Laser beam pointing tests for lunar communication and rangefinding and tracking using Surveyor 7 vidicon camer

Alley, C. O.

Currie, D. G.

English

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Laser beam pointing tests

Laser beam pointing tests using Surveyor vidicon camera for future point to point communication system

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C. O. Alley and D. G. Currie
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TECHNICAL REPORT #885
(This paper is included in the Surveyor VII Mission Report)
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June 1968
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UNIVERSITY OF MARYLAND
DEPARTMENT OF PHYSICS AND ASTRONOMY
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DEPARTMENT OF PHYSICS AND ASTRONOMY
UNIVERSITY OF MARYLAND
COLLEGE PARK, MARYLAND
20742
U.S.A.
.
1
LASER BEAM POINTING TESTS
C. 0. Alley and D. G. Currie
Technical Report #885
(This paper is included in the Surveyor VII
Mission Report)
June 1968
Financial support for this work has come from NASA Grant NGR 21-002-109.
UNIVERSITY OF MARYLAND
Department of Physics and Astronomy
College Park, Maryland
TABLE OF CONTENTS
Introduction
A.	 Laser Transmitting Stations
1. Lunar Schedule of Tests
2. Detection of Laser Beams
B.	 Conclusions
Acknowledgements
References
Figure 1	 Simulation of the Earth as Seen from
Surveyor VII with Stations Located
Figure 2	 The Earth as Seen from Surveyor VII During Tests
LASER BEAM POINTING TESTS
C. 0. Alley and D. G. Currie
An opportunity to verify the ability of earth stations for direct-
ing very narrow laser beams to a specific location on the lunar surface
was provided by the detection sensitivity of the Surveyor VII vidicon
camera operating in its integration mode. Such tests were of interest
primarily because of a planned Apollo lunar surface experiment in which
an astronaut will emplace a corner reflector array to provide a fixed
point for very precise laser ranging. The successful monitoring of
point-to-point earth-moon distances to the expected accuracy of + 15 cm
would provide: (1) a definitive test of the conjectured slow decrease
of the gravitational constant; (2) an experimental study of whether
continental drift is occurring now; (3) new knowledge on the physical
librations, size and shape, and orbital motions of the moon; and (4) new
information on the rotation of the earth (Refs. XI-1 through XI-4). An
additional factor in testing narrow laser beam pointing and tracking tech-
niques lies in their potential use in space communications systems.
The idea of using a Surveyor TV camera for such tests occurred to
the authors during a discussion on whether an astronaut could see the
pulsed ruby laser beam planned for the Apollo laser ranging retro-reflector
experiment. Measurements on the wavelength sensitivity of the vidicon
surface were conducted in November 1967 at the Jet Propulsion Laboratory
(JPL) and indicated a decrease from the peak sensitivity by a factor 1/300
for the ruby laser wavelength of 6943 A, making detection marginal for
existing and planned ruby laser systems. However, the availability of
argon-ion lasers operating in the blue-green (main wavelengths at 4880
and 5345 A), within the peak of the vidicon sensitivity with average
power of a few watts, suggested their use for the tests. The pointing
and tracking techniques would be similar to those used with pulsed lasers.
Estimates of the power density on the moon of a 10-W (transmitted)
argon-ion laser beam contained within a divergence cone angle (half) of
10 sec of arc yielded a value 2.25 times the power density of a magnitude
0 star, or nearly magnitude -1. The power density would scale directly
as the power transmitted and inversely as the square of the beam angle.
Experinece with star observations on previous Surveyor missions (p. 15
of Ref. XI-5) indicated that the laser beams could be easily observed
if they were directed to illuminate the spacecraft. The diameter of the
illuminated area on the moon is about 2 km per arc second of divergence.
-2-
A.	 Laser Transmitting Stations
Six transmitting stations were established; each consisted of an
argon-ion laser with a suitable optical system for collimating and aiming
the laser beam. All six stations used the technique of directing the laser
beam backward through a telescope to reduce the beam divergence. However,
