U.S. patent application number 09/783756 was filed with the patent office on 2001-10-18 for high resolution imaging instrument.
This patent application is currently assigned to Pinecone Imaging Corporation. Invention is credited to Rhoads, Geoffrey B..
Application Number | 20010030277 09/783756 |
Document ID | / |
Family ID | 46256952 |
Filed Date | 2001-10-18 |
United States Patent
Application |
20010030277 |
Kind Code |
A1 |
Rhoads, Geoffrey B. |
October 18, 2001 |
High resolution imaging instrument
Abstract
An imaging instrument includes plural spaced-apart photon
collectors, whose sampled outputs are correlated in pairs to yield
brightness line integrals across a remote object being imaged.
These integrals can be subject to matrix decomposition to yield a
2D array of image data corresponding to the object. Another imaging
instrument includes plural spaced-apart reflectors, each having an
optical fiber end at its focal point. The precise positions of the
optical fiber ends are controlled by a control system (e.g.,
including piezo-electric positioners) that can be operated to
counter-act the effect of atmospheric turbulence. The other ends of
the fibers terminate at an image plane and serve to provide an
output image.
Inventors: |
Rhoads, Geoffrey B.; (West
Linn, OR) |
Correspondence
Address: |
William Y. Conwell
Pinecone Imaging Corporation
6224 SW Tower Way
Portland
OR
97221
US
|
Assignee: |
Pinecone Imaging
Corporation
|
Family ID: |
46256952 |
Appl. No.: |
09/783756 |
Filed: |
February 13, 2001 |
Related U.S. Patent Documents
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Application
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Filing Date |
Patent Number |
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09783756 |
Feb 13, 2001 |
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09507877 |
Feb 22, 2000 |
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6225619 |
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09507877 |
Feb 22, 2000 |
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09021853 |
Feb 11, 1998 |
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6028300 |
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09507877 |
Feb 22, 2000 |
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08875505 |
Jul 29, 1997 |
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6084227 |
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08875505 |
Jul 29, 1997 |
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08171661 |
Dec 20, 1993 |
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5448053 |
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08171661 |
Dec 20, 1993 |
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08024738 |
Mar 1, 1993 |
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5412200 |
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60037541 |
Feb 11, 1997 |
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Current U.S.
Class: |
250/208.1 |
Current CPC
Class: |
G02B 26/06 20130101 |
Class at
Publication: |
250/208.1 |
International
Class: |
H01L 027/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 30, 1995 |
US |
PCT/US95/02101 |
Claims
We claim:
1. A high resolution imaging system comprising: a data processing
unit including a memory, a correlator, and a matrix decomposition
system; a time base; and plural spaced-apart sensors coupled both
to the data processing unit and to the time base; wherein the time
base associates time data with information acquired by the sensors,
and the data processing unit correlates the information from the
plural sensors using the correlator, and processes same using the
matrix decomposition system to yield high resolution image
data.
2. The system of claim 1 in which the correlator comprises a
processor programmed in accordance with software.
3. The system of claim 1 in which the matrix decomposition system
comprises a processor programmed in accordance with software.
4. A method of generating image data corresponding to a remote
object, comprising: acquiring a first set of photon data from the
remote object using a first sensor; acquiring a second set of
photon data from the remote object using a second sensor spaced
apart from the first; and correlating the first and second sets of
photon data to yield a brightness line integral representing a scan
of image data across said object, a direction of said scan being a
function of the relative locations of the first and second sensors
and the remote object.
5. The method of claim 4 that includes repeating the method with
other pairs of first and second sensors, and processing all of the
image data thereby produced to yield two dimensional image
data.
6. The method of claim 5 in which the processing includes using
matrix algebra to synthesize the scan image data from the
respective pairs of sensors to yield the two dimensional image
data.
7. The method of claim 4 that further includes quantifying
atmospheric distortion through which the remote object is being
imaged, and processing the image data to compensate for at least
part of said distortion.
Description
RELATED APPLICATION DATA
[0001] This application is a continuation of application Ser. No.
09/507,877, filed Feb. 22, 2000, which is a division of application
Ser. No. 09/021,853, filed Feb. 11, 1998 (now U.S. Pat. No.
6,028,300), which claims benefit of application Ser. No.
60/037,541, filed Feb. 11, 1997. Application Ser. No. 09/507,877 is
also a continuation-in-part of application Ser. No. 08/875,505,
filed Jul. 29, 1997, which is the U.S. nationalization of
international application PCT/US95/01201, filed Jan. 30, 1995.
