US3409777A - Circular scanner having superimposed dither - Google Patents

Description

NOV. 5, 1968 M, F. COHEN ET AL CIRCULAR SCANNER HAVING SUPERIMPOSED DITHER 5 Sheets-Sheet l Filed Feb.

CEA/fie 0f SCI/V NOV. 5, 1968 M F. COHEN ET Al. 3,409,777

CIRCULAR SCANNER HAVING SUPERIMPOSED DITHER Filed Feb. 1966 5 Sheets-Sheet 2 Ir-E'. 3.

NGV. 5, 1968 M, F COHEN ET AL 3,409,777

CIRCULAR SCANNER HAVING SUPERIMPOSED DITHER Filed Feb. 1966 5 Sheets-Sheet 5 Iz-n.-

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NOV. 5, 1968 M, F, COHEN ET AL 3,409,777

CIRCULAR SCANNER HAVING SUPERIMPOSED DITHER Filed Feb. 3, 1966 5 Sheets-Sheet 4 M. F. COHEN ET AL 5 Sheets-Sheet 5 Nov. 5, 1968 CIRCULAR SCANNER HAVING SUPERIMPOSED DlTRRR Filed Feb. e, 196e United States Patent O Fice 3,409,777 CIRCULAR SCANNER HAVING SUPERIMPOSED DITHER Murray F. Cohen, Roslyn Heights, Irvin S. Englander, Jackson Heights, and Sheldon Girsch, Bayside, N.Y., assignors to Kollsman Instrument Corporation, Elmhurst, N.Y., a corporation of New York Filed Feb. 8, 1966, Ser. No. 525,927 9 Claims. (Cl. Z50- 203) ABSTRACT F THE DISCLOSURE A tracking device using an image dissector tube for tracking around an illuminated disk, such as a planet, using a circular scanning pattern having a superimposed dither thereon. The tracking device operates in a spiral acquistion mode to acquire the planet or body to be tracked, and after acquisition, is automatically placed in a tine track mode for pointing an axis toward the center of the body being tracked. Radial sizing circuits are provided to adjust the effective scan circle diameter, with the scanning process operable even though an image is in a crescent phase.

This invention relates to a novel tracker device, and more particularly relates to a novel tracking device for tracking a planet or other equivalent illuminated disk or disk portion whereby the tracking device can determine the coordinates of the center of the illuminated disk or portion thereof.

In accordance with the present invention, the tracking device is so arranged as to observe the outline of the illuminated disk through the means of a circular scan having superimposed dither. That is to say, a circular scan is applied which generally follows the outline of the illaminated disk being tracked where a superimposed sinusoidal modulation is applied to the circular scanning path so that the scan direction continually oscillates into and out of the periphery of the disk being tracked. This modulation shall be hereinafter referred to as dither.

When using the novel dither concept, a novel system is further used in connection therewith so that a best circular scan t will be obtained for the outline of an illuminated disk such as a lunar or planet image as it appears on the cathode face of an image dissector tube. For a nearly circular image; for example, the appearance of the full moon at a large distance, the displacement of the sean circle and the image results in an error signal at the fundamental circular scanning frequency. For an oblate planet such as Jupiter which appears as an elliptical disk in the focal plane, a null condition can be obtained which corresponds to the center of the two foci of the ellipse.

In the case of the nearly circular planet image, -copending application Ser. No. 158,300, filed Dec, 1l, 1961, in the name of Jacob S. Zuckerbraun, entitled Photosensitive Horizon Scanner for Space Vehicle, and assigned to the assignee of the present invention (now U.S. Patent 3,246,160), provides a planet tracking technique whereby at least three points are located on the periphery of the disk with the center and radius of the planet being determined by computational methods.

It has been found that with this technique the presence of various anomalies such as dark spots on or near the image edge necessitate the sampling of many points for greater weighted accuracy.

In accordance with the invention, a circular scan pattern with a high frequency radially directed dither permits the sampling of many points during a single cycle around the planet edge. Appropriate signal processing is then provided to handle the various sample data points 3,409,777 Patented Nov. 5, 1968 in each cycle, and an analog centering technique is then used to determine the planet center rather than the digital computation required in the aforementioned copending application Ser. No. 158,300.

lt should be noted that the planet tracker of above noted copending application Ser. No. 158,300 is not a true edge tracker, since a true edge tracker will follow the shape of the edge regardless of the edge contour. Thus, if the planet were in its crescent phase, the edge tracker would outline the exact shape of the crescent. If dark spots were present in the planet rim, the edge tracker would move inside the planet disk to nd the bright edge.

In accordance with the present invention, the novel tracking technique using a scan pattern with a circular constraint can be made to find the best circular match to the planet edge.

