosure to provide the desired band or frequency of light energy wherever desired. What is important to understand is the optical telescope  100  allows the simultaneous acquisition of two different bands or frequencies along two different optical paths at all times.  
         [0087]     The IR turret  32   a  includes a third optical configuration (that it is rotated into) that does not include any elements in the IR turret  32   a  or otherwise along the first secondary optical path intermediate the dichroic beam splitter  30  and the IR fold mirror  33   c.  The third optical configuration is provided for use when the secondary mirror  24  is disposed over the primary mirror  14  as in  FIG. 1 , and the longest focal length in IR (and visible) is provided along the first secondary optical path  32 .  
         [0088]     In the third optical configuration position, the IR turret  32   a  is rotated so that no lenses in the IR turret  32   a  affect the ray bundle of IR that passes through the IR turret  32   a.  The density of the IR ray bundle that impinges on the IR fold mirror  33   c  has, preferably, nearly the same density for all FOV&#39;s (i.e., wide, mid, narrow).  
         [0089]     Magnets are attached to an outside of the IR turret  32   a  at two of the three optical configuration locations, as desired, and these magnets are precisely located and are sufficiently strong to retain the IR turret  32   a  in each of two positions, absent a torque being applied by the electrical motor that would rotate the IR turret  32   a.  The electrical motor is used only to rotate the IR turret  32   a  from one optical configuration position to another and then back again.  
         [0090]     The magnets are used in the first and second optical configuration locations to precisely maintain the upper and lower groups of lenses of the IR turret  32   a  in position. However, when the IR turret  32   a  is in the third position (i.e., when no lenses in the IR turret  32   a  are disposed along the first secondary optical path  32 ), the magnets may be eliminated and the IR turret  32   a  allowed to oscillate slightly from side to side as allowed by the tolerances of the drive shaft  32   b  and those inherent with the electrical motor. This is because the beam path of the first secondary optical path  32  is not affected by the slight wobble of the IR turret  32   a  when the primary mirror  14  and secondary mirror  24  are included as part of the first secondary optical path  32 .  
         [0091]     It is preferable that when the IR turret  32   a  is in the third or center position (i.e., in the “no lens” position) that the magnets are included and used to better hold the IR turret  32   a  in this position. It is preferable that when the IR turret  32   a  is in either the first or second optical configuration locations (i.e., when the IR turret  32   a  is rotated to its maximum in either direction and when lenses in the IR turret  32   a  are disposed along the first secondary optical path  32 ) that hard stops be also used and that the magnets used to apply a force to the IR turret  32   a  that secures it against the hard stops. This is preferred in order to more precisely control the position of the IR turret  32   a  when it is introducing lens groups into the first secondary optical path  32 .  
         [0092]     In the first and second optical configuration locations for the IR turret  32   a,  the turret  26  has first been moved into the second position with the secondary mirror  24  out of the optical path. In this instance, the incident light  28   a  does not reflect off the primary mirror  14 , strike the secondary mirror  24 , and then impinge on the dichroic beam splitter  30 . The second position for the turret  26  is shown in  FIG. 5 .  
         [0093]     The second position in which the turret  26  has been rotated so that the center of the secondary mirror  24  does not align with the center longitudinal axis  16  as shown in  FIG. 5  provides a pair of second primary optical paths (as shown by arrow ends  50  and  52 ) that are open to receive the ambient incident light  28   a  directly and therefore features a short focal length (and wide field of view). The pair of second primary optical paths  50 ,  52  originate with the ambient incident light  28   a  first striking the surfaces as shown. There is no prior reflecting or bending of the incident light  28   a,  therefore the pair of second optical paths  50 ,  52  become primary paths when the turret  26  is in the second position.  
         [0094]     Arrow end  50  ( FIG. 5 ) shows incident radiation striking the dichroic beam splitter  30 . The longer wavelengths (infrared) pass through the beam splitter  30  along a path similar to that of the first secondary optical path  32  where they are optimally utilized by the optical element(s)  32   d  in the IR turret  32   a.    
         [0095]     The incident light  28   a  passes down along and around the center longitudinal axis  16  and impinges directly on the dichroic beam splitter  30  (arrow  50 ) where the longer wavelengths (IR) pass through to form the first secondary optical path  32  and the shorter wavelengths are reflected back into space.  