Application Ser. No. 08/875,505 is also a continuation of
application Ser. No. 08/171,661, filed Dec. 20, 1993 (now U.S. Pat.
No. 5,448,053), which is a continuation-in-part of application Ser.
No. 08/024,738, filed Mar. 1, 1993 (now U.S. Pat. No. 5,412,200).
Priority is claimed to these prior applications under 35 USC
Section 120, and the issued patents are incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] The present invention relates to astronomical imaging, and
more particularly relates to methods and apparatuses for
astronomical imaging using multiple sensors.
BACKGROUND AND SUMMARY OF THE INVENTION
[0003] In accordance with one aspect of the present invention, an
astronomical imaging array is formed of several widely spaced
photon collectors (e.g. photodiodes). Each collector has associated
with it a digitizing sampler that collects a stream of sample data
from the photodiode in response to a trigger signal provided by a
time source. Samplers at different photon collectors are triggered
at different instants in accordance with their spacing, and their
relative optical path differences from the object being imaged. In
particular, each sampler is triggered to collect a record of
samples when a given phase front of light from the object being
imaged is expected to pass the photon collector.
[0004] Using intensity interferometry techniques, the sampled data
from each photon collector is correlated with data from other
collectors, yielding a waveform whose individual values represent a
brightness line integral through the object. Using different photon
collector pairs, different sets of brightness line integrals
through the object are produced. Matrix algebra is then employed to
synthesize the collected set of line integrals into a two
dimensional image representing the brightness of the object being
imaged.
[0005] Intensity interferometry was invented by Hanbury Brown and
Twiss, and is exemplified by the following articles (all by Hanbury
Brown et al): "Correlation Between Photons in Two Coherent Beams of
Light" (Nature, Vol. 177, pp. 27-29, Jan. 7, 1956); "A Test of a
New Type of Stellar Interferometer on Sirius," (Nature, Vol. 178,
pp. 1046-48, Nov. 10, 1956); "The Stellar Interferometer at
Narrabri Observatory; I: A Description of the Instrument and the
Observational Procedure" (Monthly Notices of the Royal Astronomical
Society, Vol. 137, pp. 375-392, 1967); and "The Angular Diameters
of 32 Stars," (Monthly Notices of the Royal Astronomical Society,
Vol. 167, pp. 121-136, 1974). (These articles, and others landmarks
in the field of interferometry, are reprinted in the recent volume
"Selected Papers on Long Baseline Stellar Interferometry," SPIE
Milestone Series, Vol. MS 139, 1997, edited by Lawson.)
[0006] No one, to our knowledge, has employed Hanbury Brown-Twiss
(HBT) techniques as the basis for an imaging instrument. Instead,
astronomical use of such techniques (for forty years) has been
limited to the determination of stellar diameters.
[0007] We believe one factor contributing to the failure of others
to employ HBT techniques in imaging applications may be certain
interpretations that are traditionally accorded the original
Hanbury Brown papers. We note some possible alternative
interpretations that may be central to the functioning of the
below-described embodiments.
[0008] One area in which HBT's work may have been misunderstood is
the common impression that intensity interferometry requires that
the two sensors be equidistant from the object being measured (c.f.
section 3.3 of their 1967 paper). HBT state that correlation
diminishes about 10% with the first 1 nanosecond delay between the
two collectors (corresponding to a path length difference of about
one foot), and diminishes exponentially as this delay is
increased.
[0009] (To provide this equidistant spacing, the observatory at
Narrabri employed two detectors on a circular track having a radius
of about 100 m. By this arrangement, the two detectors could be
placed equidistant from the star being measured, while also
providing a range of spacing from 0-200 meters.)
[0010] We presently believe that intensity interferometry
techniques can be used with arbitrarily positioned detectors,
including arbitrary three dimensional arrangements (e.g. one or
more in space).
[0011] Another possible misinterpretation of HBT's original
findings is the impression that the variation in correlation, as a
function of detector spacing, is a single-lobed function (c.f. FIG.
5 of HBT's 1967 paper), trailing to zero at a normalized spacing of
about 3.5. We believe it is likely that this function instead
exhibits multiple side lobes, of diminished amplitude, extending
far beyond the 0-3.5 range contemplated by HBT. (Our belief is
based, in part, on Fraunhofer analysis of the stellar disk being
imaged.) The existence of such secondary lobes would begin to
suggest that detectors can be spaced much further apart than
previously thought possible.
