GB2344951A

GB2344951A - Distant object tracker

Info

Publication number: GB2344951A
Application number: GB9827819A
Authority: GB
Inventors: Michael Andrew Hattey; Terrence Christopher Platt
Original assignee: STARLIGHT XPRESS Ltd
Current assignee: STARLIGHT XPRESS Ltd
Priority date: 1998-12-17
Filing date: 1998-12-17
Publication date: 2000-06-21
Anticipated expiration: 2018-12-17
Also published as: GB2344951B; GB9827819D0

Abstract

A device for use in astronomical observations combines an imaging function with an automatic tracking function. An interline CCD array is used as an imaging element and the output of one of its two fields is used to provide the tracking control data while the other provides the imaging data. The tracking control is produced by comparing the position of an identified object in two successive images from the same field, taken over a period of time. If the positional error exceeds a predetermined threshold then a position correction signal is generated to adjust the position of the telescope. Also disclosed is an image shifting device (fig 5 not shown) that allows the tracking of a star without the need to move the telescope.

Description

METHOD AND APPARATUS FOR TRACKING A SELECTED DISTANT OBJECT This invention relates to a method and apparatus for tracking a selected distant object.

The invention could be applied in the field of astronomy where, for example, the tracking device includes a telescope moved by drive means to track an astral body such as a star. However, the tracking device does not need to move to follow the object, because the position of an optical image can be shifted when using a stationary telescope. Whilst the invention can be used to track a star, by telescope, to compensate for the earth's rotation, objects other than stars could be tracked. For example, earth surveying satellites could be tracked where the invention is used for dish guidance. Moreover, terrestrial objects can be tracked and in this case, the earth's motion is not a factor which needs to be taken into account.

At least preferred embodiments of the invention are directed at tracking objects which are effectively at an optical infinity, i. e. where ranging information is not required and where only the position of a very distant and slowly moving object needs to be followed. (Although an object like a star may be considered to be stationary, there is relative motion, between a telescope and the star, due to the Earth's rotation, which creates the impression of a moving object.) A particular embodiment of the invention uses a CCD camera for either telescopic guidance, or for image shifting with a stationary telescope. This is explained in more detail below, but first a description will be given of the background to the invention.

Electronic imaging cameras using Charge Coupled Devices (CCDs) are now commonly used for making astronomical observations, due to their high sensitivity and the convenience of digital image processing and storage. However, a very common problem encountered when imaging the night sky, is the difficulty of precisely counteracting the rotation of the Earth during what may be an exposure time of many minutes. The standard technique is to place the telescope and camera on an "Equatorial"mounting, which has its main axis arranged to be parallel to that of the Earth, and then rotating the assembly from East to West at a rate which matches that of the sky.

This works quite well, but is subject to the mechanical inaccuracies of the bearings, alignment, gearing and the atmospheric refraction of light. These effects conspire to cause many exposures to be"trailed"and poorly defined.

Some attempts have been made to counteract these problems by the use of electronic "autoguiders"of various types, most of which employ two CCD imaging chips (as shown in Fig. 2). The CCD is used to record images of a bright field star at short intervals and the circuit issues corrections to the mounting drives, based on drifts in the guide star position on the CCD sensor. Such autoguiders can work well, but have the problem that two CCD sensors are required (one for guiding and one for imaging) and that both sensors have to be mounted in close proximity to each other, so as to "see"the same telescope star field. This increases the cost of an autoguiding CCD camera and can be a problem if one needs to guide on an object which is moving independently of the background stars. This can be the case when a comet or asteroid is the subject, as the autoguider chip sees the"fixed"stars at the edge of the field, not the slowly moving object, which is to be imaged.

At least in its preferred embodiments, the invention seeks to provide an improved method of acquiring tracking data from a CCD camera which uses only a single sensor. This can eliminate all of the problems described above.

According to one aspect of the invention, there is provided an optical imaging method for tracking a selected distant object in a given field of view, in which method a tracking device compensates for relative motion between the tracking device and the object; the method including: (a) focusing an image of the object onto a screen of an imaging device, the screen having an imaging field; (b) deriving a tracking control signal from one part of the imaging field of the screen and using it to track the selected distant object, or to track another distant object in the same field of view; (c) deriving an imaging signal from another part of the same screen and using it to provide imaging information for the selected distant object; and (d) using the tracking control signal to compensate for the relative motion.