each station used a different method for aiming the laser beam. A brief
description of each station is given below.
(1) Kitt Peak National Observatory, Tucson, Arizona. The
McMath Solar Telescope (60 in., f/60, heliostat confi-
guration) and a 2-W laser were used. The telescope was
was used in the normal direction for aiming. The guide
beam and the laser beam were separated by a specially
constructed, divided-mirror beam splitter placed near
the telescope focal plane. A reticle, which was designed
for the purpose and which permitted offset guiding from
nearby lunar features, and a field lens were placed in
the focal plane.
(2) Table Mountain Observatory, Wrightwood, California. The
JPL 24-in telescope, utilized at its f/36 Coudt'! focus, and
a 2-W laser were used. A beam splitter with a pinhole was
placed in the telescope focal plane to separate the guide
beam from the laser beam. A 2.5 magnification microscope
with a crosshair reticle was used as a viewing eyepiece.
(3) Wesleyan University, Raytheon Research Laboratory, Waltham, Mass.
A 6-in two-mirror coelostat directed the beam from a specially
constructed 4-in, f/15 telescope toward the moon; a 60-W laser
was used. The guide beam and the laser beam were separated using
a clear pellicle beam splitter located ahead of the primary
focal plane. The use of an appropriate glass filter over the
eyepiece permitted continuous viewing of the crosshair reticle.
(4) Lincoln Laboratories, Lexington, Massachusetts. A beam from
a 3.5-W laser collimated with a 3-in. telescope was directed
using a special servo driver az-el flat mirror. Guiding was
accomplished using a second 3-in telescope, which was boresighted
to the first telescope.
(5)Goddard Space Flight Center, Greenbelt, Maryland. An existing
mobile laser satellite ranging system was used; the pulsed ruby
laser was replaced by a 10-W argon-ion laser. A series of mirrors
guided the beam along the rotation axes of the az-el mount
through a 5 1/2 in. output aperture. Viewing of the moon was
accomplished by an image orthicon television display from a bore-
sighted 16-in. telescope.
(6) Perkin-Elmer Corporation, Norwalk, Conn. A portable 2-W laser was
attached at the Cassegrain focus of a 24-in telescope. Aiming
was accomplished by the 6-in guide telescope, which was bore-
sighted to the main telescope.
-3-
To aid in locating Surveyor VII on the lunar surface, Lunar Orbiter
photographs of the region around Tycho and ACIC Lunar Chart LAC 112 were
supplied to all stations. The initial estimates of the landing coordinates,
as well as the accurate location of Surveyor VII (see Section III of this
Report), were communicated with respect to both the Lunar Orbiter photo-
graphs and the lunar chart.
1. Lunar Schedule of Tests
The heavy demands on the Surveyor camera resulted in the initial
allocation of only one 10-min block of laser observing time on each of four
different nights. By combining the laser observations with the planned earth-
light polarization observations, it was possible to increase the length of
observing periods and to have a second period on Day 020. Time windows
were chosen so that stations on both East and West Coasts could be observed
simultaneously during control of the spacecraft by the Goldstone Deep
Space Tracking Station; the primary constraint was that no station be too
close to the terminator. During the window, the laser stations were responsive
to the availability of the television camera. Communication was handled by
a telephone network connecting all stations with the JPL Space Flight Opera-
tions Facility.
The first few days after touchdown were needed for other Surveyor
activities and were used for final preparations at the stations. With the
exception of the Norwalk and Greenbelt Stations, the first test period
was held at 04:30 GMT on Day 014. It was necessary to interrupt the tests
during the period near lunar noon because of glare in the camera caused
by the proximity of the earth and sun. This time was used to modify techni-
ques at some of the stations on the basis of the first test. Test periods
were resumed on Day 019, and continued on Days 020 and 021. This last
window was chosen to maximize the probability of observing stations on the
East Coast by having them far from the terminator even though, for the West
Coast stations, it placed the moon very low in the sky.