[0012] We caution that our foregoing critiques of the HBT work are
preliminary but are believed to explain the operation of our
detailed embodiment. Further study, however, may reveal other or
additional rationales.
[0013] Throughout our work we encounter wave/photon conundrums. For
example, classical photon theory holds that a single photon can
only be sensed once, e.g., when it kicks an electron out of a
valence band in a photodiode. Thereafter, it ceases to exist. Yet
HBT interferometry seems to illustrate the contrary, by evidencing
intensity correlations between spaced-apart optical detectors.
[0014] Hanbury Brown and Twiss acknowledged these conundrums, but
posed no answer. In the intervening forty years, no satisfactory
resolution of the conflicting photon and wave theories has been
found. We offer none, and instead rely exclusively on classical
wave theory in analyzing operation of our system.
[0015] So that the present invention may be better appreciated, it
may be helpful to review other work in the field of astronomical
interferometry.
[0016] Most astronomical interferometry traces its origins back to
Michelson's work in the late 1800s. Michelson showed that light
from a single source, traveling different paths, can be combined to
produce fringe patterns. This is the principle on which modern
radio telescope arrays work. For example, the New Mexico Very Long
Array (VLA) includes 27 antennas (each with a 25 m reflector)
movably positioned within a Y-pattern of up to 22 miles across. The
data from each pair of telescopes is combined (often after
recording on tape with a time synchronization signal) to form
interference patterns. The structures of these patterns, and their
changes with time (as the Earth rotates) reflect the structures of
radio sources in the sky. By applying Fourier techniques to the
resulting interferometric patterns, conventional imagery can be
produced. (Such radio astronomy interferometry/imaging is well
detailed in extensive literature familiar to those active in the
field. A bibliography of writings on the topic can be found at the
internet address
http://sgra.jpl.nasa.gov/mosaic_v0.0/Spacevlbi_lib.html.)
[0017] The resolution of imagery obtained by such interferometric
techniques depends on the size of the array (the baseline). The
fully extended VLA, for example, has a resolution of 0.04
arcseconds at 43 GHz. To obtain still better resolution, longer
baselines are required. An example of a transcontinental baseline
is the Very Long Baseline Array, which employs ten identical
radiotelescopes spread from Hawaii to the U.S. Virgin Islands.
[0018] A decade ago, first steps were made to extend
interferometric baselines still further--into space. During
1986-1988, the NASA TDRSS satellite observatory--working in
conjunction with ground-based telescopes in Japan and
Australia--proved the feasibility of such systems by recording
interferometric fringes from six radioemitting sources.
[0019] Several space VLBI (very long baseline interferometry)
projects have since been proposed but have not been completed,
among them the IVS and QUASAT programs. The costs and difficulties
associated with placing complex optical telescopes into orbit
played large roles in the demise of these projects.
[0020] Just recently, the first fully-operational space VLBI
project began operation: the Japanese Institute of Space and
Astronautical Science's VSOP mission ("VLBI Space Observatory
Program," space antenna deployed Feb. 27, 1997, first fringes
produced May, 1997). Another imminent program is the Russian
RadioAstron mission, developed by the Astro Space Center of the
Lebedev Physical Institute and scheduled for launch later in 1998.
Each of these programs utilizes a single 8-10 meter radio telescope
in an elliptical Earth orbit, in conjunction with ground
radiotelescopes. Each observes in the 22, 5, and 1.6 GHz bands.
(Imagery from, and information about, the VSOP mission is publicly
available on the world wide web at http://www.vsop.isas.ac.jp/.
Information about the RadioAstron mission is publicly available at
http ://sgra.jpl.nasa. gov/mosaic_v0.0/RadioAstron.html.)
[0021] (The movement of an antenna in a space VLBI array, together
with the changing path length of the space-to-ground data link,
complicates the array's operation. In particular, signals from the
moving antenna must be temporally correlated with those from the
ground telescopes before they can be combined to generate the
interferometric data. However, such problems can be redressed by
known techniques described in the literature. To facilitate
description of the present invention, an array of fixed photon
collectors is described--it being understood that techniques
borrowed from this space radiotelescope prior art can be used to
compensate for the dynamic effects introduced by placing one or
more optical sensors into space.)
[0022] The interferometric principles employed in radio telescope
arrays can likewise be extended to arrays of optical telescopes.