         [0096]     In the wide and mid rage FOV positions, the secondary mirror  24  is disposed away from the center longitudinal axis  16  and, accordingly, the first secondary optical path  32  looks directly into space. The IR band of the collimated ambient light that strikes the dichroic beam splitter  30  is then focused, as needed, by the upper and lower lens groups of the IR turret  32   a  when it is in either the first or the second optical configuration positions.  
         [0097]     The effect on the visible spectrum when the turret  26  is in either the first or second position is described in greater detail hereinafter.  
         [0098]     After deflection off the IR fold mirror  33   c  the first secondary optical path  32  continues to an IR focus group of lenses, identified in general by the reference numeral  33   d.  The IR focus group  33   d  is adjusted as needed to compensate for distance to the subject. The IR focus group  33   d  also compensates when the turret  33   a  is moved between any of the three optical configuration locations (i.e., positions).  
         [0099]     A fixed group of IR lenses  33   e  receives the ray bundle from the IR focus group and directs it into a Dewar  33   f.  The Dewar is a commercially available assembly that is cryogenically cooled. An IR focal plane array  33   g  (FPA) that is sensitive in the IR band receives the inverted image and its output is captured. The image can be viewed in real-time or it may be electronically transmitted and stored on a preferred media for future use, as desired.  
         [0100]     It is important to note that the optical telescope  100  always allows for simultaneous viewing (i.e., accessing) of at least two channels (i.e., bands) of light and that the infrared band that passes through the dichroic beam splitter  30  and the center aperture  15  along the first secondary optical path  32  is always active regardless of the focal length that the optical telescope  100  is set to.  
         [0101]     Other changes and modifications to the first secondary optical path  32  are, of course, possible to those now having had benefit of the instant disclosure.  
         [0102]     Referring now to  FIG. 3 , which shows an interior of the turret  26 , the second secondary optical path  34  also includes an optical element group  38  (lenses) that are disposed proximate the second center aperture  36 . The optical element group  38  is a correction group of three lenses used correct errors introduced by the Cassegrain configuration (i.e., the primary mirror  14  and the secondary mirror  24 ).  
         [0103]     The second secondary optical path  34  is active only when the turret  26  is in the first position (i.e., when the secondary mirror  24  is disposed over the primary mirror  14 ).  
         [0104]     The second secondary optical path  34  continues up inside of the turret assembly  26  until it impinges on an angled mirror  40 . The angled mirror  40  acts as a first visible fold mirror that redirects the second secondary optical path  34  approximately ninety degrees so that it is now perpendicular with respect to the center longitudinal axis  16 .  
         [0105]     The second secondary optical path  34  then passes through four lens elements in a collimation group, identified in general by the reference numeral  35 . A first element  35   a  of the collimation group  35  extends a substantial amount along a longitudinal axis, through three other elements  35   b,    35   c,    35   d  and out into the turret  26  interior.  
         [0106]     The second secondary optical path  34  is collimated when it leaves the fourth element  35   d  of the collimation group  35 . Accordingly, if one were to place their eye and look backward into the second secondary optical path  34 , it would look like an ambient image, only magnified by the focal length provided by the second secondary optical path  34 . The image, according to a preferred embodiment, would appear to be approximately twenty times its normal size anywhere along the collimated portion of the second secondary optical path  34 . Accordingly, the eye would be able to focus the magnified image because the human eye is inherently designed to focus collimated ambient light.  
         [0107]     The collimated second secondary optical path  34  then continues to pass longitudinally along the length of the turret assembly  26  arm where a second visible fold mirror  42   a  further redirects the collimated second secondary optical path  34  in the turret  26  at a right angle.  
         [0108]     The redirected second secondary optical path  34  is still perpendicular with respect to the center longitudinal axis  16  and it now extends along a portion of a section of  FIG. 2  that is identified as section C-C until the second secondary optical path  34  visible light impinges on a third visible fold mirror  42   b  where it is now redirected at another ninety degree angle downward in a direction that is substantially parallel to that of the center longitudinal axis  16  but beyond the outer circumference of the primary mirror  14 .  
         [0109]     Section C-C extends beyond that portion that the second secondary optical path  34  traverses. Only the portion of section C-C that is disposed between the second visible fold mirror  42   a  and the third visible fold mirror  42   a  corresponds with the actual path taken by the second secondary optical path  34 .  
