Download - OPERATING EXPERIENCE AT THE NATIONAL RADIO ASTRONOMY OBSERVATORY

OPERATING EXPERIENCE AT THE NATIONAL RADIO ASTRONOMY OBSERVATORY *

John W. Findlay National Radio Astronomy Observatory, Green Bank, West Virginia

The Telescopes at the NRAO

The National Radio Astronomy Observatory is located in a fairly flat valley near the village of Green Bank, West Virginia. The site covers about 2,700 acres of land at an elevation of about 2,700 feet. The surrounding mountains rise to about 4,500 feet to the east, west and north, and are somewhat lower to the south. The mountain crests are at elevation angles of 2 degrees to 4 degrees as viewed from the site, and thus, although they do not interfere with the radio observations, they provide an excellent screen against radio interference from distant transmitters. FIGURE 1 shows a view of the site from a nearby mountain. The center of the site is at the following geographical position.

+79" 50.2' Longitude

4-38' 26' 17" Latitude

Since the work of building the observatory began in 1957, several parabolic reflector antennas have been completed. TABLE 1 shows the presently available antennas at the NRAO.

From the antennas listed in this table, we will select the 85-foot and the 300- foot as the large instruments for which good operational experience has been obtained.

The 85-foot Polar Mounted Antenna

This instrument was completed in the early part of 1959, and has performed very satisfactorily since that time. A brief specification of the instrument follows:

Dish diameter: Focal length:

Surface: Sky coverage:

Drive system:

Indicator system:

Constructed by:

85 feet 36 feet ( f /D = 0.424) Aluminum sheet panels 6h east to 6h west in hour angle, except as limited by the horizon. Declination motion is from the North Pole to within 2' of the south horizon, i.e., f90 ' to -50". Electric motor drive in slew, scan, and track about the polar axis; slew and scan about the declination axis. A geared synchro system, reading to 30" of arc is gen- erally used. A high precision (3" of arc) inductosyn system is available for special work. Blaw-Knox Equipment Division, Pittsburgh, Pennsylva- nia.

Operated by the Associated Universities, Inc., under contract with the National Science Foundation.

25

26 Annals New York Academy of Sciences

x e 8 m E b 0

0

2

Findlay: NRAO Operating Experience 27 TABLE 1

ANTENNAS AT THE NRAO

5 P e of receivers

Antenna, type, size and steering

equatorial mount Paraboloid 25.9M

Paraboloid 42.7M equatorial mount under construction

Beam scan Operating

frequencies Type of

observations

HA 2 6 hrs. Dec. -52"

to +90°

Planetary, galactic and extragalactic sources

Planetary, gaalctic and extragalactic sources

400 Mc/s 8,000 Mc/s

up to 10 kMc/s

TWT, parametric, crystal mixers, Masers

- ~

Dee. -50° to +90°

HA 2 6 hrs.

Paraboloid 92M meridian transit

1 6 0 " zenith distance

2 9 0 ° zenith distance

Planetary, gaalctic and extragalactic sources

sources Selected radio

Up to 1,427 Mc/s

750 and 1,400 Mc/s

Crystal mixer and parametric

Paraboloid 12.2M Crystal mixer

Paraboloid 9.2M alt-azimuth

Standard gain horn 36.5M long, collecting area 10 sq. m.

Paraboloid 3.7M

All sky

Fixed Cas A trans-

its only

Interference monitor

Cas A flux

Up to 3,000 Mc/s

600 Mc/s- 1,450 Mc/s

8,000 Mc/s

Crystal mixer

Crystal mixer

Fixed Atmospheric radiation

TWT

Paraboloid 6.1M Fixed Atmospheric radiation

5,000 Mc/s TWT

Paraboloid 1.5M ~ HA f 6 hrs. Dec. -52"

to +90°

Planetary, galactic, extragalactic, and atmospheric radiation

250 Mc/s GE bolometer

Service conditions: ( a ) Ambient temperature. Operating - 10'F. to 105'F. Nonoperating - 30'F. to 4- 120'F. ( b ) Wind. Survival -reflector in any position-87 m.p.h. Survival-reflector stowed-100 m.p.h. ( c ) Ice. To withstand a one inch ice load together with a 60 m.p.h. wind with the reflector stowed. ( d ) Snow. No specification in view of the wind and ice loads already specified.