Recent efforts in the optical interferometry domain include the
Cambridge Optical Aperture Synthesis Telescope (COAST) and the
Sydney University Stellar Interferometer (SUSI).
[0023] A further interferometric technique is amplitude
interferometry as described, e.g., in Currie et al, "Four
Stellar-Diameter Measurements by a New Technique: Amplitude
hiteferometry," Astrophysical Journal, Vol. 187(1), Part 1, pp.
131,134 (Jan. 1, 1974). Amplitude interferometry is a variant of
Michelson interferometry, designed to better measure stellar
diameters in the presence of atmospheric turbulence.
[0024] All of the Michelson-based interferometry systems--whether
radio or optical--rely on the wave conception of radiation, i.e.
that light/radio waves exhibit localized maxima and minima which
can be combined to constructively or destructively interfere and
produce fringe patterns. This imposes on all such systems a high
degree of physical precision (e.g. of sub-wavelength dimension)
because received waves must be combined in known phase
relationships in order for the resulting fringe patterns to have
the desired meanings. At optical wavelengths, for examples, the
lengths of connecting optical fibers must be physically or
synthetically matched to within millionths of an inch. If such
arrays are extended to space (c.f. NASA's New Millenium
Interferometer: http://huey.jpl.nasa.gov/nmi/index.html), the
locations of the space sensors must be ascertained to the
nanometer--a specification which NASA acknowledges will require "a
significant capability enhancement."
[0025] In contrast, the interferometry aspects of the present
invention do not rely on these wave-based constructive/destructive
interference forms of interferometry, with their attendant
sub-wavelengths tolerances. Instead, HBT interferometry is
concerned with the correlation of stellar intensity signals over
time. The accurate measurement of time is a far easier task than
the attainment and maintenance of sub-wavelength physical
tolerances. Moreover, the nature of correlation operations is
forgiving of many temporal errors (i.e. sample records are time
shifted as necessary to obtain optimum correlation, removing small
errors in triggering times).
[0026] By eliminating the strict physical tolerances inherent in
Michelson-based systems, costs are reduced and reliability is
increased. Such detectors are thus well adapted for use in arrays
whose baselines extend into space. Like Michelson systems,
arbitrarily fine angular resolutions can be obtained by spacing the
detectors accordingly.
[0027] Also disclosed in the following specification, and sharing
the attribute of plural spaced detectors, is a more traditional
optical imaging system (i.e. no interferometry). This system
employs techniques described in the above-cited patents to
characterize/model, atmospheric turbulence through which a
telescope is seeing.
[0028] In accordance with this further aspect of the present
invention, these turbulence modeling techniques are used in an
array of widely spaced small (e.g. three to twelve inch reflector)
telescopes to characterize atmospheric turbulence above each
telescope. (Each telescope is pointed at the same general region of
the sky.) However, instead of using the resulting data to "unblur"
each telescope's imagery, as in the patents' preferred embodiments,
it is used to dynamically reposition--at essentially real time
(e.g. 100 Hz repositioning)--the end of an optical fiber in each
telescope's focal region. Such fibers are run from the plural
telescopes to a central collection facility, where their opposite
ends terminate in an image plane. This collected group of fiber
terminations presents an image that can be viewed directly, or
magnified/processed by traditional optics as desired.
(Alternatively, the image can be sampled/recorded several times a
second, and the resulting static images can be combined to
synthesize a more accurate image.)
[0029] The foregoing arrangement yields extremely high angular
resolution imagery (the exact resolution depends on the extent of
the telescope array) while employing inexpensive optical components
(e.g. the small component telescopes).
[0030] The foregoing and additional features and advantages will be
more readily apparent from the following detailed description,
which proceeds with reference to the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a schematic block diagram of an intensity
interferometry imaging system according to one embodiment of the
present invention.
[0032] FIG. 2 is a diagram showing the geometrical relationship
between a pair of detectors, and a line integral across the object
being observed, in the system of FIG. 1.
[0033] FIG. 3 is another view showing how a line integral relates
to features on the object being imaged.
[0034] FIG. 4 is a schematic block diagram of another multi-sensor
imaging system according to another embodiment of the present
invention.
DETAILED DESCRIPTION
[0035] Referring to FIG. 1, an intensity inteferometry imaging
system 10 according to one embodiment of the present invention
includes plural spaced-apart photon detection subsystems 12, and a
data processing subsystem 14.