         [0110]     Referring now to  FIG. 4 , the still collimated second secondary optical path  34  enters into a linear tubular structure  46  that is attached to the upright member  18 .  
         [0111]     The second secondary optical path  34  is collimated until it enters a visible focus group of lenses  47   a.  The focus group  47   a  is adjusted to provide range correction for the distance of the object that is being viewed. The light in the second secondary optical path  34  is no longer collimated.  
         [0112]     The second secondary optical path  34  leaves the focus group  47   a  and enters into a front zoom group of lenses  47   b.  The front zoom group  47   b  cooperates with a back zoom group  47   c  to provide zoom capability in visible wavelengths.  
         [0113]     The front zoom group  47   b  moves in unison toward the back zoom group  47   c  as the back zoom group  47   c  moves in unison simultaneously toward the front zoom group  47   b  to provide a longer focal length (i.e., increased magnification or zoom).  
         [0114]     Conversely, the front zoom group  47   b  moves away from the back zoom group  47   c  as the back zoom group  47   c  moves simultaneously away from the front zoom group  47   b  to provide a shorter focal length.  
         [0115]     Accordingly, a variable zoom capability is provided. The position of the front zoom group  47   b  and the back zoom group  47   c  as shown in  FIG. 4 , shows the minimum focal length that is possible for the two zoom groups  47   b,    47   c.    
         [0116]     The zoom capability provided by the optical components in the front group  47   b  and the back group  47   c  provide a progressive zoom capability, where exactly the desired degree of zoom is realized by progressively moving select elements of the two zoom groupings  47   b,    47   c.    
         [0117]     Continuous zoom is better than a step function for tracking in the visible spectrum in that any object of interest in the visible spectrum is always visible when zooming, whereas by way of contrast, the object disappears from view momentarily when “stepping” from one focal length (magnification) to another focal length (magnification) and it must then be re-acquired which can be difficult as it will not look the same (it will be either larger or smaller or only a portion of it may be visible) which can impede identification and rapid re-acquisition.  
         [0118]     In order to achieve the desired visible zoom capability with a high quality optical arrangement, it is necessary to arrange the optical components of each zoom group  47   b,    47   c  along a relatively long linear path so that the components in each grouping  47   b,    47   c  can be moved longitudinally a required amount in order to affect the field of view (i.e., to zoom in or out).  
         [0119]     The light of the second secondary optical path  34  leaves the back zoom group  47   c  and enters a visible fixed element group  47   d.  The fixed element group  47   d  focuses the visible light on a CCD  49 . The CCD  49  is a type of FPA and it provides real-time data acquisition for viewing or storage in a desired media for future use.  
         [0120]     The first and second secondary optical paths  32 ,  34  and of course, the first primary optical path  28  features a long focal length (i.e., greater magnification).  
         [0121]     Referring again to  FIG. 5 , with the turret  26  in the second position, arrow end  52  shows incident radiation striking the optical components of the focus group  47   a  that are disposed at the top of the tubular structure  46 , which is now open to ambient light  28   a.    
         [0122]     The shorter wavelengths pass through the optical groups  47   a - 47   d  along a path similar to that of the second secondary optical path  34  where they are optimally utilized. Other wavelengths may also pass through the optical groups  47   a - 47   d  or they may be blocked by filters or coatings applied to any of the elements in any of the groups  47   a - 47   d,  as desired.  
         [0123]     In this mode (i.e., when the turret  26  is in the second position as shown in  FIG. 5 ), the optical telescope  100  has a wider field of view for both infrared and visible that can be used to scan for objects of interest simultaneously in both spectrums of light.  
         [0124]     It is important to note that when the turret  26  is in either the first or second position, that two optical paths always remain active allowing for simultaneous two-channel viewing in both IR and visible.  
         [0125]     Of course, depending upon how the optical telescope  100  is to be used, it is possible to modify the components so that any of the optical paths (i.e., the first secondary optical path  32 , the second secondary optical path  34  or either of a pair of second primary optical paths  50 ,  52 ) can be configured to include any spectrum of light that is desired.  