The general design of the antenna is shown by FIGURE 2. The polar motion is provided through the polar gear, 48 feet in diameter, and the declination motion through a gear 20 feet in diameter. The drive system permits the telescope to be moved fairly rapidly (slewed) in hour angle and declination at 20' per minute to move it from one observing position to another. The fine setting of the telescope is controlled by drives with speeds variable up to 8 O per minute, and these same drives are used to scan the telescope across the small area of sky being examined. The telescope may also be moved from east to west hour angles by constant speed drives at either the solar or the sidereal rate. Thus, at the sidereal rate, for example, the telescope remains pointed always at the same area of sky.


FIGURE 2. The Howard E. Tatel 85-foot radio telescope at the NRAO.

Observations with such a telescope are often made by keeping the telescope fixed in hour angle and declination, and allowing the radio source to move through the beam. Sky areas may be observed i n this way also, or by moving the telescope at the sidereal rate in hour angle, but scanning in declination at a uniform speed. The rate at which the telescope moves during an observation must be chosen to allow of adequate integration time being used in the receiver to achieve the sensitivity that the observations require. For observations of very

Findlay: NRAO Operating Experience

Toward zenith 3.16 mm.

Toward horizon -I 5.71 mm.

29

35.883 2.006 feet

35.748 2.0015 feet

TABLE 2 THE SURFACE OF THE 85-FOOT TELESCOPE

I RMS departure from Position of reflector best fit paraboloid 1 best fit paraboloid

Focal length of

~ -~ -

Greatest departure RMS departure Position of dish, from best fit from best fit zenith distance paraboloid paraboloid

3.3 cm. 1.07 crn.

30" 2.8 cm. 1.27 cm.

51" 24' 3.4 c m . 1 ~~~ ~ 0.95 crn.

0"

.-

Focal length of best fit paraboloid

128.145 k.012 feet

127.980 t . 0 1 1 feet

127.782 &.009 feet

~~

_____

weak sources, long integration times of a minute or so are used, and for these the telescope is either set to track the source or to move at a rate very nearly the sidereal rate so that the apparent drift of the source through the beam is slow.

The 85-foot telescope was designed for use at frequencies as high as 10 kMc, and very many observations have been made with it at a frequency of 8 kMc (3.75 cm.). The surface of the reflector was carefully set to approximate as closely as possible to its correct parabolic shape when the dish was pointing toward the zenith. This setting was made by using measurements of the surface position by an accurate theodolite and a surveyor's steel tape, in a way very essentially similar to that described later for the 300-foot telescope. A check of the accuracy of the surface setting, of the stability of this setting with time, and of the way the surface deflects as the dish is tilted, is provided in FIGURE 3. This shows the contours of equal deviation of the surface from a best-fit parabolic surface. They were obtained from a photogrammetric survey* of the telescope surface made in late 1962. The surface panels had previously been adjusted by the tape and theodolite method in 1960. These photogrammetric results are summarized in TABLE 2.

The 300-foot Transit Telescope This instrument was designed and constructed in a short time for a rather

modest cost in order to fill some definite needs in radio astronomical research. It is described more fully by Findlay (1963). The following is a brief specification of the instrument.

Dish diameter: 300 feet Focal length:

Surface: 128.5 feet ( f /D = 0.428) Expanded aluminum mesh (Squarex) 0.625" X 0.091''

From the North Pole through the zenith to 60" south of the zenith. (Declination range 4- 90' to - 2 1 O . )

Mount: Meridian transit Sky coverage:


FIGURE 30.

Photogrammetric measurements of the surface of the NRAO 85-foot telescope. The contour interval is 0.005 feet or 1:52 mm. ( a ) The dish pointing to the zenith; ( b ) the dish pointing to the horiion.

FIGURE 3.

Drive system: Two speed electric drive through quadruple roller chain giving positioning speeds of 10" per minute and 2.5" per minute.

