OPERATING EXPERIENCE AT THE NATIONAL RADIO ASTRONOMY OBSERVATORY

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<ul><li><p>OPERATING EXPERIENCE AT THE NATIONAL RADIO ASTRONOMY OBSERVATORY * </p><p>John W. Findlay National Radio Astronomy Observatory, Green Bank, West Virginia </p><p>The Telescopes at the NRAO </p><p>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. </p><p>+79" 50.2' Longitude </p><p>4-38' 26' 17" Latitude </p><p>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. </p><p>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. </p><p>The 85-foot Polar Mounted Antenna </p><p>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: </p><p>Dish diameter: Focal length: </p><p>Surface: Sky coverage: </p><p>Drive system: </p><p>Indicator system: </p><p>Constructed by: </p><p>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 sys- tem is available for special work. Blaw-Knox Equipment Division, Pittsburgh, Pennsylva- nia. </p><p>Operated by the Associated Universities, Inc., under contract with the National Science Foundation. </p><p>25 </p></li><li><p>26 Annals New York Academy of Sciences </p><p>x e 8 m E b 0 </p><p>0 </p><p>2 </p></li><li><p>Findlay: NRAO Operating Experience 27 TABLE 1 </p><p>ANTENNAS AT THE NRAO </p><p>5 P e of receivers </p><p>Antenna, type, size and steering </p><p>equatorial mount Paraboloid 25.9M </p><p>Paraboloid 42.7M equatorial mount under construction </p><p>Beam scan Operating </p><p>frequencies Type of </p><p>observations </p><p>HA 2 6 hrs. Dec. -52" </p><p>to +90 </p><p>Planetary, galactic and extra- galactic sources </p><p>Planetary, gaalctic and extra- galactic sources </p><p>400 Mc/s 8,000 Mc/s </p><p>up to 10 kMc/s </p><p>TWT, parametric, crystal mixers, Masers </p><p>- ~ </p><p>Dee. -50 to +90 </p><p>HA 2 6 hrs. </p><p>Paraboloid 92M meridian transit </p><p>1 6 0 " zenith distance </p><p>2 9 0 zenith distance </p><p>Planetary, gaalctic and extra- galactic sources </p><p>sources Selected radio </p><p>Up to 1,427 Mc/s </p><p>750 and 1,400 Mc/s </p><p>Crystal mixer and parametric </p><p>Paraboloid 12.2M Crystal mixer </p><p>Paraboloid 9.2M alt-azimuth </p><p>Standard gain horn 36.5M long, collecting area 10 sq. m. </p><p>Paraboloid 3.7M </p><p>All sky </p><p>Fixed Cas A trans- </p><p>its only </p><p>Interference monitor </p><p>Cas A flux </p><p>Up to 3,000 Mc/s </p><p>600 Mc/s- 1,450 Mc/s </p><p>8,000 Mc/s </p><p>Crystal mixer </p><p>Crystal mixer </p><p>Fixed Atmospheric radiation </p><p>TWT </p><p>Paraboloid 6.1M Fixed Atmospheric radiation </p><p>5,000 Mc/s TWT </p><p>Paraboloid 1.5M ~ HA f 6 hrs. Dec. -52" </p><p>to +90 </p><p>Planetary, galactic, extragalactic, and atmospheric radiation </p><p>250 Mc/s GE bolometer </p><p>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-reflec- tor 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. </p><p>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 mo- tion 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. </p></li><li><p>28 Annals New York Academy of Sciences </p><p>FIGURE 2. The Howard E. Tatel 85-foot radio telescope at the NRAO. </p><p>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 </p></li><li><p>Findlay: NRAO Operating Experience </p><p>Toward zenith 3.16 mm. </p><p>Toward horizon -I 5.71 mm. </p><p>29 </p><p>35.883 2.006 feet </p><p>35.748 2.0015 feet </p><p>TABLE 2 THE SURFACE OF THE 85-FOOT TELESCOPE </p><p>I RMS departure from Position of reflector best fit paraboloid 1 best fit paraboloid Focal length of </p><p>~ -~ - </p><p>Greatest departure RMS departure Position of dish, from best fit from best fit zenith distance paraboloid paraboloid </p><p>3.3 cm. 1.07 crn. </p><p>30" 2.8 cm. 1.27 cm. </p><p>51" 24' 3.4 c m . 1 ~~~ ~ 0.95 crn. </p><p>0" </p><p>.- </p><p>Focal length of best fit paraboloid </p><p>128.145 k.012 feet </p><p>127.980 t . 