[0036] Each photon detection subsystem includes a reflector 16, a
sensor 18, an amplifier 20, a high speed A/D converter (sampler)
22, a time source 24, and a memory 26.
[0037] The reflector 16 in each illustrated photon detection
subsystem is a parabolic mirror that serves to collect incident
light and direct it onto the sensor 18. The design of the mirror is
not critical--its purpose is simply to intercept as many photons as
possible from the source being imaged; high precision optics are
not required. In the illustrated embodiment, the mirror has a one
meter diameter. Smaller or larger mirrors can, of course, be
used.
[0038] The sensor 18 can take numerous forms, depending on the
particular application being served. In the illustrated embodiment,
a simple photodiode is used.
[0039] The output signal produced by the sensor 18 is provided to
an amplifier 20, which scales the signal preparatory to A/D
conversion. The illustrated amplifier has a bandwidth of 100 MHz.
Such a high bandwidth amplifier is most important for high
intensity sources, such as our sun, or for implementations
employing very large photon collection mirrors. When imaging
fainter objects, or using smaller mirrors, the bandwidth of the
amplifier is less critical (e.g. a 10 MHz bandwidth amplifier will
suffice for most applications).
[0040] The output of amplifier 20 is provided to the input of a
fast A/D sampler 22. A 1 GHz digital oscilloscope (available from
Tektronix) is used in the illustrated embodiment; many other
devices can likewise be used. The digital oscilloscope produces
8-bit output samples, quantifying the instantaneous amplifier
output on a scale of 0-255. 16- or 32-bit sampling devices can of
course be used.
[0041] The digitized output samples from the sampler 22 are written
into memory 26. Due to the 1 GHz sample rate, data is provided to
the memory 26 at a rate of 1 megabyte per millisecond, so a fairly
large memory is desirably employed. A 32 megabyte memory, for
example, can store a total of 32 million samples (spanning a total
sampling time of 32 milliseconds).
[0042] The digitizing operation of sampler 22, and the writing of
data into memory 26, is initiated by a trigger signal provided to a
triggering input of the digital oscilloscope. This triggering
signal is provided from the time source 24.
[0043] Digressing a moment, a quasi-hypothetical wavefront of light
from the astronomical object being imaged will reach different of
the photon detection subsystems 12 at different times, depending on
their physical locations. ("Quasi-hypothetical" simply pays homage
to the equivocation surrounding the concept of an optical
wavefront.) The purpose of the time source 24 is to synchronize the
acquisition of sample data at each of these detection subsystems
relative to a common wavefront.
[0044] For example, if two detection subsystems 12 are positioned a
kilometer apart on a north-south line, and are used to image a star
45 degrees above the horizon, due south, the relative geometry
indicates that light from the star will reach the northern
detection subsystem 2.35 microseconds after it reaches the southern
detection subsystem. If the time source 24 at the southern
detection subsystem is set to trigger its data acquisition at time
T, the time source at the northern detection subsystem should be
set to trigger its data acquisition at time T+2.35
microseconds.
[0045] In the illustrated embodiment, each sampler 22 is triggered
such that the center of its data record coincides with the arrival
of a singular quasi-hypothetical optical wavefront emanating from
the center of the object. In simpler terms, each digitizer is
triggered to end half a record length time after the arrival of the
singular wavefront, producing half the data record before arrival
and half after arrival. Precision and accuracy issues produce some
small variations about this ideal "half."
[0046] In the preferred embodiment, the triggering time for each
detection subsystem is programmed from a central control station 28
(which typically houses the data processing subsystem 14), and is
relayed to the subsystems by telephone or wireless data
communication. Sampling then begins when the local time source
reaches the specified time. (The end of sampling is also controlled
by a signal from the time source, a programmable interval after the
sampling starts.) In other embodiments, direct control of the
triggering operations can be effected from the central station 28,
e.g. through matched fiber optic lines. In some applications,
Doppler-compensated triggering times may be used.
[0047] Time sources 24 in the illustrated embodiments are Global
Positioning System receivers, designed to receive time data from
the array of 26 GPS satellites in low earth orbit. The time signals
available to civilians from the GPS satellites are accurate only to
about 100 nanoseconds. Authorized government users can decode the
time signals to produce data accurate to within 10 nanoseconds.
Accuracy better than the 100 nanosecond civilian standard can be
achieved by averaging several intervals of the 100
nanosecond-accurate clock signals.