A digital encoder disk giving a 10 second of arc digit interval operating a declination display and providing also printed and punched tape outputs.

Robert D. Hall and Edgar R. Faelten

Service conditions: ( a ) Amhient temperature. Operating O'F. to 105°F. Nonoperating - 20'F. to f 120°F. ( b ) Wind. Survival-reflector stowed - 65 m.p.h. Re- flector capable of being moved at winds up to 45 m.p.h.

Indicator system:

Designers:


Y

t 31

---X

FIGURE 36. FEED SUPPORT

Observations possible up to 25 m.p.h. Full accuracy of instrument achieved up to 15 m.p.h. (c) Snow and ice loads. The reflector surface should withstand a uniform load of 10 lbs. per square foot due to snow or ice in the stowed position.

The astronomical problems required that the telescope perform well at wave lengths of 21 cm. and longer, and it was realized that it would be difficult and costly to achieve an accuracy compatible with this in the whole steel structure. The main structure was therefore designed to be fabricated and erected to the accuracies normally achieved in standard engineering practice, and the surface was designed so that it could be fitted to the required accuracy onto the steel supporting members. To reduce the cost and size of the project, it was also agreed to accept the limitations of a transit mount, the fact that precise operation would only be possible in winds up to 15 miles per hour and that the 30' of sky above the southern horizon would not be observed by the telescope. The instru-


FIGU

RE

4.

The

300

-fOot

tran

sit te

lesc

ope a

t the

NR

AO

.


i 33

‘X

FIGURE 5a.

Photogrammetric measurements of the surface of the 300-foot transit telescope. The contour interval is 0.2 feet or 6.1 mm. (a) dish at the zenith; ( b ) dish at a zenith angle of 30°; (c) dish at a zenith angle of 51 24’.

FIGURE 5 .

ment is shown in FIGURE 4. It was completed and started operation 23 months after the beginning of the design, and by now has been used for almost exactly one year.

The first setting of the surface was based on tape and transit observations, but a photogrammetric survey* was carried out at the end of 1962. The results of this survey, in the form of contours of equal deviation of the surface from a best-fit parabolic surface, are given in FIGURE 5, and the results are summarized in

Operating Experience with the Two Telescopes

The experience of time lost on the two telescopes due to various causes is summarized in TABLE 4. This experience is given for three “summer” months

TABLE 3.

The survey was made by D. Brown Associates, Inc., Eau Gallie, Florida.

34 Annals New York Academy of Sciences V

i

- -x

FIGURE Sh.

(April, May and June) and for three “winter” months (January, February and March). I t will be seen that for both telescopes the observing time lost due to weather limitations is of the same order as that lost due to equipment difficulties. In fact, in the particular example chosen for the 85-foot, winter quarter equipment failures were considerably greater than normal, since this period coincided with the use of a radiometer with a high rate of unreliability in a particular type of traveling wave tube.

The experience indicated in TABLE 4 is fairly typical. A well scheduled, large radio telescope, such as the 300-foot, used for scientific research should be employed for a large fraction of thc available time. The hours scheduled for the telescopes represent about 85 per cent of the total available time (24 hours a day, seven days a week) during three months. A telescope is regarded as scheduled when an observing program, for which the electronic equipment is installed and the telescope is tested and calibrated, is given to the staff operating the telescope to conduct the observations.

The type of operating experience shown in the TABLE also seems to justify the

Findlay: NRAO Operating Experience 35

use of radio telescopes for radio astronomy in which the instruments are designed to give their best performance only in a rather gentle meteorological environment. This, of course, is only possible when the type of observation permits some deIay in achieving the results required. Programs which require continuous observation of a radio source, such as of the sun or of a flare star, in order to monitor short term sporadic effects would be hampered by the use of instruments with relatively low ability to operate in a rigorous meteorological environment.