Preferably, the screen is a surface of a CCD imaging chip, in particular an interline CCD that provides an interlaced data readout, the tracking control signal being derived from an even set of signals in the interlaced readout and the imaging signal being derived from an odd set of signals in the interlaced readout, or vice versa.

In the preferred embodiment of the invention, the special nature of an interlaced interline CCD sensor can be employed advantageously. Most astronomical cameras have ignored this type of CCD, as it is designed for daylight video imaging and, in its original form, it had a relatively low sensitivity. However, the design of these sensors has greatly improved during the last ten years, and they now match, or exceed, the performance of many"scientific"CCD sensors.

The difference between a conventional ("frame transfer") CCD and an"interline" CCD, is mainly in the method of reading out the stored image data. In the frame transfer CCD, the light image is projected onto an array of photosensitive cells (pixels), which then fill with an electronic charge, proportional to the number of photons received. This charge is then"read out"by cycling the voltages applied to the pixel array, so that the electrons are shifted down and across the array to an output gate. The fact that pixels are part of the readout array, makes it impossible to read out the charge selectively, without disturbing the entire image.

In the"interline"CCD, the pixels consist of charge collecting photodiodes, which are interdigitated with CCD registers, similar to the pixel array of a frame transfer sensor.

Photocharge collects in the pixels of the diode array during an exposure, but it is read out by applying a voltage pulse to the diodes, which moves the accumulated charge into the readout registers. The CCD can therefore continue integrating a new image, while the previous image is being read out and processed for guidance information.

Besides the latter features of the interline CCD, most interline devices are designed for "interlaced"TV readout. Interlacing consists of reading out all of the"even"lines of a TV image in one"field", followed by reading out all of the"odd"lines in a second field. These two fields are transferred to the readout array by separate voltage pulses and so it is possible to repetitively read out one field for guidance, while integrating the other field for imaging.

Because an interlaced readout is not used for astronomical imaging (two consecutive exposures would be needed), there is no significant loss of resolution resulting from the use of only one field for the image, and the only effect is to reduce the CCD sensitivity by 50%. This is acceptable, when the guidance action of the camera will allow much longer exposures to be made, to compensate.

In another embodiment of the invention, sets of even and odd signals are alternated in consecutive frames so that alternate even and odd signals are combined to form a single image with twice the resolution, the tracking control signal and the imaging signal being derived from respective combined single images. Thus, two successive images of the object are captured, one using the"even numbered"pixel rows and one using the"odd numbered"pixel rows. These two images can then be mathematically combined to yield a single image, with odd and even rows, thereby having twice the vertical resolution of each starting image. This technique would also permit a colour CCD chip to be used in the imager/guider, as the two frames combined will contain all of the colour filter data from the colour matrix CCD, without any loss caused by using only one half of the available pixels.

In an alternative embodiment, the CCD imaging chip is a frame transfer CCD that provides a full frame readout, the imaging signal being derived from a set of signals in a part of the full frame readout which corresponds with an area of the screen on which an image of said selected object is received, the tracking control signal being derived from a set of signals in another part of the full frame readout which corresponds with an area of the screen on which an image of said other object is received.

Although the interline CCD is ideally suited to autoguiding/imaging, it would be possible to use instead a frame-transfer, or full-frame CCD. Preferably, with such a CCD, a narrow strip at the edge closest to the readout register, is rapidly read out to the controlling computer and then the remaining image charge pattern is returned to the original pixels by"reverse clocking"the vertical registers by the appropriate number of pixels. In the astronomical application, the narrow strip of pixels will normally contain at least one star of sufficient brightness to allow it to be used for autoguiding and the rest of the image frame will be relatively unaffected by the repeated readout of the guiding strip. The main disadvantage of this method is the loss of a portion of the image frame (perhaps 10-20%) for guiding, and the possibility of introducing image"smear"if the guider strip readout is not performed very rapidly.

Some large CCDs have dual readout registers and split imaging areas, which would allow the use of one half of the chip for imaging, while the other half is guiding.

Whilst the invention can be used to control the movement of a tracking device, such as a telescope, so that it continually images a star on a predetermined area of the CCD screen, the telescope need not be moved. Instead, in another embodiment of the invention, means are used for correcting the position of the image, and it is unnecessary to move the entire telescope, or optical assembly. This overcomes the problem of interfacing control or correction signals (e. g. from the camera), with the electronics of the telescope drive system, which is not always easily possible. In addition, it can be difficult to make sufficiently fine adjustments to the positioning, when using the fast, and often inaccurate, motors and gearboxes of the mounting.

