Abstract

We have built and tested a 30 × 5 cm aperture telescope which used six movable mirrors to compensate for atmospherically induced phase distortion. A feedback system adjusts the mirrors in real time to maximize the intensity of light passing through a narrow slit in the image plane. We have achieved essentially diffraction-limited performance when imaging both laser and white-light objects through 250 m of turbulent atmosphere. The behavior of our telescope was accurately predicted by computer simulations. The system has yet to achieve its full potential, but has already operated successfully for objects as dim as 5th magnitude.

© 1977 Optical Society of America

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References

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  1. R. A. Muller and A. Buffington, J. Opt. Soc. Am. 64, 1200–1210 (1974).
    [CrossRef]
  2. H. W. Babcock, Publ. Astron. Soc. Pac. 65, 229 (1953); H. W. Babcock, J. Opt. Soc. Am. 48, 500 (1958).
    [CrossRef]
  3. Partial descriptions of these systems may be found in the Digests of Technical Papers for the Topical Meeting on Optical Propagation Turbulence, July 9–11, 1974, Boulder, Colorado, and also for the Topical Meeting on Imaging in Astronomy, June 18–21, 1975, Cambridge, Massachusetts, both sponsored by the Optical Society of America. Additional details (including some further details of this work) will be presented at the S. P. I. E./S. P. S. E. Technical Symposium East, Vol. 75, March 22–23, 1976.
  4. F. J. Dyson, J. Opt. Soc. Am. 65, 551 (1975).
    [CrossRef]
  5. J. L. Gumnick and C. D. Hollish, IEEE Trans. Nucl. Sci. 13, 72 (1966).
    [CrossRef]

1975 (1)

1974 (1)

1966 (1)

J. L. Gumnick and C. D. Hollish, IEEE Trans. Nucl. Sci. 13, 72 (1966).
[CrossRef]

1953 (1)

H. W. Babcock, Publ. Astron. Soc. Pac. 65, 229 (1953); H. W. Babcock, J. Opt. Soc. Am. 48, 500 (1958).
[CrossRef]

Babcock, H. W.

H. W. Babcock, Publ. Astron. Soc. Pac. 65, 229 (1953); H. W. Babcock, J. Opt. Soc. Am. 48, 500 (1958).
[CrossRef]

Buffington, A.

Dyson, F. J.

Gumnick, J. L.

J. L. Gumnick and C. D. Hollish, IEEE Trans. Nucl. Sci. 13, 72 (1966).
[CrossRef]

Hollish, C. D.

J. L. Gumnick and C. D. Hollish, IEEE Trans. Nucl. Sci. 13, 72 (1966).
[CrossRef]

Muller, R. A.

IEEE Trans. Nucl. Sci. (1)

J. L. Gumnick and C. D. Hollish, IEEE Trans. Nucl. Sci. 13, 72 (1966).
[CrossRef]

J. Opt. Soc. Am. (2)

Publ. Astron. Soc. Pac. (1)

H. W. Babcock, Publ. Astron. Soc. Pac. 65, 229 (1953); H. W. Babcock, J. Opt. Soc. Am. 48, 500 (1958).
[CrossRef]

Other (1)

Partial descriptions of these systems may be found in the Digests of Technical Papers for the Topical Meeting on Optical Propagation Turbulence, July 9–11, 1974, Boulder, Colorado, and also for the Topical Meeting on Imaging in Astronomy, June 18–21, 1975, Cambridge, Massachusetts, both sponsored by the Optical Society of America. Additional details (including some further details of this work) will be presented at the S. P. I. E./S. P. S. E. Technical Symposium East, Vol. 75, March 22–23, 1976.