The Radio Properties of the Telescopes

One aspect only of the radio properties of the telescopes will be discussed, the relationship between the measured gain of the antennas and the measured accuracy of their surfaces. The simplest theory of this effect has already been described in an earlier paper on Radio Astronomy. Measurements of the aperture efficiency of the 85-foot and the 300-foot radio telescopes have been made by F. D. Drake, C. M. Wade, and P. G. Mezger, and their results are summarized in TABLE 5. This TABLE shows how the measured aperture efficiency of the antennas

36

85-Foot Telescope

Summer Winter quarter quarter

Annals New York Academy of Sciences TABLE 4

3M-Foot Telescope

Summer Winter quarter quarter

Balance of hours in quarter, some unscheduled, remainder used on equipment installation, calibration and maintenance

Hoursscheduled during quarter I 1,122 I 1,871 I 1,904 I 1,880

1,062

~

Scheduled hours lost due to 27 126 17 25 1 1,122 1 1,871 I 1,904 I 1,880 weather = 2.4% of = 6.7% of = 0.9% of = 1.3% of

Telescope

85-f00t

3M)-fOOt

Scheduled hours lost due to 40 278 34 24 I 1,122 1 1,871 I 1,904 1 1,880 equipment and power failure = 3.6% of = 14.9% of = 1.8% of = 1.3% of

-___ _ _ ~

Measured I Calculated Weighted RMS aperture a ecture surface errors Frequency efficiency egwency

0.275 cm. 1,420 Mc/s 56% 5 7 %

3,000 ML/S 52% 52%

5,000 Mc/s 45 % 42%

7,600 Mc/s 32% 2896

1.20 cm. 750 Mc/s 59% 58 %

41% 1,400 Mc/s 1 40%

- _ _ ~

______-

~

_ _ ~ _ - ~~ ~~

Scheduled hours lost due to 5 2,6 12 0 I 1,122 1 1,871 I 1,904 I 1,880 radio interference = 0.4% of = 1.4% of = 0.6% of z 0% of

varies with frequency, and compares the measured efficiency with that calculated from the theory of Ruze ( 1952). Mezger, in work shortly to be published, has modified this theory to take into account the variation of the primary feed illumination over the surface of the dish. This can be done if the feed pattern is known by using a weighted value of the measured RMS surface deviations, and it is such a weighted value which appears in the second column of TABLE 5.

The good agreement between the measured and calculated values for the aperture efficiency shown in TABLE 5 shows that a simple theory of the effect of phase errors can adequately predict the performance of a parabolic dish insofar as its aperture efficiency is concerned.

Acknowledgment

I am grateful to Dr. P. G. Mezger for his permission to use his measurements of the NRAO dishes in advance of publication.

Findlay: NRAO Operating Experience 37 References

FINDLAY, J. W. 1963. The 300-foot radio telescope at Green Bank. Sky and Telescope. 25: 68.

RUZE, J. 1952. The effect of aperture errors on the antenna radiation pattern. Nuovo Cimento. Suppl. 9(3): 364-380.

Discussion of the Paper

JOHN RUZE (Massachusetts Institute of Technology, Lincoln Laboratory, Lexington, Mass.) : The gain of a parabolic reflector may be represented by the formula

where q-is the aperture efficiency. Typical values range from 0.5 to 0.65. This re-

duction from full utilization of the aperture area is due to non-uniform illumination, energy loss over the edge of the reflector and blockage of the aperture by the feed and supports.

D-dish diameter. A-the operating wave length. The range of interest for large dishes extends

€-is the reflector rms deviations from the best-fit paraboloid expressed in the

This formula is derived by statistical methods on the assumption that (1) the deviations from the best-fit paraboloid are random and distributed in a Gaussian manner, (2) that the errors are uniformly distributed over the antenna aperture, (3) that the region over which the errors are substantially constant is large com- pared to a wave length; that is, the reflector is smooth in wave length measure, (4) that the number of such uncorrelated regions in the aperture is large (at least ten).

The formula does not require that the aperture illumination be uniform nor that the errors be small. It would at first appear that the theory would be of limited applicability to large parabolic structures where the errors are not random but to a large part are due to calculable strains. However, Equation 1 still holds provided that the deviations are distributed in a Gaussian manner and holds ap- proximately i f distributed in a bell-shape curve. Other aspects of the theory such as the prediction of increased side-lobe level requires the random error assumption.