According to another aspect of the invention, optical imaging apparatus for tracking and imaging a selected distant object in a given field of view comprises: a tracking device which includes: an imaging device having a screen with an image field for receiving an image of the selected object, means for focusing an image of the object onto the screen of the imaging device; and means for deriving a tracking control signal from one part of the image field of the screen, which is used to track the selected distant object, or another distant object in the same field of view, and also for deriving an imaging signal from another part of the same screen to provide imaging information for the selected distant object; the tracking device using the tracking control signal to compensate for the relative motion.

Preferably, the screen is a surface of a CCD imaging chip, in particular an interline CCD that provides an interlaced data readout, the tracking control signal being derived from one set of signals in the interlaced readout and the imaging signal being derived from the other set of signals in the interlaced readout.

Alternatively, the CCD imaging chip is a frame transfer CCD that provides a full frame readout, the imaging signal being derived from a set of signals in a part of the full frame readout which corresponds with an area of the screen on which an image of said selected object is received, the tracking control signal being derived from a set of signals in another part of the full frame readout which corresponds with an area of the screen on which an image of said other object is received.

Where the object or objects are astral bodies, such as stars, the tracking device includes a telescope, and either drive means for maintaining the tracking of the telescope when it needs to be moved, or means for shifting the image on the screen when the telescope is stationary.

The latter means preferably includes a light refracting device, which is positioned in an optical path between an objective of the telescope and the imaging device, for causing a shift in the position of the image on the screen in response to the tracking control signal, said shift being caused by changing the angle of incidence on the refracting device.

According to another aspect of the invention, there is provided an electronic camera comprising a CCD imaging chip, a surface of which acts as a screen for receiving an image from an optical focusing system, said chip being a single interline CCD that provides an interlaced data readout, the camera having data connection means for computer which is programmed to derive a tracking control signal from one set of signals in the interlaced readout and an imaging signal from the other set of signals in the interlaced readout.

According to another aspect of the invention, there is provided image shifting means for interposition between an optical device (such as a camera) having a screen for receiving a focused image, and an image focusing system (such as a telescope) for focusing the image on the screen, said image shifting means including: a housing means which can be coupled between said optical device and said image focusing system, said housing means having an optical axis therethrough; light refracting means for refracting light on said optical axis, said light refracting means including for means for changing the angle of incidence of input light from said image focusing system whereby the position of the image on said the screen can be shifted.

According to another aspect of the invention, there is provided a motion tracking and guidance system which includes the latter electronic camera and the latter image shifting means, said image shifting means being removably coupled between said camera and a telescope; the system further including a laptop computer programmed to derive said tracking control signal from said interlaced readout in response to movement of said focused image on said screen.

Preferred embodiments of the invention will now be described with reference to some of the accompanying schematic drawings, in which: Fig. 1 shows a simplified guidance system for a telescope; Fig. 2 illustrates the operation of a CCD imaging chip used in a normal"astronomical imaging"non interlaced readout mode; Fig. 3 illustrates the operation of an interlaced, interline transfer CCD when used in an"astronomical self-guiding"mode in an embodiment of the invention.

Fig. 4 illustrates the operation of a frame transfer CCD; Fig. 5 shows a preferred image steering optical system which can be used with a stationary telescope; and Fig. 6 is a diagram illustrating the principle of the image steering used in Fig. 4.

Referring to Fig. 1 of the drawings, one embodiment of the invention employs a CCD camera 1 to guide a movable telescope 2, by means of an automatic guiding system 3,4,5,6 and 7. The telescope is supported by an equatorial mounting, whereby (a) it is pivotally supported by a forked member 8 for adjusting its declination (with respect of the horizon), and (b) the forked member 8 is rotatably supported on a mounting 9, which, due to its fixed inclination, is used to adjust ascension. The angle of declination is adjusted by a declination motor 6 which is controlled by motor controller 5. A right ascension motor 5 rotates the forked member 8 about the inclined axis 9a and is also controlled by means of motor controller 5. The mounting 9 is attached to a fixed inclined support 9b, which provides an initial inclination of the angle of latitude for the location of the telescope and pointing due North, so that the axis is parallel to that of the Earth. This provides a reference point for positioning the telescope 2 for astronomy. The construction and operation of motor controller 5, motors 6 and 7 and the pivotal and rotational mountings is well known in the art and therefore no further detailed description will be given.