Many star trackers and planet trackers have been further developed in the past which use various tracking techniques. Thus, it is known that a star may be tracked by interrupting or oscillating the star image with respect to a defining aperture. Such an arrangement is shown in copending application Ser. No. 244,665, tiled Dec. 14, 1962, in the name of Burt Walker, entitled Star Tracker Scanning System Using a Circular Scanning Pattern and a Square Aperture, -and assigned to the assignee of the instant invention now U.S: Patent 3,244,896. In this arrangement, signals are developed by a detector, and the star is centered by appropriate processing methods. Thus, the tracking center will correspond to the center of illumination as defined by the central maximum of the image pattern. Clearly, at great distances, a planet will appear as a small object in the focal plane of this type of star tracker optics. By increasing the F number of the optics, the planet can be magnified whereby its geometrical properties are recognizable. The planet can still be tracked by star tracking techniques of the above noted type if tracking to the center of illumination proves sufticient.

However, since it is normally the geometrical center of the planet which is desired, obtaining the center of illumination may result in significant errors caused by the planet phases and variations in surface retiectance or emissivity over the illuminated planet portion.

In accordance with the present invention, the novel tracker structure will be provided with the ability to sample the planets rim at a large number of points around the circle of the illuminated disk so that various anomalies in the sampling operation can be integrated out. This arrangement is obtained through the use of a circular scan which has superimposed thereon a novel dither variation.

A high resolution optical system of novel construction is further provided which will greatly magnify the image to provide an extended image in a very small field of view. This, in combination with a high signal-to-noise ratio which is obtainable in the electronic portion of the apparatus, contribute directly to tracking accuracy. In addition, the novel system of the invention eliminates all mechanical moving parts and provides an ability to obtain a directional reading even though the object being tracked is ot the optical axis of the system.

Accordingly, a primary object of this invention is to provide a novel technique for tracking illuminated disks or portions thereof.

Another object of this invention is to provide a novel high accuracy planet tracker.

A still further object of this invention is to provide a novel high accuracy planet tracker which has no moving parts.

Another object of this invention is to provide a novel tracking mode for radiant energy tracking devices in which the device to be tracked is presented as an illuminated disk or portion thereof which is circularly scanned in an optical scanning pattern which has a high frequency dither superimposed thereon.

These and other objects of this invention will become apparent from the following description when taken in connection with the drawings, in which:

FIGURE la schematically illustrates a typical image dissector in cross-sectional view.

FIGURE lb illustrates the spiral image acquiring mode of scanning operation used in the image dissector of FIG- URE la during the acquisition mode of operation.

FIGURE 1c illustrates the fine tracking mode of operation in which a circular scanning pattern having a superimposed dither thereon is matched to the illuminated planet image.

FIGURE 2 illustrates the scan geometry or the displacement of the scan circle center from the planet center during the tine tracking mode of operation.

FIGURE 3 schematically illustrates a pulse width demodulator which is used to determine the misalignment between the scanning pattern and the planet image being tracked.

FIGURES 4a, 4b and 4c are to be taken in conjunction with FIGURES 5a, 5b and 5c, respectively, to illustrate the manner in which the planet image may be sized to the scan radius.

FIGURES 6a and 6b illustrate the manner in which a light terminator on a planet crescent is determined.

FIGURES 7a through 7d illustrates the tracking con- 9 ditions from full illumination of the planet to its quarter phase.

FIGURE 8 is a functional block diagram of the electronic control circuitry used in accordance with the invention.

FIGURE 9 illustrates the novel double Cassegrain objective used in accordance with the invention.

FIGURE 10 shows a novel modification of the Cassegrain objective of FIGURE 9 in which the lens surfaces are merged and the optical operation is improved.

While the concepts of the present invention may be carried out with various types of scanning mechanisms, it shall be described herein with reference to an image dissector arrangement. Image dissectors are well known to the art, and the image dissector as used in accordance with the invention is schematically illustrated in FIG- URE la.

Thus, in FIGURE la, a suitable optical system having an optical axis 20a which is directed toward the object to be tracked by a suitable servoing system (not shown) is located in front of the photocathode 21 of an image dissector tube 22. The image dissector tube 22 will include suitable detiection plates or coils, schematically illustrated as plates 23 and 24, for altering the position of the electron beam emitted from the rear of photocathode 21 and toward the image defining aperture 25 (which may be anodic) in orthogonal directions. That is to say, in the schematic illustration of FIGURE 1a, plate 23 may be deemed to cause vertical positioning of the electron beam, while plate 24 will cause horizontal positioning of the beam. Image dissectors of this type are well known to the art and the function and construction of plates 23 and 24 need no further description.