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Figures (9)

FIG. 1
FIG. 1

Mechanical layout of the apparatus. Incident light passes through the entrance aperture, is focused by the primary mirror, reflects off the first and second diagonal mirrors, and comes to an image just in front of the projection lens. The first diagonal is mounted on piezoelectric columns which allow steering of the image in the dimension of good resolution. The second diagonal is made of six independent mirrors which can be moved perpendicular to the mirror plane. The projection lens refocuses the image onto the slits in front of the photomultipliers PM-1 and PM-2. PM-1 measures the sharpness and is used to provide feedback to the six mirrors of the second diagonal. PM-2 and its slit are mechanically translated perpendicular to the beam lines and are used to record the image. PM-3 and PM-4 receive the light which does not pass through the slit for PM-1, and they are used to steer the first diagonal mirror to keep the image centered upon the sharpness slit. The telescope mount (not shown) allows rotation of the entire telescope about the axis of its barrel, thus permitting reorientation of the direction of good resolution.

FIG. 2
FIG. 2

Design for the flexible mirror. Mirror plane angles are adjusted by the set-screws, while positions are adjusted by bias voltages placed on the piezoelectric crystal. Angles and mean mirror positions are fixed in the initial calibrations of the mirror, while rapid perturbations in mirror position about the mean perform the active wave-front phase correction.

FIG. 3
FIG. 3

Photograph of the flexible mirror. Mirrors, piezoelectric columns, and mounting base are visible, as are the access holes for half of the 18 angle adjustment set-screws. The electrical connections to the mirror take place through the circuit-board strip just visible below the main mirror mounting base.

FIG. 4
FIG. 4

Mounting of photomultipliers behind slits. The incident light is introduced into the face of the photomultiplier through a prism glued to the face. The light becomes trapped within the face, undergoing typically five reflective encounters with the photocathode before reaching the opposite side of the tube. The slit is made by evaporating aluminum onto a glass microscope slide except in a narrow line which was covered during the evaporation process.

FIG. 5
FIG. 5

Block diagram of the electronics. The incident light reflects off the first and second diagonal mirrors and then proceeds to the four photomultipliers. The sharpness slit with its photomultiplier PM-1 defines the desired image location and size. The beam splitter sends 20% of the light to the scanning photomultiplier PM-2 and its slit. These can be moved mechanically across the image plane to record the image. The potentiometer P encodes the position of the slit, and the output is integrated with typically several seconds integration time by the capacitor C. The X-Y plotter graphs the data. The steering photomultipliers PM-3 and PM-4 detect the light not passing through the sharpness slit and adjust the angle of the first diagonal mirror to center the image on the slit.

FIG. 6
FIG. 6

Images of laser light viewed through 250 m of turbulent atmosphere. The agreement between corrected image and Monte Carlo calculation is good, and the corrected central diffraction peak is nearly a decade improved over the uncorrected image. The curves are normalized to the same area.

FIG. 7
FIG. 7

Determination of dimmest correctable objects. The conditions are similar to those of Fig. 6, except neutral-density filters have been placed in the optical path to reduce the amount of light. The image quality in (b) is nearly as good as that of Fig. 6 even though the amount of light has been reduced by 100 times. However, the image quality degraded when the light was reduced in (c) to a level close to N2 = 36(N being the number of movable mirrors) where theory predicts that degradation should set in. Curve (b) has a somewhat smaller normalization than curves (a) and (c).

FIG. 8
FIG. 8

Measurements of the autocorrelation function A(τ). These curves show the characteristic coherence time for speckles from the uncorrected laser image drifting and changing at the sharpness slit. The curves chosen represent a typical range of seeing experienced at our laboratory, with a high-quality window installed to isolate the work area from the outside. These curves illustrate the wide range of conditions that can occur within a relatively short time period.

FIG. 9
FIG. 9

White-light images. An incandescent bulb with a 1 mm filament was placed 250 m from the telescope. In (a) and (b) a narrow mask was placed in front of the bulb to make an unresolvable object. In (c) the slit was removed to make a resolvable object with the expected FWHM as shown.

Tables (1)

Tables Icon

TABLE I Program logic for the flexible telescope system.

Equations (4)

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S 1 = d x d y I 2 ( x , y )
S 2 = d x d y I ( x , y ) M ( x , y ) ,
B = ( 4 × 10 6 ) 10 - m / 2.5 photons / cm 2 s ,
A ( τ ) = 0 T d t I ( t ) I ( t + τ ) ,