In the prior art, the camera 1 is fitted with two CCD imaging chips each of which is of the form shown in Fig. 2. One chip is used for guiding telescope 2 and the other chip is used for imaging. Each chip includes super pixels SP1 and SP2 which output to respective vertical registers VI (and similar pixels output to vertical registers V2) both vertical registers having outputs to horizontal registers H1, H2 connected in sequence. The operation of the two chips for guidance and imaging is known and the difference between a prior art system using these two chips and a system according to the invention, is that only one CCD chip is used, preferably with means to enable it to operate in an interlaced read-out mode. This, together with the general operation of the guidance system will now be described in more detail with reference to Fig. 3.

The motor controller 5 is connected to a computer 4, preferably a laptop computer, by a connecting lead for supplying drive signals and data to controller 5. Camera 1, is a CCD imaging camera in which is provided the components shown in the dotted line box 3. These include CCD imager 3a, CCD clock generator 3b analog to digital converter 3c and video amplifier 3d. The A/D converter (3c) is also connected to computer (4) as shown in Fig. 1.

This embodiment of the invention employs the CCD camera with a single CCD chip and a control system to guide the movable telescope, so as to track a star over a period of time in which the motion of the Earth would otherwise cause the star image to move away from a focal point on the imaging plane of the camera. The camera is attached to the eyepiece of the telescope 2 and its CCD output is connected as shown to the computer 4 which can process the output signals so as to provide a display 4a and so as to operate the servo-motors to cause them to tilt the telescope and bodily rotate it around the inclined axis 9b, in order to follow the (apparent) star motion.

The way in which signals are readout from the CCD camera for the purpose of tracking and imaging will now be described in more detail and with reference to Fig.

3.

Fig. 3 shows schematically, the operation of an interlaced, interline transfer CCD when used, for example, in the system shown in Fig. 1. The CCD screen is shown as having imaging pixels IP in Field 1, which each output to respective portions of vertical registers V 1, V2 and auto-guiding pixels AP, which each output to respective portions of vertical registers VI, V2. The vertical registers are connected to horizontal registers H1, H2, which are connected to an output amplifier OP. These components are part of/or are connected to interface circuitry 3 in the camera head.

Guidance of the telescope is largely controlled directly by the computer 4 which communicates with the interface circuitry 3. An exposure is started by sending a "clear"signal to the interface circuitry, which applies a pulse to the imaging substrate of the CCD imager to remove all of the image charge from the pixels. This begins the exposure with a"clean"CCD.

When the exposure begins, the image received on the screen of the CCD causes photons to generate electron-hole pairs in pixel wells and electrons are trapped. After a short interval of sufficient length to develop a detectable charge from an object being tracked, a"read Field 1"pulse is issued from the computer and the stored electrons in Field 1 of the interlaced array are dumped into vertical registers V 1, V2 of the CCD.

Field 2 continues to integrate charge without being affected.

The computer now issues a repeating sequence of 1 vertical clock cycle, followed by about 500 horizontal clock cycles (or as many as there are pixels in the horizontal plane of the CCD). After each horizontal clock, the analog to digital converter is triggered to convert its input voltage into a 16 bit binary number and this number is stored in a memory array within the computer. Thus for each pixel in the CCD array, a number corresponding to the charge within that pixel, is stored in a digital format.

When a line has been fully read out, a new vertical clock is issued and the process repeats for the next line of the CCD array.

After a short time (typically 2 seconds), all of the Field 1 pixels have been read into the memory array and a new field 1 image has begun to integrate in the CCD. The software then examines the stored data for the presence of a star image, or similar well defined feature, and determines its"centroid". This is the apparent centre of the inevitably, diffused light spot, which can be determined to the sub-pixel level. Once this point has been located, the information is stored and will be used for comparison with future Field 1 images, to determine the degree and direction of any positional drift.

This process now repeats, after a similar exposure time, and the new star image is read out and stored as above. A new centroid is found and the positional errors are calculated. If the error exceeds a predetermined amount, a drive correction signal is generated and sent to the telescope motor controller 5 to cause an appropriate shift in the telescope aim. This is usually in the form of a serial data burst containing direction and duration data for the motors 6 and 7.