In addition to these control plates, suitable accelerating electrodes will be provided for accelerating the electron beam toward aperture 25, and similarly suitable focusing electrode means will be provided for focusing the beam toward a focal plane which is contained in the plane of opening 25.

A suitable electron detection means such as a photomultiplier 26 having output electrodes 27 and 28 is thcn located behind aperture 25, thereby to receive an electron stream when the focusing and control electrodes of the image dissector cause the electron beam issuing from photocathode 21 to pass through aperture 25.

In describing the acquisition pattern of scanning and the tracking pattern of scanning in FIGURES lb and 1c. reference may be made to the analogy of oscillation of the aperture 25. It should be noted, however, that there is no physical movement of aperture 25, but that different points of the photocathode 21 will be observed passing through aperture 25 by suitably controlling the position of the electron beam issuing from any particular point of photocathode 21 through the control of control electrodes 23 and 24.

Thus, in the acquisition mode of operation, as illustrated in FIGURE 1b, suitable control potentials are applied to deflection plates 23 and 24 so that control conditions make it possible to spirally sample areas of photocathode 21 so that when a point is reached which contains an image in the photocathode 21, the electron beam issuing therefrom shall pass through aperture 25.

In a similar manner, and in FIGURE 1c, a circular scanning pattern is shown, which has a dither superimposed thereon, which scans an image 31 formed on photocathode 21 by the objective where the scanning line 30 is again a sampling scanning of the electron image issuing from photocathode 21 by suitable control of the deflection electrodes 23 and 24.

A cquisilion mode In virtually all types of tracking devices, it is first necessary to acquire the signal or object which is to be tracked. A novel acquisition mode of operation is provided in the present invention which includes the use of a spiral scan pattern. Thus, in the acquisition mode, the optics 20 will focus the planetary disk somewhere on the cathode 21 of the image dissector tube 22 if it is in the total tield of view of the apparatus. An electron stream will then issue from the photocathode at the point where the image is received.

In the search mode, the control electrodes 23 and 24 will cause this electron beam to move in a spiral pattern, as shown in FIGURE lb, where this spiral pattern is independent of the specific location on photocathode 21 from which the electron stream emanates. This spiral action will, in effect, be scanned by aperture 25 so that if the image appears, for example, in shaded area of FIGURE lb, when the scanning pattern reaches this position in the spiral, the electron beam will pass through aperture 25 and be sensed by photo-multiplier 26.

In the search mode, this scan is in the form of an inwardly directed spiral, as illustrated by the arrows in FIGURE lb, with a signal pulse generated by photomultiplier 26 being compared to the sweep signals applied to electrodes 23 and 24, thereby the determine the relative position of image 40 on the photocathode 21 so that a preliminary determination may be made of the planet coordinates with respect to the optical axis 20a of the telescope used for the tracker. Suitable D-C current slgnals may then be applied to the deflection plates 23 and 24 (or deection coils, if coils are used) which will drive the spiral scan center to a position somewhere within the planetary disk.

The equipment may then be automatically placed in its fine track mode of operation to point the optical axis 20a directly at the effective center of the disk 40.

Fine track mode of operation In the fine track mode of operation, as illustrated in FIGURE 1c, the scanning mode is provided by a 200 cycle per second circular scan which has superimposed thereon a 10 kilocycle sinusoidal modulation to result in the dither scanning pattern 30 of FIGURE lc. Moreover, the radius of the circular scan will coincide with the radius of the target image 31 in FIGURE lc.

As will be described more fully hereinafter, when the respective centers of the scan circle 30 and the planet image 31 are aligned and the planet and scan radii are matched, the dither scan will be equally divided in time between being out of the scan image and into the scan image (the duty cycle of the effective scanning aperture will be evenly divided between the illuminated and dark regions of space). Where alignment exists, the output of the image dissector photo-multiplier 26, after the use of appropriate shaping circuitry, will be a series of square pulses of equal duration. However, if the respective centers of the scan circle and planet image are displaced, a pulse width modulated rectangular wave train will be developed with a period identical to that of the 200 cycle per second scan circle, but with a pulse width modulation corresponding to the displacement between the centers of circles and 31 in FIGURE 1c.

FIGURE 2 illustrates the geometry of the scanning conditions when the center of the scanning pattern 30 is displaced from the center of the illuminated disk 31 by a distance d in the negative y direction. The variable I describes the instantaneous distance of the scan circle from the planet edge as a function of the phase angle 0.