[0048] Although FIG. 1 shows just a few photon collection
subsystems 12, in actual practice dozens, hundreds, or thousands of
such subsystems are employed. Also, the clipart of FIG. 1 suggests
a fairly sophisticated collection system. In practice, simpler is
generally better. The main criteria for the reflector/sensor are
(1) collecting as many photons as practicable, and (2) ensuring--to
the degree practicable--that light received by a sensor emanates
only from the object being imaged. (Realistically, it suffices if
light is collected from a small neighborhood around the object
being imaged.) Desirably, the reflector/sensor should also permit
tracking of the target object as the earth rotates, although this
is not essential.
[0049] Generally speaking, no spectral filtering is performed;
every photon helps. (There are situations in which limited
filtering may be appropriate. An example is the differential phase
shifts that different wavelengths of light may experience as they
travel through different columns of the earth's atmosphere to
different of the photon collection subsystems. Such vagaries may be
reduced by filtering out certain wavelengths prior to the
sensor.)
[0050] The photon detection subsystems 12 can be sparsely arrayed
in relative random placements, or systematically arrayed, or some
combination--driven as much by the randomness of human population
center locations as anything else, with typical pair-distances
being anywhere from a few hundred meters to thousands of kilometers
or much, much larger still.
[0051] It is desirable to have extremely long record lengths in the
samplers 22, with relatively fast transfers to mass storage,
facilitating high repetition rates in the acquisition of finite
segment waveforms. In an illustrative embodiment, 5,000-10,000
samples are acquired each time a photon collection subsystem 12 is
triggered. Desirably, tens, hundreds, or thousands of such data
records are acquired within a brief interval. This interval should
generally be as short as possible, usually less than a second.
(Depending on the processing capabilities of the photon detector
subsystem, a thousand 10,000 sample records might be acquired in 50
milliseconds or less.) These extras sets of waveforms are collected
in order to increase the signal to noise ratio in the ultimate
resulting image, assuming that the total collecting time is short
enough so that the object changes inappreciably.
[0052] The data from the memories 26 is transferred to the data
processing subsystem 14 for post processing.
[0053] Data processing subsystem 14 is illustrated as including a
memory 30, a correlator 32, and a matrix decomposition element 34.
Although dedicated hardware can be used for these latter two
elements, other embodiments implement these functions in software
on a general purpose computer.
[0054] Data records from each of the memories 26 is copied into
memory 30, which is suitably large for this purpose. Memory 30
desirably includes disk storage for storing the complete set of
input data, together with a large random access memory for storing
data records currently being processed.
[0055] Correlator 32 performs standard correlation operations
between data records produced by selected pairs of photon detector
subsystems 12. Such correlation, when applied to any pair of
waveforms produced by the same optical wavefront, results in a new
waveform whose individual values each represents a total brightness
line integral through the object, where the line in question is
perpendicular the line axis drawn between the given pair of
detector subsystems. This will be more readily apparent by
reference to FIGS. 2 and 3.
[0056] FIG. 3, for example, shows the correspondence between a star
40 being imaged, and a brightness line integral 42 (not to scale)
corresponding thereto. Line integral 42 is produced by correlator
32 in response to data records produced by a pair of photon
detector subsystems whose axis is parallel to line X-X through the
star 40. Different vertical "bands" 44 are illustrated through star
40, with arrows 46 showing the portions of the line integral 42
corresponding thereto.
[0057] If star 40 were of uniform brightness across its extent,
brightness line integral 42 would be a smooth, sinusoidal function,
beginning at zero at the end of the disk, growing to a maximum
value, and then tapering off to zero at the opposite end of the
disk.
[0058] In reality, the star 40 is not of uniform brightness. The
illustrated star, for example, has dark splotches 48 (e.g.
sunspots). These irregularities in the star's brightness are
manifested as irregularities 50 in the brightness line integral
42.
[0059] It will be recognized that the positions of irregularities
50 in the brightness line integral 42 indicate one dimension of the
splotches' position on the stellar disk (i.e. the left/right
position in the figure). However, line integral 42 does not provide
any data about the splotches' position in the other dimension
(up/down). But by correlating a different pair of data records,
from a pair of detection subsystems oriented along a different
axis, a different brightness profile can be obtained, e.g. one
along line Y-Y. Similarly, yet another pairing can provide
brightness data along line Z-Z, etc.