As the characteristics of the mechanical amplification of the motor/telescope system may need to be learnt, it is often necessary to run a"training"session on the camera system, where a known length of motor burst is sent and the corresponding image shift is measured from the CCD readout. A"pixels per second"factor can thus be determined and stored for use during the actual imaging cycle.

After the selected image exposure time has elapsed, the computer issues a"read Field 2"pulse and the actual image data is read out in the same way as the tracking data, and stored in the computer. This data may then be displayed as an image on the monitor and saved to disk for future use.

Although a full frame imager can be used to perform a similar function, the readout process is different. In this case, which is illustrated by Fig. 4, the"read Field 1"and "read Field 2"pulses are not used and the tracking readout consists of a rapid read of, say 20 lines, followed by"backward clocking"of the remaining data. This puts the stored image back into the same pixels as it started in, but the 20 lines are lost from the image edge. The 20 line data block is examined for star images, as before, and error determined in the same way. Fig. 4 illustrates pixels/vertical registers P/V which output to horizontal registers H1, H2 connected to output amplifier OP.

Fig. 5, schematically illustrates an image shifting device that can be used with a fixed telescope (now shown). In Fig. 4, a short tube, or box 10 is interposed between the CCD camera 1 and the optical output 11 of the telescope (not shown). This tube 10 contains a negative input lens 12, two rotatable plane-parallel optical windows 13 and 14, and a positive output lens 15. The input and output lenses are of equal, and opposite, power and are used to transfer the converging image beam through the movable windows as parallel light, and then re-converge it in its original form. This prevents any significant optical aberration of the output image being caused by the tilt of the windows and optically"removes"the length of the assembly from the light path.

This optical effect is valuable, as it is often impossible to reach the focal point of a telescope if too long an optical path length is needed by the guiding unit. A flip mirror 16 is pivoted to hinge 17, whereby input light is reflected into a finder eyepiece 18.

The two windows are typically made of 2mm thick crown glass, with anti-reflection coatings on both surfaces. Each window can be rotated about an axis in its plane, through an angle of, typically +/-30 degrees, and the rotation axes of the two windows are arranged to be at 90 degrees to each other. A small servo motor assembly 13a and 14a on each axis permits the window rotations to be applied electrically by error signals from the CCD camera autoguider software (not shown).

As is well known, a light beam passing through a plane parallel glass plate, undergoes refraction, and this will displace the light beam from its original path by an amount which is proportional to the angle of incidence and the refractive index of the glass.

This principle is schematically illustrated in Fig. 5, where a rotatable deviator plate 20, positioned between a negative collimator 21 and a re-imaging lens 22, can be tilted so as to shift the image in the focal plane of lens 22. The arrow 23 represents the image position when the plate 20 is vertical (to the drawing) and perpendicular to the beam. The arrow 24 represents the position of the image with the plate 20 inclined (as shown in the drawing).

It can be calculated that the maximum displacement caused by a 2mm crown glass window at an angle of 30 degrees to the beam, will be approximately 374uM, this being proportional to the sine of the angle of tilt. Most CCD cameras have a pixel pitch of between 5 and 30uM and so this displacement is sufficient to move the image by between +/-75 and 13 pixels at the CCD surface. The tracking error of a typical telescope mounting is rarely more than +/-10 pixels at the focal lengths usually used and so the mechanism described can completely compensate for the errors encountered, with no input to the mounting drives.

A particular advantage of the image shifting device is that it avoids having to move the telescope bodily to track a star, thus avoiding all of mechanical entrainment problems of the prior art.

The image shifting device can also be used independently and with systems other than those described above with regard to automatic tracking.