The variable l which is the instantaneous displacement of the center of oscillation from the planetary rim is derived from triangle OPO as follows:

where R is the radius of the planet disk and scan circle for' matched conditions, while a equals some function of 0.

of this sinusoid will depend on the orientation of the planet with relation to the scan center. When the orientation of the planet displacement changes by some particular angle, the phase of the sinusoid will shift accordingly. Thus, the position of the planet can be determined by phase detection of the fundamental harmonic component of the pulse width modulated scan signals.

One technique for demodulating the pulse width modulated wave prior to a synchronous phase detection process is illustrated in FIGURE 3. FIGURE 3 illustrates an input terminal 40 to which the pulse width modulated train is applied to parallel connected constant current sources 41 and 42 which have additive series connected polarities. The output of the parallel connected constant current sources 41 and 42 are then connected to a common integrator 43 which has an output terminal 44 which will present a sinusoidal wave train whose relative phase will be dependent upon the angle 0 in FIGURE 2.

Thus, in FIGURE 3, the signal pulse train at input 40 will swing between ground and some fixed negative voltage so that the signal will swing equally in the positive f(t) K[Ti (on) -Tt' (06)] Radial sizing It will be apparent that operation in the fine scan mode 6 requires the matching of the circular scan radius 30 to the planet image radius 31 in FIGURES 1c and 2.

In accordance with the invention, the nature of the novel scan pattern 30 permits the generation of an error signal proportional to a size difference in the radial sizes of the planet image radius 31 and the scan radius 30. Moreover, the polarity of this error signal will indicate whether the scan radius is smaller or larger than the planet image radius. Furthermore, these error signals do not depend on maintaining alignment between the planetary and scan centers.

FIGURES 4a, 4b and 4c illustrate the geometry of the fine track mode with the scan center and planet image centers aligned in each case and with ,matching radii (FIGURE 4a) with the radius of the scanning pattern smaller than the planetary image (FIGURE 4b), and with the radius of the scanning pattern greater than the radius of the planetary image (FIGURE 4c).

If, as in FIGURE 4a, the planet and scan radii are equal, then as the aperture is oscillated in and around the planet disk, the dwell time spent within the disk will be equal to the dwell time spent outside the disk.

The generated waveform in this case will be that shown in FIGURE 5a which plots output voltage from the photo-multiplier 26 as a function of time where the generated waveform will be seen to be a square wave for the condition of FIGURE 4a.

Where the scan radius differs from the planet radius, however, as in FIGURES 4b and 4c, the waveform becomes unsymmetrical, as shown in FIGURES 5b and 5c. Thus, in FIGURES 5b and 5c, it can be seen that the waveform of FIGURE 5a changes from a longer pulse duration (FIGURE Sb) to a shorter pulse duration (FIG- URE 5c) when the scan radius is smaller and larger, respectively, than the planetary image.

It can be easily shown by a Fourier analysis that an asymmetrical rectangular waveform contains the second harmonic of the fundamental frequency signal. The polarity of this second harmonic will then differ in the cases of FIGURES 5b and 5c, thereby to define an error sizing signal. Note that the frequency of this second harmonic component will be 20 kiocycles since the fundamental for this particular situation would be the l0 kilocycle dither.

Accordingly, once the planet is acquired, means are automatically provided wthen the dither mode of scanning becomes operative to appropriately size the scanning radius to the planetary image.

Operation when the planetary image is in a crescent phase In the foregoing, it has been presumed that a full disk is available in the scanning operation. It will often occur, however, that the planet to be tracked will be in one of its various illumination phases as determined by the solar and line-of-sight angle changes with respect to the planet so that problems could be created as compared to automatic trackingof the planet vertical.

It should be further noted that the planet tracker may operate in regions other than the visible light regions and could also operate in the ultraviolet or near infrared portion of the spectrum. Even then, however, such problems could occur as due to uneven planet heating and the like, so that it becomes necessary to provide means for tracking, even though the planet is in any of its various phases including the crescent phase.

As previously described, the determination of the center of illumination of a disk-shaped object could result in large position uncertainties due to surface albedo variations and phase variations. However, the novel scanning apparatus of the present invention can be used for all illumination phases at a suicient distance from the planet so that at least a sc micircular outer edge of the planet will be presented through the employment of suitable logic and signal processing circuitry.

Before discussing this circuitry, it is useful to understand the geometric properties of the planet light terminator as described in FIGURES 6a and 6b which illustrate a projected geometric technique for determining the shape of the planet terminator. l

Assuming that sunlight shining on the planet is in the direction shown by the labeled arrows in FIGURE 6b, a terminator 51 will be defined which marks the boundary between light and dark on the planet 50. This terminator 51 can, of course, be observed from various angles by the scanning system depending upon the relative position of the vehicle carrying the scanner with respect to the planet 50.