[0060] By processing the different line integrals produced by
correlator 32, along different axes through the object being
imaged, the position of surface features on object 40 can be
determined. While two orthogonal line integrals theoretically
contain all the information needed to fully resolve the two
dimensional image brightness profile, in practice, many such
brightness line integrals are collectively processed to obtain this
data.
[0061] The collective processing of the different brightness line
integrals to yield the two dimensional brightness data is performed
by the matrix decomposition element 34. It will be recognized that
the brightness value at any position (x,y) on the object 40 can be
determined by examining the projection of point (x,y) onto each of
the different line integrals 42 (i.e. along line X-X, along line
Y-Y, along line Z-Z, etc.), and solving the associated linear
equations to determine what brightness value would yield the
observed values at each of the projected points.
[0062] As noted earlier, each time source has some inherent error
(e.g. 1-100 nanoseconds). However, when correlating thousands of
sets of data samples, a self consistency about where each waveform
exists relative to the others emerges. This self-consistency allows
the sets of data samples to be matched to within one sample period
(i.e. 1 nanosecond). The time error associated with each set of
data samples can then be determined.
[0063] The post-processing operations detailed above were expressed
in the earlier-filed provisional application as follows:
[0064] A section of the correlated waveform, generally centered in
the data record to some degree of accuracy, thus contains a set of
such (contiguous) line integral values, essentially covering the
object in total, with some overlap between line integrals related
to the sampling rates of the digitizers. Except in extremely high
resolution imaging where slight dynamic rotation of the axes are
often desired (as in very distant object imaging), the collected
set of correlation waveforms from the, e.g. one minute set, can be
added to effect a higher signal to noise ratio set of covering line
integral values. It can be appreciated that a second pair of
detectors will produce a second set of covering line integrals in
the identical fashion, with, in general, a different orientation
and spacing (spacing being a function of the lateral distance
between photodetectors and the sampling rates of the digitizers
[here assumed equal]). Likewise, a third pair and a fourth pair and
a fifth pair and so on each produce a generally unique set of line
integral values, until all combinations of pairs are exhausted.
[0065] The entire set of line integrals thus represents a rather
simple system of linear equations (with certain basic "bias"
differences compensated for) over any garden variety set of image
basis functions (such as square pixels), a system whose solution
produces an image of the object where the resolution ("pixel
spacing") of the resulting image is generally equivalent to the
breadth of a line integral provided there are sufficient numbers of
pair combinations and sufficient rotation orientation randomness to
ensure a full independent set of linear equations. For example, 50
scattered photodetectors with sufficiently large separations should
be capable of producing a 1K by 1K image.
[0066] It will be recognized that imaging system 10 provides high
resolution imagery, with extremely simple sensors. Moreover, the
data processing needed to generate the imagery is straightforward
and does not involve, e.g., the Fourier (or other domain)
transformations required in radio astronomy and the like.
[0067] Turning now to FIG. 4, a different form of spaced-sensor
astronomical imaging system is shown.
[0068] The FIG. 4 system 60 includes plural telescopes 62, each
equipped with a system to characterize the atmospheric distortion
through which the telescope is viewing. Such a system is disclosed
in the earlier-cited patents. The outputs from the telescopes 62
are combined by a central station 64 to produce the final
image.
[0069] Desirably, each of the telescopes 62 is fairly small (e.g.
less than a meter) so that the tips/tilts of received wavefronts
are not too complex. (If large reflectors are used, more complex
atmospheric distortion results.) In an exemplary system, the
reflectors are on the order of the Fried distance, r.sub.o or
less.
[0070] The telescope outputs which make up the final image are not
two-dimensional sets of image data. Rather, they are single mode
optical fibers 66 which couple light from a single point in the
telescope's focal region. The fiber end at the telescope (typically
planar) is adaptively moved within this focal region by a
three-dimensional piezo electric positioning system, in response to
real-time data about the turbulence of the atmosphere through which
the telescope is viewing (e.g. repositioned 100 times per second).
This positioning system maintains the fiber end at the point where
light from the source being imaged is focused, regardless of how
intervening atmospheric turbulence may cause this point to
shift.
[0071] More particularly, each telescope 62 includes a dichroic
mirror between the reflector and the fiber end. The mirror couples
light into a fast (e.g. 100 frame/second) camera using, for
example, a 512.times.512 pixel CCD (e.g. of the sort described in
U.S. Pat. No. 5,444,280, incorporated herein by reference). Data
from this CCD is used, in accordance with the disclosure of the
earlier-cited patents, to produce tip/tilt data for a region, e.g.,
a half degree across. (A half degree of space usually includes
sufficient bright stars to provide meaningful tip/tilt data.) This
tip/tilt data determines the movement of the fiber end.