         [0126]     It may at times be desirable to scan in a wide field of view, possibly looking for objects of interest in the infrared spectrum. When an IR source is detected, it may be desirable to engage in further observations in the visible spectrum, possibly zooming in. If more magnification is desired, it may desirable to rotate the turret  26  into the first position (i.e., the long focal length as shown in  FIGS. 1, 3 , and  4 ) and study the object closer (with even more magnification) in either or both the IR or visible spectrums, while again zooming in or out as desired in visible. The IR turret  32   a  allows for a stepped zoom capability in IR. Other optical components or groupings (not shown) can be added, if desired, along the first secondary optical path  32  to provide zoom capability in IR.  
         [0127]     It is also very important to note that when the turret  26  is in the second position, the secondary mirror  24  is not disposed over the dichroic beam splitter  30 , which then becomes the aperture for shorter focal length (wide field of view) observations in IR. This is extremely important because no energy is lost by impinging on any of the component parts of the turret  26  when it is in the second position, neither over the beam splitter  30  nor over the optical groups  47   a - 47   d  in the tubular structure  46 .  
         [0128]     It is also especially important to note that the turret  26  only places a single group of optical elements  38 ,  40  in the field of view along the second secondary optical path  34 . As it rotates it either places the single group of optical elements  38 ,  40  there or removes it from the field of view. Of course, the secondary mirror  24  is also similarly inserted or removed from the first optical path  28  simultaneous with the optical elements  38 ,  40  that are being inserted or removed from the second secondary optical path  34 . This provides two important primary benefits.  
         [0129]     First, there are no additional optical groups that may be disposed over the primary mirror  14  when it is actively being used which could lessen the amount of light striking the primary mirror  14 . When the primary mirror  14  is in use, the turret  26  and secondary mirror  24  provide minimum obstruction. This is because the turret  26  aligns with the upright member  18  when the turret  26  is in the first position.  
         [0130]     The size of the turret  26  is preferably as close to that of the first end  18   a  of the upright member  18  (i.e., that portion of the upright member  18  that extends over the primary mirror  14 ) as is possible to either minimize or eliminate any additional loss of incident light  28   a  when the turret  26  is disposed in the first position.  
         [0131]     Second, because the turret  26  does not have to introduce numerous optical groups into and out of the field of view, it is simple to include a reliable stop that ensures repeatability in the positioning of the turret  26  when the secondary mirror  24  and primary mirror  14  are in use and in alignment. When multiple optical groups are inserted or removed from an active relationship with a different configuration of a primary mirror (not shown herein, consistent with certain types of prior art), multiple stops are required that contribute to variability in positioning of the multiple optical groups.  
         [0132]     Other modifications are possible to achieve other benefits. For example, the optical telescope  100  is designed to optimally utilize the inherent diffraction-limited focal lengths of the IR band (which require a shorter IR focal length than visible for any given aperture size). As a result of this inherent limitation, the widest FOV in visible is narrower than the narrowest FOV in IR.  
         [0133]     This prevents having the greatest possible focal lengths in visible (which is highly desirable) and having those visible focal lengths correlate directly with the focal length in IR. Certain applications of the optical telescope  100  benefit when there is direct correlation in focal length between visible and IR in that an analysis or viewing that switches back and forth from one band to the other benefits with the object of interest is the same size.  
         [0134]     If desired, a secondary zoom visible telescope, shown in dashed lines and identified in general by the reference numeral  102 , and having a focal length range comparable to that of the IR band of the optical telescope  100  may be added as a separate component, preferably disposed proximate the optical telescope  100  and parallel thereto. The secondary zoom visible telescope  102  is also varied in focal length whenever the focal length in IR is changed (i.e. when the IR turret  32   a  is moved) so that an approximate correlation in focal lengths in IR and visible is provided. The optical telescope  100  provides considerably greater focal length capability in visible for even more detailed viewing than is possible with the secondary zoom visible telescope  102 .  
         [0135]     The invention has been shown, described, and illustrated in substantial detail with reference to the presently preferred embodiment. It will be understood by those skilled in this art that other and further changes and modifications may be made without departing from the spirit and scope of the invention which is defined by the claims appended hereto.  
         [0136]     For example, a simplified IR turret that only includes two positions may be used instead of the preferred type of IR turret  32   a.  The simplified IR turret may include groups of lenses in each of two positions or one of the two positions may omit any lenses, as desired. Conversely, a four position IR turret may similarly be designed if a greater number of steps are required in IR.