FIGURE 6a shows a projected ortho-normal grid which can be constru; ed by the projection lines extending from FIGURE 6b to FIGURE 6a, and then connecting the intersected grid lines to generate an ellipse which describes the planet terminator. That is to say, vertical lines 52, 53, 54 and 55 taken from FIGURE 6b, are drawn in FIGURE 6a as circles 52a, 53a, 54a and 55a. Horizontal lines 52h, 53h, 5412 and 55b are then projec.cd from the intersections of lines 52 through 55 with terminator 51 to intersect respective circles 52a through 55u with these intersecting points defining the terminator ellipse 56 in FIGURE 6a.

The semi-major axis of ellipse 56 will always correspond to the radius of the planet S0. while the eccentricity of the ellipse will vary from one at quarter phase to zero at fu l phase. The actual terminator of the planet will depart from an ellipse due to planet oblateness and surface anomalies. but for purposes of the present invention. it is suflicient to presume that the terminator will be elliptical.

When using the novel rotating dither scan of FIGURE lc, signals will be available where the dither amplitude is sufficiently large to penetrate the illuminated portion of the planet image.

Thus. in FIGURES 7a through 7d which show various tracking conditions, in the condiLions of FIGURES 7a and 7b, signals from the dither pattern 30 will be obtained for a full cycle (in the condition of FIGURE 7b, the dither pattern always enters the terminator line) while in the conditions of FIGURES 7c and 7d, dither signals -are obtained for only approximately I/2 cycle. However,

in each of conditions 7a through 7d, the center of the planet can be tracked without ambiguity since the signals obtained are obtained from circular portions of the planet.

Note that in the condition of FIGURE 7b, there is no relative posiion of a scan circle and planet where all members of the pulse train will have the same width.

However, a tracking null position can be obtained when error signals in opposite quadrants will balance out. Thus,-

the null track posiion for the condilion of FIGURE 7b may be biased off-center and to the right in favor of the elliptical portion of the disk. Since one-half of the disk is always circular, however, a discrimination technique may be used to track only the circular portion.

More particularly, and where the condition of FIGURE 7b is obtained, the signal delivering information from thc photo-multiplier 26 of FIGURE la can be turned on for 1/2 of its. duty cycle. This duty cycle may be controlled by suitable switching means which is initially phased at 6:0 with reference to the scanning circle.

When tracking the circular side of a planet, the waveform obtained for a small angular displacement of the planet center and scan center can be shown to be of the form of l/z sine wave. The harmonic content of the V2 sine wave is well known, and is as follows:

Thusl when tracking non-circular portions, the harmonic content of the pulse width demodulated wave will depart from the 1/2 sine wave and odd harmonics will appear since the terminator in FIGURE '7/1 is elliptical Cit in shape. Therefore, it becomes possible for the circuitry to distinguish between the tracking of the planet edge and the elliptical terminator in the condition of FIG- URE 7b.

Functional description of complete operating system The complete electronics and other circuitry used in the control and processing of the output signals of the photo-multiplier 26 of FIGURE la are described in FIGURE 8 in detail. It will be noted, however, that for purposes of simplicity, the various components are illustrated in block diagram form with all of the block components being of types well known to their various respective arts and which could be easily and readily constructed by those skilled in the various arts.

Refering now to FIGURE 8, the circuitry associated with the novel scanning system includes suitable power sources, which are not shown, along with a 1 cycle astable multivibrator 101, a 400 astable multivibrator 102 and a 2 kilocycle astable multivibrator 103. The 1 cycle multivibrator 101 is connected to a fiip-tiop 104g which, in turn, drives a sawtooth generator 105 which is connected to an acquisition disconnect switching means 106. The acquisition disconnect switching means 106 is then connected to a variable gain amplifier which is a sawtooth modulator 107. The 400 cycle astable multivibrator 102 is connected through a frequency divider 108 to a 200 cycle filter and amplifier 109, which, in turn, is connected to a gain contol input of the variable gain amplifier 107. The 2 kilocycle astable multivibrator 103 is then connected to a 10 kilocycle filter and amplifier 104. The variable gain amplifier 107 is then connected to a vertical ring modulator 110 and a 90 phase shifter 111. The output of 10 kilocycle amplifier 104 is connected to the control electrode of vertical ring modulator 110 with the output of vertical ring modulator 110 and amplifier 107 connected to the adder 112. The adder 112 is then connected to a vertical defiection amplifier 113 which is subsequently connected to the vertical defiection electrode of the image dissector of FIGURE la such as electrode 23.