[0072] At the central station, the other ends of these fibers are
terminated in an image plane. This collected group of fiber
terminations presents an image that can be viewed, recorded, or
processed as desired. For example, the image can be projected,
using glass lenses, onto a viewing screen. Alternatively, the image
can be sampled/recorded several times a second, and the resulting
static images combined to synthesize a more accurate image, e.g. by
a weighted average or the like. (Weighting can be advantageously
employed because the turbulence characterizing system can assess
the relative merit of each recorded frame by reference to the
turbulence at that instant. The weight given to any image frame can
be inversely proportional to the degree of turbulence under which
the image was produced.)
[0073] It is important that the optical path lengths from the
object being imaged, to the termini of the fibers at the central
station, be matched. The fiber lengths coupling the telescope feed
points to the central station can be made equal. But additional
optical delay must be provided to account for the differing path
length from the object to each of the telescopes.
[0074] This additional optical delay can be provided by various
means. One suitable arrangement is to interrupt the fiber from each
telescope and interpose a fiber optic delay line. An arrangement
much like a pair of cooperating rotary electrical switches can be
employed. In particular, the fiber from the telescope can terminate
at the periphery of a rotatable disk. Radially arrayed about this
disk are first ends of plural delay lines. The disk can be rotated
so as to couple light from the end of the telescope fiber into any
of these delay lines.
[0075] A reciprocal arrangement is used on the second part of the
interrupted fiber. That end is routed to the periphery of a second
rotatable disk, which has the second ends of the optical delay
lines radially arrayed therearound. The two disks are turned to
couple light into, and out of, the desired one of the delay lines.
(A variety of optical and mechanical considerations come into play
in the implementation of this system including, e.g., how to
utilize index matching fluids between the fiber ends when the fiber
ends are physically movable. Such details are within the
capabilities of an artisan in the optical fiber field.)
[0076] A similar arrangement can be employed in which the optical
delay lines do not terminate radially at the periphery of the disk,
but rather are normal thereto and near the edge thereof. The
interrupted telescope optical fiber can be terminated near the edge
of the disk to couple into and out of such delay lines.
[0077] In alternative embodiments, electro-optic techniques can be
used to switch the light through different delay elements. Still
further, at such time as analog optical delay devices become
commonplace (e.g. devices which control the refractive index of a
media in response to a control signal), they can be employed to
advantageous effect.
[0078] Starlight collected by the telescopes is desirably used to
aid in matching of the optical path lengths using, e.g.,
conventional Michelson interferometric techniques. Gross precision
(e.g. on the order of a millimeter or a centimeter) is achieved by
the physical components. Adjustments can then be made, e.g., by
servo mechanisms which reposition each of the telescopes 62 in
three dimensions, to fine tune match the optical path lengths.
[0079] As in the FIG. 1 embodiment, the telescopes 62 can be
arrayed in any fashion over an arbitrarily large area. (Current
single mode fiber technology presently limits the distribution of
the telescopes to a maximum distance of about 100 kilometers from
the central station 64. As improvements are made to the fibers,
further separations will become practical.)
[0080] Due to the wide distribution of the telescopes 62, the
turbulence of the atmosphere can be tomographically modeled over a
similarly wide area. This information can be advantageously used,
for example, by aviation authorities who can use it to direct
aircraft to areas of low turbulence. (Turbulence data will be
spotty at low altitudes if the telescopes are widely spaced, since
the viewing cones of each will not overlap until higher altitudes.
If such a system is employed for aviation purposes, it would be
desirable to have more closely spaced telescopes around the
periphery of the array. This would permit identification of low
altitude turbulence as it moves into (and out of) the area
monitored by the array.)
[0081] From the foregoing, it will be recognized that system 60
provides real-time imaging of astronomical objects using
inexpensive component telescopes. The angular resolution provided
by the system depends on the dimensions of the telescope array;
exceedingly high resolutions are possible.
[0082] Having described the principles of our invention with
reference to preferred embodiments and several variations thereof,
it should be apparent that the embodiments can be modified in
arrangement and detail without departing from such principles.
Accordingly, we claim as our invention all such modifications as
may come within the scope and spirit of the following claims, and
equivalents thereto.
* * * * *
References