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NAIC/AO Newsletter, November 1997

The Telescope Upgrading Project

Don Campbell

True first light for the new telescope was achieved on October 18. The telescope's tie down system was used on October 17 to lower the suspended structure of the telescope approximately 18 inches to bring the Gregorian optics into focus. October 18 was also the official start of the commissioning of the telescope. During the first few days of commissioning, observations were made at 1410 MHz to check the focus, to measure the system sensitivity, beamwidth and sidelobe levels, and to develop an initial pointing model. The very first observations of a radio source indicated that the beamwidth was roughly correct, that the sensitivity was at least 9 K/Jy and that the sidelobe level was at least 10 dB down. Subsequent measurements gave beamwidths of 3.8 arcmin in zenith angle and 3.4 arcmin in azimuth, in good agreement with the slightly elliptical 700 x 776 ft. illumination pattern. After adjustments in the position of the horn and tertiary reflector to remove a small coma lobe, an zenith angle scan gave the beam plot shown in Figure 1. The sidelobes in this plane are 15 dB down, the design value.


Fig.1. This Zenith angle cut through the Gregorian beam pattern was taken during the first week of commissioning. Sensitivity, beamwidth and side lobes are very close to the expected values.

A first cut at a pointing correction model was obtained by making a series of azimuth and zenith angles scans for eight sources over the telescope's hour angle coverage for each source. The sources have declinations roughly evenly distributed over the telescope's declination coverage. The resultant model gave a tracking accuracy of about 20 arcsec for the L-band system over the full range of the telescope's azimuth and zenith angle coverage. Refinements of the model and additional observations should improve this considerably.

We have not yet made definitive measurements of the telescope's sensitivity at L-band. However, initial measurements give 10ħħ1 K/Jy, close to the design value of 11 K/Jy. The height of the suspended structure is only approximately correct and varies with temperature as the tie down cables are not being actively tensioned. The gain also varies systematically up to 20% with zenith angle, possibly because we are currently relying on platform tilt under the weight of the dome to keep the system in focus. This variation can be tuned out by adjusting the tertiary reflector or the position of the carriage house as the dome tracks through its zenith angle range.


Fig.2. A 120-second integration at L-band of the fastest millisecond pulsar, PSR B1937+21, with the Penn State Pulsar Machine.

A few initial test observations have been carried out to check system performance. Figure 2 shows an observation at 1420 MHz of the first millisecond pulsar, PSR 1937+21. The integration time was 120 seconds. Figure 3 shows the first HI spectrum of a galaxy, UGC1033. On October 15 the 2380 MHz planetary radar system was partially tested by transmitting to Titan with the reflected signal being received by the NASA/JPL Goldstone 70m antenna. The transmitted power was 800 kW. At a round trip time of 2 hours 20 minutes only a very weak echo was expected at best, and none was detected on this first attempt.


Fig.3. The First post-upgrade HI spectrum.

There had been considerable activity leading up to the lowering of the platform and the beginning of commissioning. During July and August the Observatory's maintenance department aligned the telescope's elevation rails and rack gear to a best fit smooth curve. At some future date some additional alignment work may be needed to bring the rails to a precise radius of curvature. In September the drive system contractor returned to the observatory to complete the commissioning of the dome, carriage house and azimuth drive systems. This visit resolved the outstanding problems with the system and it now meets all its specifications. During September and early October, NAIC electronics and computer staff completed work on the tie down control system, and the Observatory's maintenance department aligned the tie down masts and booms and properly tensioned all the cables.

We currently have receiver systems installed in the dome at 430 MHz, 1400 MHz and 5 GHz. During the early stages of the telescope's commissioning, pointing and sensitivity tests will be carried out for these systems as well as for the 430 MHz line feed system on the carriage house. A wide band, 1.1 to 1.7 GHz, L-band receiver will be installed in the dome in the near future.

NAIC/AO Newsletter, November 97
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