The 90 phase shifter 111 is then connected along with the output of 10 kilocycle amplifier 104 to the horizontal ring modulator 114 and the output of horizontal ring modulator 114 and 90 phase shifter 111 are then connected to adder 115 which is, in turn, connected to the horizontal defiection amplifier 116 which is connected to the horizontal deection electrode of the image dissector of FIGURE la such as electrode 24.

The output of the photo-multiplier 26 in FIGURE la is schematically illustrated in the upper left-hand corner of FIGURE 8 as connected to a wide band amplifier 120. The output of amplifier 120 is connected to the narrow band amplifier 121 which amplifies about the second harmonic output signal of amplifier 120. Amplifier 121 is then connected to synchronous demodulator 122 which is, in turn, connected through the low pass filter 123 to variable gain amplifier 107, thereby to generate the radial sizing signal.

Synchronous demodulator 122 is, in turn. connected to frequency doubler 124 which receives a 10 kilocycle signal from the l0 kilocycle filter and amplifier 125 which is connected to the 10 kilocycle adder 126. The 10 kilocycle adder 126 then has applied thereto the signals from the outputs of vertical deection amplifier 113 and horizontal deflection amplifier 116. These output signals are further connected to synchronous demodulators 127 and 128, respectively, which are each connected to the image dissector output signal from amplifier 120 taken through the pulse width demodulator 129 which may be of the type shown in FIGURE 3.

Each of synchronous demodulators 127 and 128 are further connected to low pass filters 130 and 131, respectively, to generate output signals which are functionally related to the coordinates of the center of the planet being tracked and could, for example, be connected to a servo system to keep the optical axis of the tracker pointed toward their center position.

The amplified image tube output signal from amplifier 120 is further connected to the monostable multivibrator 140 and to one input terminal of a compare gate 141. The output of monostable multivibrator 140 is connected to the other terminal of compare gate 141 with the output of compare gate 141 connected to the variable current source 142 and to the acquisition disconnect switching means 106.

In addition, the output of compare gate 141 is connected to the x point storage means 143 and the y point storage means 144 which are each connected to the 200 cycle sampling signal and proportional D-C signal outputs of vertical deflection amplifier 113 and horizontal defiection amplifier 116, respectively.

In the following description of operation of FIGURE 8, it will be more convenient to regard the aperture of the image dissector tube of FIGURE 1a as being the element which scans over the image plane of the photocathode 21 instead of the actual scanning of the entire electron image analog of the optical image. Thus, where reference is made hereinafter to spiralling or oscillating apertures, it is to be understood that the electron image analog of the planet is the moving element with the aperture 2S remaining fixed.

The initial scan pattern in the operation of the device is the spiral scan pattern (FIGURE 1b) which searches for a target in the field of view of the photocathode 21 of FIGURE la. The spiral scan mode is generated by the 1 cycle astable multivibrator 101, and the asymmetric fiip-fiop 104a which actuates the sawtooth generator 105. The sawtooth waveform output of generator 105 serves as a linear gain control for the variable gain amplifier 107 which also receives a sine wave control signal at 200 cycles per second from the input branch including blocks 102, 108 and 109.

The output of the variable gain amplifier 107 will then be a sawtooth modulated sine wave which is connected directly to the vertical defiection amplifier 113 and horizontal defiection amplifier 116 with an appropriate 90 phase shift obtained by the phase shifter 111. These two signals will then produce the spiral scan mode, as schematically illustrated in FIGURE 1b.

In order to obtain the rotating `dither pattern, a 10 kc. signal is superimposed upon a 200 cycle waveform. Thus, the output of 10 kc. amplifier 104 is fed directly to the vertical ring modulator and horizontal ring modulator 114 to appear directly at the output of amplifiers 113 and 116 independently of the phase shift required for the spiral acquisition mode of operation.

The reason for modulating the 10 kilocycle signal in the manner shown in FIGURE 8 can be understood from the following:

The resultant waveform in the fine track mode is to be a circle whose radius matches the planet disk radius. A dither pattern is superimposed on this circle whose arnplitude is constant around the circle and whose amplitude vector is always pointed radially toward the center of the circle. The large circular pattern is the resultant of the amplitude vector of two sine waves whose phases are displaced by 90. Similarly, the superimposed dither pattern is also the vector resultant of two sine waves which, as pointed out above, are in phase.

Thus, at the axis points on the scan reference circle, a maximum value of x must be accompanied by a minimum value of y. The reverse is true at the iy axis points. Similarly, at the 1r/4, 31r/4 and 31r/2 positions, the x and y values must both be equal to 1/\/2 in order to maintain an equal amplitude and radial direction.