Claims

CLAIMS 1. An optical imaging method of tracking a selected distant object, in a given field of view, where a tracking device compensates for relative motion between the tracking device and the object; the method including: focusing an image of the object onto a screen of an imaging device, the screen having an imaging field; deriving a tracking control signal from one part of the imaging field of the screen and using it to track the selected distant object, or to track another distant object in the same field of view, and deriving an imaging signal from another part of the same screen and using it to provide imaging information for the selected distant object; and using the tracking control signal to compensate for the relative motion.
2. A method according to claim 1, in which the screen is a surface of a CCD imaging chip.
3. A method according to claim 2, in which the CCD imaging chip is an interline CCD that provides an interlaced data readout, the tracking control signal being derived from an even set of signals in the interlaced readout and the imaging signal being derived from an odd set of signals in the interlaced readout, or vice versa.
4. A method according to claim 3, in which the sets of even and odd signals are alternated in consecutive frames so that alternate even and odd signals are combined to form a single image with twice the resolution, the tracking control signal and the imaging signal being derived from respective combined single images.
5. A method according to claim 2, in which the CCD imaging chip is a frame transfer CCD that provides a full frame readout, the imaging signal being derived from a set of signals in a part of the full frame readout which corresponds with an area of the screen on which an image of said selected object is received, the tracking control signal being derived from a set of signals in another part of the full frame readout which corresponds with an area of the screen on which an image of said other object is received.
6. A method according to any preceding claim when used in astronomy, wherein the object or objects are astral bodies, such as stars, and wherein the tracking device includes a telescope and drive means for moving the telescope which respond to the control signal.
7. A method according to any of claims 1-5 when used in astronomy, wherein the object or objects are astral bodies, such as stars, and wherein the tracking device includes means for shifting the image of the object on the screen, and a telescope coupled to the image shifting means.
8.'A method according to claim 7, wherein the image shifting means includes a light refracting device, which is positioned in an optical path between an objective of the telescope and the imaging device, for shifting the position of the image on the screen in response to the tracking control signal, said shift being caused by changing the angle of incidence on the refracting device.
9. Optical imaging apparatus for tracking and imaging a selected distant object in a given field of view, the apparatus comprising: a tracking device which includes: an imaging device having a screen with an image field for receiving an image of the selected object, means for focusing an image of the object onto the screen of the imaging device; and means for deriving a tracking control signal from one part of the image field of the screen, which is used to track the selected distant object, or another distant object in the same field of view, and also for deriving an imaging signal from another part of the same screen to provide imaging information for the selected distant object; the tracking device using the tracking control signal to compensate for the relative motion.
10. Apparatus according to claim 9, in which the screen is a surface of a CCD imaging chip.
11. Apparatus according to claim 10, in which the CCD imaging chip is an interline CCD that provides an interlaced data readout, the tracking control signal being derived from one set of signals in the interlaced readout and the imaging signal being derived from the other set of signals in the interlaced readout.
12. Apparatus according to claim 10, in which the CCD imaging chip is a frame transfer CCD that provides a full frame readout, the imaging signal being derived from a set of signals in a part of the full frame readout which corresponds with an area of the screen on which an image of said selected object is received, the tracking control signal being derived from a set of signals in another part of the full frame readout which corresponds with an area of the screen on which an image of said other object is received.
13. Apparatus according to any of claims 9-12, wherein the object or objects are astral bodies, such as stars, the tracking device includes a telescope, and either drive means for maintaining the tracking of the telescope when it needs to be moved, or means for shifting the image on the screen when the telescope is stationary.
14. Apparatus according to claim 13, wherein the image shifting means includes a light refracting device, which is positioned in an optical path between an objective of the telescope and the imaging device, for causing a shift in the position of the image on the screen in response to the tracking control signal, said shift being caused by changing the angle of incidence on the refracting device.
15. An electronic camera comprising a CCD imaging chip, a surface of which acts as a screen for receiving an image from an optical focusing system, said chip being a single interline CCD that provides an interlaced data readout, the camera having data connection means for computer which is programmed to derive a tracking control signal from one set of signals in the interlaced readout and an imaging signal from the other set of signals in the interlaced readout.
16. Image shifting means for interposition between an optical device (such as a camera) having a screen for receiving a focused image, and an image focusing system (such as a telescope) for focusing the image on the screen, said image shifting means including: a housing means which can be coupled between said optical device and said image focusing system, said housing means having an optical axis therethrough ; light refracting means for refracting light on said optical axis, said light refracting means including for means for changing the angle of incidence of input light from said image focusing system whereby the position of the image on said the screen can be shifted.
17. A motion tracking and guidance system which includes the electronic camera of claim 15 and the image shifting means of claim 16, said image shifting means being removably coupled between said camera and a telescope; the system further including a computer programmed to derive said tracking control signal from said interlaced readout in response to movement of said focused image on said screen.