NAIC/AO Newsletter, November 97
On July 28th, 1997, Arecibo made its first l 18-cm observation in support of the recently launched 8-m orbiting antenna, HALCA, of the Japanese VSOP (VLBI Space Observatory Programme) Project. Also taking part in the run was the 32-m telescope in Noto, Sicily. We note that this observation also represents the FIRST scheduled post-upgrade radio-astronomy experiment with the Gregorian feed system. In fact, so early in the telescope's return to service were these measurements, that the antenna did not yet have even a pointing model evaluated. In addition, the new L-band had been returned to Ithaca shortly before this for the retro-fitting of some components, and Edgar Castro, Head of Electronics, had done a wonderful job at replacing it with our pre-upgrade 1.4-GHz receiver tuning its turnstile junction to the limit in order to get it to perform within the required range of 1648 - 1680 MHz. Further, the platform had not yet been pulled down to its expected focal height by the new tie-down system, and was in fact the best part of a foot above focus! During the days prior to the run, Phil Perillat made intensive measurements of the VSOP target source, J1527+312, plus a number of other sources at a similar declination. Through cross-scan measurements from Arecibo rise to set on these objects, Phil was able to determine the telescope pointing along the trajectory of the source and confirm that this remained stable on the time scale of days.
Come the evening of July 28th, Ray González set up the recorder ready to take the data, and then joined the crew of "dedicated nail biters", Tapasi Ghosh, Mike Davis and Chris Salter, clustered around Phil Perillat as he commanded the telescope through its preparatory calibrations. The system temperature came out close to 40 K, while the sensitivity of 1.4 K/Jy reflected the height of the feed above its true focus; when the system is brought "into focus", a value almost an order of magnitude larger is anticipated. At 22:15 UT the recorder was started, and soon afterwards the chart recorder showed the source rising above the Arecibo "horizon" of zenith angle 20 deg. Now usually VLBI is not the most exciting experiment to monitor, but on this historic occasion every single "wiggle" of the total-power trace was a topic of discussion. Gene Lauria and Kiriaki Xilouris had checked out the hand of circular polarization the day before, but as this was a test run, we decided to play safe and flip the hand for ten minutes during the run, "just in case". At a later time we even moved briefly off-source to be sure it was still there, and indeed, there was a pleasing change of level as we did so. At 00:33 UT, the source set (Fig 4), and the S2 recorder was stopped. Ray returned to the recorder and packed up the 8 VHS tapes, preparing them then and there to be shipped to Penticton for correlation. Just two and a quarter hours had elapsed since J1527+312 rose, but for those of us there it was a major milestone in bringing the upgraded telescope back on line.
Figure 4. The total-power chart record for the final hour of the space VLBI run on J1527+312. The general rise in level as the source sets is due to the expected small increase in Tsys with Zenith angle. Also seen is the step at the end of the eun as the source sets across the "Arecibo Horizon"; the flux density of J1527+312 is about 600 mJy. The "notch" on the left occured when we moved the telescope off source by 5 arcmin (HPBW ~3 arcmin) just to be sure that our pointing really was "spot on".
However, this is not the end of the good news of VLBI from Arecibo. In Newsletter No. 21, we mentioned that we had submitted a consortium proposal (with NRAO as consortium partner) to the NSF Major Research Instrumentation (MRI) program for the purchase of a VLBA compatible system for Arecibo. We have now had official confirmation that this proposal has been completely successful. The MRI program requires that cost sharing at the level of 30% is required from the proposing institution. This implies that of the total project costs of $502,000, about $150,000 will be provided by Cornell University, and the rest by the MRI Program. The money involved has recently been received, and orders for the equipment will be placed in the immediate future. When this is in place at the Observatory, the 305-m telescope will be capable of joining in a wide range of VLBI observations in conjunction with the VLBA, the global VLBI network, and future space VLBI, as well as ad-hoc arrays of the world's largest frequency-agile telescopes.
The Arecibo Observatory still has a way to go, including the need to build up both our experience and our technical resources, before we will arrive as full players on the VLBI stage. However, that happy situation is measurably closer today than it was at this time last year.