As the theory is of a statistical nature, the gain reduction from a perfect dish is not predicted exactly but as a statistical quantity with a definite mean or expected value expressed by Equation 1. However, the distribution of gains of seemingly identical reflectors is sharply peaked, increasing with the number of uncorrelated error regions and the smallness of the deviations. I t is estimated that for typical reflectors this gain-spread is less than the precision of normal gain measurements.

A major contribution of Findlay’s paper is that he has been able to show by precision photogrammetric measurements on an actual large dish that (1) the error deviations are essentially Gaussian and (2 ) that they are not markedly effected by radial dependence, although the gravity droop is evident. Furthermore, by measurements a t a number of wave lengths he has obtained general agreement

from 0.025 to 10 feet (35,000 to 100 Mc).

same units as A.


with Equation 1. In other words, he has provided substantial experimental verifi- cation on actually constructed large dishes (85 and 300’) of the theoretical equation and the assumptions on which it is based.

The gain reduction expressed by Equation 1 has been subject to some confusion due to the different conditions under which it has been plotted. At times it has been expressed as a phase-front error which is closely twice the reflector error; it has been expressed in wave lengths, radians and degrees, and in terms of the rms error and the contractually more significant peak error. Recent mechanical measurements on actual dishes has indicated that the peak error is roughly three times the rms error about the best-fit paraboloid.

O n this basis we have prepared FIGURE 1 which shows the loss of gain as a function of both the rms reflector error and the peak error. On the basis that gain is the criteria and that we require that the gain be at least 80 per cent of a perfect dish we obtain the tolerance of A/25 rms or 2 A/S peak. This tolerance may be too loose in some low-noise applications or in expensive dishes where we would not wish 20 per cent of the output or basically purchase price devoted to mechanical tolerances. However, a paraholoidal reflector will operate with much looser tolerances and provide useful data-but a point is soon reached where for operation at this wave length it would be more economical to build a smaller but more precise dish. The larger and coarser dish would still be preferable a t the lower frequencies (larger wave lengths).

In general no simple answer can be given as to how the dish tolerance should be set. The answer depends on the application, is intimately related to the cost and to possible future operation a t higher frequencies.

FIGURE 1

Findlay : NRAO Operating Experience 39

/ogPo

FIGURE 2.

To place these tolerance considerations in evidence we have plotted in FIGURE 2 the expected gain as a function of frequency for a 480-foot dish with a 0.500-inch tolerance, for a 120-foot with a 0.032 tolerance and a 0.008-inch, 30-foot parabola. The curves show the frequency range where the larger and coarser dish is superior to the smaller and finer structure.

In general the gain of a given dish increases with frequency until the tolerance effects take over and then a rapid gain decreases and beam distortion occurs. Maximum gain occurs at the waye length defined by

A, = 4 m at which point the gain has been reduced by 4.3 db or by a tolerance factor of 0.368. The value of this maximum gain is

G , = 0 . 0 2 4 ~ (:y and depends on the square of the precision of manufacture (D/E). On this basis it has been estimated that with conventional methods of construction the maximum gain obtainable is around 70 db. However, due to the strong dependence of the maximum realizable gain on the manufacturing precision unconventional methods such as stress compensation, deflection control and/or protected structure should find future interest.

References

RUZE, J. 1952. Physical limitations on antenna. Technical Report Number 248. Research Laboratory of Electronics. MIT. The effect of aperture errors on the antenna radiation pattern. Supplemento a1 Nuovo Cimento. 9 ( 3 ) : 364-380.

40 Annals New York Academy of Sciences ROBIEUX, I. 1956. Influence de la precision de fabrication d’une anteane sur ses perfor-

BATES, R. H. T. 1959. Random error in aperture distributions. IRE Trans. Antennas Propa-

BRACEWELL, R. N. 1961. Tolerance theory of large antennas. IRE Trans. Antennas Propa-

mances. Ann. Radioelect. : 29-56.

gation. Vol. AP-7, (4): 369-372.

gation. AP-9 ( 1 ) : 49-58.

Download - OPERATING EXPERIENCE AT THE NATIONAL RADIO ASTRONOMY OBSERVATORY

Top Related