The reason why the dither pattern is also superimposed on the spiral scan mode is simply that the presence of fine oscillations during the spiral acquisition will in no way degrade the search capability of the scanner and it eliminates the need for switching the pattern in at a later time.

During the spiral scan mode and as soon as the aperture crosses the planetary disk image (using the analog of a moving aperture) a pulse is generated and will pass through the amplifier 120. This pulse is then fed to a pulse width discriminating circuit comprising the monostable multivibrator 140 and compare gate 141 which determines a minimum acceptable pulse width. The function of the pulse width discriminator is to determine a position on the planet disk which is near the planet diameter. When the planet pulse width is greater than the pulse width of the monostable multivibrator 140, recognition signals are sent to the x and y points storage 143 and 144, respectively, whereby D-C bias current values corresponding to the instantaneous amplitudes of the x and y defiection plate output signals are fed back to the x and y defiection electrodes to drive the scan oscillation center to a point inside the planet disk.

At the same instant of recognition, the spiral scan generator is disabled by the acquisition disconnect switch 106 and the scan circle is made to collapse to a point and then slowly spiral outward by connecting the variable current source 142 into the sawtooth modulator 107 which acts assentially as a variable gain amplifier whose gain control is regulated by current inputs.

The slowly expanding circle continues until the fine track mode is initiated by means of a 10 kilocycle presence signal which is obtained in the output of amplier 120, and is generated by the dither crossing the edge of the planet disk.

The complete system now goes into the fine tracking mode, and performs three basic functions; tracking in the x and y or vertical and horizontal directions, radial sizing, and sampling of a circular segment of the planet image.

The l0 kilocycle signals from the image dissector output amplifier 120 are fed into the pulse width demodulator 129 and the tracking signals are synchronously demodulated in the x and y or vertical and horizontal directions by multiplication with the reference signals which are tapped from the deflection electrodes and filtered for 200 cycles.

Analytically, synchronous demodulation is described as follows:

The A-C components are filtered in the low pass filters 130 and 131 so that the tracking signals are D-C signals whose polarity will be dependent on the phase of the incoming signal.

Radial sizing is accomplished by synchronously demodulating the second harmonic of the l0 kilocycle rectangular wave. That is, the 10 kilocycle component of the scan is tapped off both defiection electrodes and connected to the 10 kilocycle adder 126 where signal addition will result in a waveform of constant amplitude. The output of adder 126 is then applied to the l0 kilocycle amplifier and filter and is doubled in the frequency doubler 124 to provide a reference signal for the synchronous demodulation of the 20 kilocycle component of the signal in the synchronous demodulator 122. As in the tracking signal case, the A-C component is filtered out, and the polarity of the D-C signal is used to obtain correct radial sizing by feeding back the error signal to the variable gain amplifier 107 through the low pass filter 123.

Optical system An important consideration for the best operation of the novel tracker of the present invention lies in a suitable optical system which can provide a long focal length in a small package. This is especially true where the tracker is to utilize an image dissector as the detecting element. In this case, and for commercially available image dissectors, the eld -will be a circle 0.7 inch in diameter, and in order to preserve tracking accuracy, a long focal length is necessary.

This becomes extremely difficult since, in the ordinary photographic telephoto lens, the so-called telephoto effect is only 0.75 to 0.85 where the telephoto effect is the ratio of the overall length of the lens from front to focal plane to the focal length of the lens.

If a eld of view of 30 minutes is used, the focal length of the objective would become 80 inches and a telephoto effect of 0.75 would result in a system 60 inches long.

It is common practice to use a Cassegrain type of objective where the telephoto effect is to be reduced where Cassegrain systems typically will give a telephoto effect of approximately 0.4 so that the overall length of the system would be reduced to about 32 inches.

In order to shorten the Cassegrain objective still further, the power of the primary mirror must be increased. This. however, has the effect of increasing the convergence of the rays reected by the primary so that the secondary moves closer to the primary with its power also increasing. When this procedure is carried too far, it results in an unbalanced system which will have low definition over the entire field of view.

In accordance with the present invention, a novel Cassegrain system is provided, as illustrated in FIGURE 9, wherein a second Cassegrain system is interposed in the light path of the primary Cassegrain system.

Thus, in FIGURE 9, a primary Cassegrain system is formed of a concave mirror 200 which directs light toward a convex mirror 201. The concave mirror 200 has an opening 202 therein, and normally the light from convex mirror 201 would focus in a plane on the righthand side of opening 202.

In accordance with the invention, however, a second Cassegrain system which includes concave mirror 203 and convex mirror 204 is interposed in the primary Cassegrain system with the light from convex mirror 204 passing through aligned opening 205 in mirror 203 and opening 202 in mirror 200 toward a focal plane which includes the photocathode of the image dissector tube.