0 1 1 feet </p><p>127.782 &amp;.009 feet </p><p>~~ </p><p>_____ </p><p>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. </p><p>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 essen- tially 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 sum- marized in TABLE 2. </p><p>The 300-foot Transit Telescope This instrument was designed and constructed in a short time for a rather </p><p>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 specifica- tion of the instrument. </p><p>Dish diameter: 300 feet Focal length: </p><p>Surface: 128.5 feet ( f /D = 0.428) Expanded aluminum mesh (Squarex) 0.625" X 0.091'' </p><p>From the North Pole through the zenith to 60" south of the zenith. (Declination range 4- 90' to - 2 1 O . ) </p><p>Mount: Meridian transit Sky coverage: </p></li><li><p>30 Annals New York Academy of Sciences </p><p>FIGURE 30. </p><p>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. </p><p>FIGURE 3. </p><p>Drive system: Two speed electric drive through quadruple roller chain giving positioning speeds of 10" per minute and 2.5" per minute. </p><p>A digital encoder disk giving a 10 second of arc digit interval operating a declination display and providing also printed and punched tape outputs. </p><p>Robert D. Hall and Edgar R. Faelten </p><p>Service conditions: ( a ) Amhient temperature. Operating O'F. to 105F. Nonoperating - 20'F. to f 120F. ( b ) Wind. Survival-reflector stowed - 65 m.p.h. Re- flector capable of being moved at winds up to 45 m.p.h. </p><p>Indicator system: </p><p>Designers: </p></li><li><p>Findlay: NRAO Operating Experience </p><p>Y </p><p>t 31 </p><p>---X </p><p>FIGURE 36. FEED SUPPORT </p><p>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. </p><p>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- </p></li><li><p>32 Annals New York Academy of Sciences </p><p>FIGU</p><p>RE </p><p>4. </p><p>The</p><p> 300</p><p>-fOot</p><p> tran</p><p>sit te</p><p>lesc</p><p>ope a</p><p>t the</p><p> NR</p><p>AO</p><p>. </p></li><li><p>Findlay: NRAO Operating Experience </p><p>i 33 </p><p>X </p><p>FIGURE 5a. </p><p>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. </p><p>FIGURE 5 . </p><p>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. </p><p>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 </p><p>Operating Experience with the Two Telescopes </p><p>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 </p><p>TABLE 3. </p><p>The survey was made by D. Brown Associates, Inc., Eau Gallie, Florida. </p></li><li><p>34 Annals New York Academy of Sciences V </p><p>i </p><p>- -x </p><p>FIGURE Sh. </p><p>(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 equip- ment 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. </p><p>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 sched- uled 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. </p><p>The type of operating experience shown in the TABLE also seems to justify the </p></li><li><p>Findlay: NRAO Operating Experience 35 </p><p>use of radio telescopes for radio astronomy in which the instruments are designed to give their best performance only in a rather gentle meteorological environ- ment. This, of course, is only possible when the type of observation permits some deIay in achieving the results required. Programs which require continuous ob- servation 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. </p><p>The Radio Properties of the Telescopes </p><p>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 ac- curacy 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 </p></li><li><p>36 </p><p>85-Foot Telescope </p><p>Summer Winter quarter quarter </p><p>Annals New York Academy of Sciences TABLE 4 </p><p>3M-Foot Telescope </p><p>Summer Winter quarter quarter </p><p>Balance of hours in quarter, some unscheduled, remainder used on equipment installation, calibration and maintenance </p><p>Hoursscheduled during quarter I 1,122 I 1,871 I 1,904 I 1,880 </p><p>1,062 </p><p>~ </p><p>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 </p><p>Telescope </p><p>85-f00t </p><p>3M)-fOOt </p><p>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 </p><p>-___ _ _ ~ </p><p>Measured I Calculated Weighted RMS aperture a ecture surface errors Frequency efficiency egwency </p><p>0.275 cm. 1,420 Mc/s 56% 5 7 % </p><p>3,000 ML/S 52% 52% </p><p>5,000 Mc/s 45 % 42% </p><p>7,600 Mc/s 32% 2896 </p><p>1.20 cm. 750 Mc/s 59% 58 % </p><p>41% 1,400 Mc/s 1 40% - _ _ ~ </p><p>______- </p><p>~ </p><p>_ _ ~ _ - ~~ ~~ </p><p>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 </p><p>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. </p><p>The good agreement between the measured and calculat...</p></li></ul>