During each test period, modes of operation for the stations were pre-
scribed with definite on and off sequences to identify stations that were
geographically close together. The aperture and exposure time were varied
to produce on the A-scope display approximately one-half the saturation voltage
level in the dark part of the earth crescent where laser beams were being
transmitted, as this maximized the sensitivity. With this setting, repeated
exposures were taken while the stations were directed to follow the above modes.
2.	 Detection of Laser Beams
Detection was achieved visually during the first observing period on
Day 020 for both Tucson and Wrightwood. Suspected laser beam spots with the
correct locations, as shown in Fig. XI-1, were observed at the JPL Space Flight
Operations Facility. Further confirmation resulted when the earth image was
shifted 3 deg within the 6.5-deg narrow field of view, the two spots shifting
-4-
with it. The on/off sequencing discussed above also served to verify the
detection of the beams. Full confirmation was obtained only with the
subsequent, detailed study of correlations in projected enlargements from
high-quality photographic negatives reproduced from the video tape recordings
by kinescope film recorder. A positive print of one of the negatives,
enhanced using a high-contrast process, is shown in Fig. XI-2. The spread
of the images over several of the video scan lines is caused by aberrations
in the optics (electron and visible) of the camera and also in the ground
reproducing equipment.
Each of the stations detected was transmitting about 1W, after telescope
and atmospheric losses. Wrightwood was systematically scanning about the
position of Surveyor VII and was limited by atmospheric "seeing" while
Tucson had deliberately spread the beam. The spots appeared with an approxi-
mate star magnitude of -1, as originally calculated. Detection of these
stations was accomplished again visually with about the same intensity during
the second run on Days 020 and 021. The approximate magnitude of the detected
beams was determined by comparing pictures of the laser beams with those of
Jupiter.
By digitization of the video pictures, it has been possible to increase
the sensitivity of detection considerably beyond the visual. It is estimated
that by stretching the digitization in regions near station locations,
intensities of laser beams directed to illuminate Surveyor VII can be detected
with 1/75 the intensity displayed by Tucson and Wrightwood. This technique
enabled easy detection of the Tucson beam on Day 019 when high winds and bad
seeing conditions had degraded the performance. (Wrightwood was not operating
on that day.)
A search for beams from the East Coast stationshas been made with the
equipment at the University of Maryland developed for visual scan of bubble
chamber pictures. No positive results were found. Examination of the stretched
digitized printouts has not given positive indication as yet, but the work is
continuing with the technique of averaging successive frames for enhancement
and looking for correlations at predicted locations. Although local weather
conditions and structural obscurations interfered with transmission from
East Coast stations (especially in the Boston area), there were periods when
contact with Surveyor VII seemed possible.
B.	 Conclusions
The primary value of these tests lies in the experience gained in a
variety of techniques for tracking and pointing laser beams,with different
types of telescopes. A report on this subject will be prepared by members of
this Surveyor Working Group.
The potential value of well collimated laser beams for space communica-
tions is emphasized by noting that the 1-W laser beams appeared as bright
stars, while the uncollimated light from major cities was not detected.
-5-
ACKNOWLEDGMENTS
We wish to thank Mr. Benjamin Milwitzky, Surveyor Program Manager, and
Mr. Steve Dwornik, Surveyor Program Scientist, for endorsing the laser pointing
test and for providing the support of the National Aeronautics and Space
Administration, which made the test possible.
Because of the short time between the initiation and the execution of
the laser pointing test, many people and organizations voluntarily partici-
pated under adverse conditions and without compensation at each of the six
stations. The essential contributions of the following persons are gratefully
