Optical reception antennas used at a small Sun-Earth-probe angle (small solar elongation E) require sunshading to prevent intolerable scattering of light from the surface of the primary mirror. An integral sunshade consisting of hexagonal tubes aligned with the segmentation of a large mirror was proposed for use down to E = 12 degrees. For smaller angles, asterisk-shaped vanes inserted into the length of the hexagonal tubes would allow operation down to about 6 degrees with a fixed obscuration of 3.6 percent. Two alternative methods are investigated to extend the usefulness of the integral sunshade to smaller angles by adding either variable-area shutters to block the tube corners that admit off-axis sunlight or by inserting slats (partial vanes) down the full length of some tubes. Slats are effective for most operations down to 6 degrees, and obscure only 1.2 percent. For E between 10.75 and 12 degrees, shutters cause even less obscuration

Devore, C. W.

Kerr, E. L.

English

NASA Technical Reports Server

N09 - 2 0 3 4  -.
TDA Progress Report 42-95 July-September 1988 
Shutters and Slats for the Integral Sunshade of an 
Optical Reception Antenna 
E.  L. Kerr and C. W. DeVore 
Communications Systems Research Section 
Optical reception antennas used at  a small Sun-Earth-probe angle (small solar elonga- 
tion E)  require sunshading to prevent intolerable scattering of light from the surface of 
the primary mirror. An integral sunshade consisting of hexagonal tubes aligned with the 
segmentation of a large mirror has been proposed for use down to E = 12 degrees. For 
smaller angles, asterisk-shaped vanes inserted into the length of the hexagonal tubes 
would allow operation down to about 6 degrees with a fixed obscuration of 3.6 percent. 
Here we investigate two alternative methods of extending the usefulness of the integral 
sunshade to smaller angles by adding either variable-area shutters to block the tube cor- 
ners that admit off-axis sunlight or by inserting slats (partial vanes) down the full length 
of some tubes. Slats are effective for most operations down to 6 degrees, and obscure 
only 1.2 percent. For E between 10.75 and 12 degrees, shutters cause even less obscuration. 
1. Introduction 
Deep-space-to-earth optical communication will require the 
development of a large-aperture ground-based reception an- 
tenna. Such an antenna, SORDA, is described in [ 11. To re- 
ceive data from a deep-space probe during the daylight hours, 
it is essential to shade the antenna primary mirror from all 
sunlight. Various sunshade designs have been considered. The 
best so far, the integral sunshade, is described in [ 2 ] .  The inte- 
gral sunshade segments the aperture with a bundle of long 
hexagonal tubes. During solar conjunction, when the space 
probe appears to be close to  the Sun, the integral sunshade 
blocks sunlight incidence on the primary mirror as long as the 
angle E seen from the Earth between the Sun and the probe (the 
SEP angle or the solar elongation) is greater than 12 degrees. 
When the solar elongation is less than 12 degrees, the inte- 
gral sunshade would admit “chinks” or oddly-shaped patches 
I 
of sunlight on the primary mirror, as in Fig. l(a). The chinks 
would grow as the solar elongation was reduced; see Fig. I(b). 
In [ 2 ]  it was proposed that vanes (six flat plates arranged in 
cross section like an asterisk) be inserted in the hexagonal 
tubes of the sunshade to  permit operation within 6 degrees of 
the Sun. The finite thickness of the vanes would obstruct the 
antenna aperture (that is, reduce its effective collecting area). 
If the width across the flat sides of the hexagonal tubes is 
1.1 1 m and the effective vane thickness is 1 cm, the area re- 
duction is 3.6 percent. 
Herein, we compare two modified approaches to  the sun- 
shading problem. Since the chinks begin from sunlight leakage 
through the extreme corners of the hexagonal tubes, the cor- 
ners may be blocked with appropriately shaped partial shut- 
ters, whose area can be increased as the solar elongation is 
reduced. These shutters would also obstruct the aperture, but 
196 
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at first not as much as the vanes would. The distribution of 
sizes and the additional obstruction caused by the partial 
shutters are analyzed in this article. Alternatively, one may 
insert a slat down the length of each tube as the decreasing 
solar elongation exposes it to  sunlight penetration. We will 
compare the effectiveness of the shutters to  the slats. 
II. Review of the Integral Sunshade 
Sunshades used with small antennas are usually an exten- 
sion of the primary lightshade between the objective and eye- 
piece, or between the secondary and primary mirrors. The 
closer the antenna is pointed toward the Sun, the longer the 
sunshade must be. End-mounted sunshades become impracti- 
cal for large antennas. For instance, at E = 12 degrees, the sun- 
shade length must be five times the aperture diameter. In one 
previous concept, the sunshade was to  be mounted on the 
exterior of the antenna dome. Later it was observed that, by 
segmenting the sunshade in a hexagonal pattern similar to  that 
proposed for the primary mirror, the sunshade may be short- 
ened enough to  mount on a fast-primary antenna and fit al- 