This novel arrangement reduces the telephoto effect of the Cassegrain objective to approximately 0.15 without.

' however, unduly affecting the definition over the field of view.

A further modication of the novel double Cassegrain of FIGURE 9 is illustrated in FIGURE 10 which reduces the number of optical surfaces required in an arrangement of the type shown in FIGURE 9. Thus, in FIGURE 10, the Cassegrain system is provided with the usual priy mary mirror 200 and secondary mirror 201 of FIGURE 9 where, however, the function of the tertiary and quaternary mirrors 203 and 204 of FIGURE 9 are merged into the primary and secondary mirrors 200 and 201, respectively. The resulting system is extremely compact and results in a telephoto effect of approximately 0.11. Note that the primary and secondary mirrors 200 and 201 in FIGURE 10 are both made to be aspheric in order to correct spherical aberration and coma. The resilient astigmatism is so small as to be completely negligible in FIGURE 10.

A negative lens 210 is then placed just in front of the focal plane of mirror 201 to fiatten the field at the photocathode of image dissector 229.

Although this invention has been described with respect to its preferred embodiments, it should be understood that many variations and modifications will now be obvious to those skilled in the art, and it is preferred, therefore, that the scope of the invention be limited not by the specific disclosure herein, but only by the appended claims.

The embodiments of the invention in which an exclusive privilege or propertly is claimed are defined as follows:

1. An illuminated body tracking device comprising telescope means for forming an image of said illuminated body; image dissector means having a photocathode, control means for controlling a beam of photo-electrons emitted from said photocathode and output circuit means for receiving said beam of photo-electrons and generating output signals in response thereto; control circuit means connected to said control means for selectively directing photo-electrons from selected regions of said photocathode toward said output circuit means to effect scanning of said photocathode; said control circuit means generating a circular scan of said photocathode having a superimposed dither thereon.

2. The device of claim 1 wherein said control circuit means includes a relatively low frequency voltage source connected to said contro] electrodes for generating a relatively Iow frequency circular scanning frequency and a relatively high frequency voltage source for superimposing a relatively high frequency dither on said relatively low frequency circular scan.

3. The device substantially as set forth in claim 2 wherein said control circuit means further includes means responsive to the second harmonic component of the output of said output circuit means connected to said control means for adjusting the radius of said circular scan to the radius of the image of said illuminated body on said photocathode.

4. The device as set forth in claim 3 wherein said circular scan has a frequency of about 200 cycles per second and said dither has a circular scan of about 10,000 cycles per second.

5. The device as set forth in claim 3 wherein said control means includes acquisition circuit means for causing a spiral scanning pattern which spirally scans the full area of said photocathode whereby an output signal from said output circuit means signals the existence of an illuminated body in the field of view of said photocathode.

6. The method of scanning the periphery of an illuminated body against a dark background; said method comprising the steps of circularly scanning around the periphery of said illuminated body, and superimposing a dither on said circular scan to cause said scanning path to continuously cross the dark to light boundary at the periphery of said illuminated body.

7. A tracking device for a planet tracker; said tracking device including means for forming an image of the planet to be tracked, and scanning means for scanning said image; said scanning means for scanning said image including means for photoelectrically observing selected discrete portions of said image and output signal generating means for generating an output signal when said means for photoelectrically observing said image detects an illuminated discrete area; said scanning means further including control means connected to said means for photoelectrically observing selected discrete portions of said image to select discrete areas of said image to be observed in a predetermined scanning pattern; said predetermined scanning pattern comprising a circle having the diameter of said image and having a high frequency dither superimposed thereon whereby, during scanning, said means for photoelectrically observing said image repetitively moves around and into and out of the periphery of said image.

8. The device as set forth in claim 7 which further includes sizing circuit means for adjusting the diameter of said scanning circle to the diameter of said image; said sizing circuit means including first circuit means responsive to the phase of the second harmonic of the dither frequency connected to said output signal generating means, and second circuit means connecting said rst circuit means to said control means for adjusting the diameter of the said circular scan to the diameter of said image.

9. The device as set forth in claim 8 which further includes acquisition control circuit means connectable to said control means; said acquisition control circuit means 13 14 driving said photoelectric observing means in a spiral 3,246,160 4/ 1966 Zuckerbraun 250-203 pattern. 3,290,505 12/1966 Stavis Z50-203 References Cited UNITED STATES PATENTS RALPH G. NILSON, Primary Examiner.

3,240,942 3/1966 Birnbaum etal Z50-203 5 M. ABRAMSON, Assislanf Examiner.

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