acknowledged:
Tucson: Dr. James Brault, Staff Astronomer of the Kitt Peak National
Observatory, and Professor S. K. Poultney of the University
of Maryland, using a Spectra-Physics Laser loaned by the
Aerospace Corp.
Wrightwood: Mr. M. S. Shumate, JPL, and Mr. J. W. Young, Table Mountain
Observatory, using a laser constructed and loaned by Hughes
Research Laboratories.
Waltham:	 Professor J. E. Faller, Wesleyan University, using a laser
from the laboratory of Dr. George De Mars, Raytheon Research
Laboratory. Ten undergraduate students from Wesleyan
University, led by Mr. D. Brr.stein, and Mr. M. Hulett assisted
Professor Faller.
Lexington:	 Dr. Robert Kingston and Dr. Hoyt Bostick, Lincoln Laboratories,
using a laser loaned by Spacerays, Inc.
Greenbelt:	 Dr. H. H. Plotkin, Mr. H. Richard, and Mr. W. Carrion of the
Optical Systems Branch, GSFC, using an existing laser satellite
tracking system incorporating an RCA laser.
Norwalk:	 Mr. H. Wishnia and Dr. Morley Lipsett of the Perkin-Elmer
Corporation, using a Perkin-Elmer laser and Mr. R. Perkin's
telescope.
The test would not have been possible without the integration mode of
the Surveyor vidicon camera. Work that led to the incorporation of the inte-
gration mode into the camera was initiated by Mr. L. H. Alien, JPL, who also
performed the vidicon sensitivity measurements at the ruby wavelength and
assisted in the overall test operations.
Appreciation is expressed to all members of the Surveyor Project at the
Jet Propulsion Laboratory for technical help in the organization and perfor-
mance of the tests. Special appreciation is extended to the following JPL
personnel involved in the television aspects of the mission: Mr. J. Strand,
Mr. T. H. Bird, Mr. J. J. Rennilson, Mr. D. L. Smythe, and Mr. C. Chocol;
and to Dr. R. Nathan and Mr. E. T. Johnson of the JPL Image Processing Laboratory.
References
XI-1, .alley, C. 0., l;f ,nder, P. I.,, llickc, R. I I,, Faller. J. E., Frautke n, l'..1.,Plotkin,
II, II., and Wilkinson, 1). '1'.. "Optival Radar Using a Corner Ile lector tan
he Moon." 1. Geophys, Iles., Vol, so, pp. 2267 20J, M).
XI-2. Alley, C. t)., and Bender, P. L., "Information Obtainable from Laser Rangy
Measurenx-nts to it Lunar Corm-r Rc W Aector," to by published in the Proceed-
ings of the 1AU, 11IGG Symposium oil Continental Hrfft, Secular 11otitrxt of
the Pelt, andRotationof thr' Kurth (William Marko^vitt and B. Cainot,
editors), sptnpo iuut lield at Stres.t, Italy, March 21 -25, 19K.
XI-3. Alley, C, 0., Bendel, P. L., Currie, D. C., I)ickc•, ll. fl., taut Faller, J. E.,
"Same Implications for Physit:s and Gcopbysia% of Laver flange SleAstire•-
ments from Earth to a Lunar Retro-Relitctor," to lw published is the Pro-
= credinsx of the N. A. T,0. Adcaraccrl Study Iaa.stitcctr• ur► tlrc ^1) licativI trf
Modern Phyxics to tlrc • Leant and Plnrrdary Interiors (S. K. lluacant, editur).
s%mlxOsitun held at the Universiq cat Newcastle upon Tyne, Marc4t 29-April
4,1967.
XI-4. MacDonald. f., J. F., "Implications for wophysic of the Precise ^teasttrr.-
ment of the Eactb*s Rotation," Sc7 nev, Vol. 15r, pp. 2(^4-20 , J ►tIy 21, 1961,
XI• . !lr •nnil,om. J. J., Dragg. J. L., Morris, E. C., Slux-naker, E. M., and Turke icii,
A., "I. mar Surface• Topography," Surveyor 1 tlission Report. Part It: Scivaa-
tifrc-1)crtu and Rtstrlts, pp, 744, Jet Propulsion-Laboratory, Pasadena, Calif.,
September 10, low.
Fig. X1-1. This photograph of a globe, with the overexposed crescent indicated by cross-hatching, simulates sl.+
earn+ as seen from Surveyor VU at 09 :04 GMT on Day 420, The station locations are indicated by black dots, and
permit ready identification of the origin of the two laser beams in Fig. X1-2 as Table Mountain Observatory near
Los Angn !4s, California, and Kilt Peak National Observatory, near Tucson, Arizona. Simulutions similar to this
photograph were prepared in advance by J. J. Unnilson, JPL, for each period of anompted laser detrci;on.
Fig. Xi-2. Laser beams with powers of approxlmately 1 W each appear as slarilke images comparable In brightness
to Sirius (magnitude,-146 in this narrow-angle, 1/4, 3-sec exposure of the earth. The crescent of the sun-lltut"Matz=d
earth is distorted becaL7 
_ 
of overexposure. This was one of the first plctvr,;s In which the beams were reezrtily visiWir
(Day 424, about 44:06 GMT).
-6-




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