most within the spherical volume swept out by the motion of 
the primary mirror itself. This configuration saves cost by 
minimizing the dome enlargement required to  accommodate 
the sunshade. It eliminates the problem of having to  remove 
and secure the sunshade during times of high winds. The inte- 
gral sunshade can also provide a strong, rigid support structure 
for the secondary reflector, instead of the usual spider. 
111. Review of Solar Conjunctions 
Most space probes, particularly those on missions to the 
planets, appear to  approach and recede from the Sun because 
of the orbiting of the Earth and of the space probe. The times 
when this happens are called epochs of solar conjunction. The 
chief reason for tracking a probe near the Sun is that the 
probe may be near a planet that has periodic solar conjunc- 
tions. An outer planet will appear to  approach the Sun from 
the east, pass close to  it above or below or perhaps exactly 
behind it, and then recede to  the west, over a period of several 
days. The path above, behind, or below the Sun depends on 
the inclination and orientation of the planet orbit relative to  
the Earth orbit. During the days of closest approach, or when 
the planet moves directly behind the Sun, there will be a 
communications outage. The duration of the outage may be 
reduced by reducing the minimum solar elongation at which 
optical communication is possible. 
IV. Design Complications 
Fitting the upper part of the sunshade to  the swing sphere 
leads to some complications, which have been turned into 
opportunities for design economies. Because of the variations 
in the lengths of the tubes, the shading characteristics are 
not uniform. In general, the tubes nearest the Sun will begin 
to  admit light at small elongation angles before the tubes 
farther away. This means that small shutters can be added on 
the corners of some tubes while others are left open, with 
little overall obstruction of the collecting area. 
The hexagonal tubes extend at one end as close as possible 
to  the envelope of the focused beam from the primary mirror, The exact shape and placement of the shutters, and the 
and t o  the reception station dome at the other end. If employed sizing of each one, depend not only on the solar elongation, 
with a segmented primary mirror, the tubes would be aligned but also on the direction of the line seen in the sky from the 
with the segmentation lines to  minimize obscuration. The probe to  the Sun. The line will appear to rotate slowly rela- 
integral sunshade is short enough t o  fit within a dome whose tive to  the segmentation pattern as the antenna moves in 
diameter is 6/5 the diameter of the primary mirror. azimuth and elevation to  track the probe over a maximum of 
about ten hours of observing, from some minimum elevation 
angle at rising to the same angle at setting. The shutters would 
also have to  move and change in size appropriately, or else 
be made a little larger than the absolute minimum necessary. 
The geometry of this problem is very complicated and still 
under study. Further investigation will also determine the 
exact conditions under which a slat will be effective in block- 
ing solar penetration. 
The current design calls for a unit composed of sixty-one 
tubes. The width across the flats of each tube is 1.1 1 meters. 
The integral sunshade has the appearance of a honeycomb, 
concave in the shape of a pyramid on the bottom, and convex 
in the shape of a sphere on the top. The sunshade and antenna 
are supported at elevation pivot points on opposite sides of the 
sunshade by a yoke whose base may be turned in azimuth. 
Within each tube, along the sides, are ring baffles to  intercept 
unabsorbed glare light reflected from the side of the tube at 
grazing incidence, and to  redirect it back out the top of the 
tube. The ring baffles make the effective wall thickness of 
the integral sunshade about equal to 1 centimeter. The in- 
tegral sunshade excludes sunlight from the primary mirror ex- 
cept when the solar elongation is less than 12 degrees. 
Operations and usage will affect the detailed design and 
implementation of the shutters. During any given day, the 
shutters may remain about the same size, but their shape and 
placement depend on the path taken by the planet during 
conjunction. The number of slats required would also be 
constant for a day, but some of them might have t o  be removed 
197 
and turned during a day (or else two slats in the form of an 
“X” would be required.) After the time of closest approach 
during the conjunction, the shutters or slats must all be 
switched to  the other side of the antenna. The switching could 
be effected most easily by making the range of the elevation 
axis a full 180 degrees, and by rotating 180 degrees in azimuth 
also. However, the difficulties of mounting the mirror seg- 
ments to  maintain their alignment during a reversal of the 
gravity vector would probably limit the elevation range to  
about 110 degrees. 
I 
V. Methods of Reducing the Minimum Solar 
Elongation for Optical Reception 
The first alternative would be to  block a corner or side of 
each vulnerable tube at the outer end. A study of this ap- 
proach has been undertaken. In the study, the integral sun- 
shade structural plates were taken to be infinitesimally thin. 
The I-centimeter thickness added by the ring baffles would 
allow operation about 0.5 degree closer to  the Sun. Thus far, 
only the worst case has been analyzed. The Sun will penetrate 
the shade most easily if the Sun is shining at an angle across 
opposite vertices of the hexagons. It will begin doing so when 
E is less than 12 degrees. (In the best case, when the Sun is 
shining perpendicularly to  opposite edges of the hexagons, 
sunlight penetration does not begin until E becomes less than 
about 10.5 degrees.) Since different tubes would require 
shade in each of the tubes would vary. Figure 2 shows the 
sizes and shapes of the shutters in the case where E = 9 degrees 
with an attendant loss of 3.6 percent of the collection area. 
The case where E = 6.5 degrees is illustrated in Fig. 3 .  Here 
the loss of signal would be 26.7 percent. The amount of the 
remaining collecting area has also been calculated and graphed 
in Fig. 4. The collecting area drops off to 73.3 percent at an 
elongation angle of 7.049 degrees. (Elongation angles were 
calculated and shown in Fig. 4 for the infinitesimally thin- 
walled sunshade. Elsewhere in this report they have been esti- 
mated and reported to  be about 0.5 degrees smaller because 
the ring baffles will be used.) Further study is necessary to 
analyze the problem of shading as the angle of the Sun in 
relation to the target changes. It appears, however, that only 
a small fraction of additional obscuration is required to permit 
I different amounts of extra shading, the size of the added 
uninterrupted observation for as long as ten hours, while the 
Sun angles change from early morning to late afternoon. 
A second alternative calls for inserting a slat into each tube 
as shading is required. This will cause a 1.2-percent loss of 
signal to the corresponding mirror segment. As the solar 
elongation decreases from 12 degrees, the total receiver area 
loss for the telescope rises from 0 to 1.2 percent. This loss is 
much more acceptable than the larger losses mentioned earlier 
for shutters. However, slats are much larger than shutters and 
potentially more difficult to install or implement for auto- 
matic placement. They require reorientation (just as the shut- 
ters do) as the Sun angle changes. 
As illustrated in Fig. 5,  there is a range of angles from 10.75 
to 12 degrees for which the shutters give slightly less obscura- 
tion than the slats. As the elongation diminishes from 10.75 
degrees, the performance of the slats relative to the shutters 
increases dramatically. 
VI. Conclusion 
As currently conceived, without any additional shading, the 
integral sunshade will block unwanted solar interference for 
any solar elongation down to 12 degrees. By attachingvariable- 
area partial shutters at the ends of some of the tubes, it will be 
possible to continue to receive optical communication from a 
space probe with a loss in signal power varying from 0 to 3.6 
percent as the elongation is reduced from 12 degrees to 9 de- 
grees. Slats inserted across the corners and along the length of 
the hexagonal tubes would cause an overall signal loss vary- 
ing from 0 to 1.2 percent as the solar elongation varies from 
12 degrees to 8 degrees. The signal loss with shutters is slightly 
less than the loss with slats for elongations from 12 degrees 
down to 10.5 degrees; for smaller angles the loss is decidedly 
greater. If the optical communications system performance 
is such that a greater signal loss can be accepted, it may be 
more convenient to use shutters instead of vanes even at 
elongations as small as 6.5 degrees. The shutters should be 
much easier to fabricate than the slats, and it should be much 
easier to actuate the shutters than to install the slats when 
needed. Both shutters and slats should be considered during 
the design of the optical reception antenna. 
198 
References 
[ l ]  E. L. Kerr, “Strawman Optical Reception Development Antenna (SORDA),” TDA 
Progress Report 42-93, Jet Propulsion Laboratory, Pasadena, California, pp. 97-1 10, 
May 15,1988. 
[ 2 ]  E. L. Kerr, “An Integral Sunshade for Optical Reception Antennas,” TDA Progress 
Report 42-95, this issue. 
199 
ORIGINAL PAGE 
BLACK AND WHITE PHOTOGRAPH 
Fig. 1. A model of the integral sunshade. The primary mirror is to 
be mounted on the back, facing forward. (a) Light incident at this 
small angle relative to the sunshade axis is admitted by the tubes on 
the upper right that show patches of the background. (b) At this 
smaller angle more tubes show the background and admit more 
light. 
Fig. 2. Partial shutters as viewed along the length of the integral 
sunshade tubes, when the projected Sun vector (arrow) is directed 
parallel to the line joining the hexagon corners from lower left to 
upper right. This figure shows shading necessary for E = 9 degrees. 
200 
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a 
a 
0 
2 
," 
W 
a: 
J 0.995 
9 
t- 
Fig. 3. Partial shutters when sunlight is incident along the same 
line as in Fig. 2. Here we see the shading necessary for E = 6.5 
degrees, an extreme case in which some shutters block a third of the 
tube area. 
0.990 
0.985 
201 
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Shutters and slats for the integral sunshade of an optical reception antenna

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