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NAIC/AO Newsletter, March 1997
Before recommencing the drift-scan searches, the pointing of the 430-MHz feed was determined using continuum drift-scan records. Estimates at a number of declinations gave a maximum error of 3.6 arcmin from the nominal pointing, i.e. about one-third of the HPBW (10 arcmin). To monitor the effects of the on-going upgrade operations, continuum data are now always recorded in parallel with the searches. Immediately after the initial pointing evaluation, the drift-scan searches were recommenced with the excellent 60-kHz frequency resolution, and ultra-short sampling interval of 80 microsec, available with the PSPM. This represents half a million samples per HPBW. The above properties open up a new area of parameter space for pulsar searching, potentially making possible evaluation of the limiting rotation periods for neutron stars. Somewhat higher resolution in frequency and time is anticipated with the newly commissioned AOFTM spectrometer. These two new backends allow searching of a deeper volume of space than previously, and together can better reveal the galactic distribution of neutron stars. (An update of the present status of the AOFTM can be found at http://www.naic.edu/~aoftm/aoftm_help.htm).
Each of the five pulsar-search groups have been assigned 8 one-degree wide declination strips. Between Oct 96 and Mar 97, over 860 hr of observing were completed, covering 68 observing sessions. Due to motion restrictions on the Carriage House, the strips covered have been arbitrary, although their distribution among the groups appears to have been relatively even. The 68 sessions cover 36 independent declination strips, representing about 1000 square deg of Arecibo sky. We await with interest the reduction of the 571 8-mm tapes containing 2.8 Tbytes of data. In early Mar 97, re-tensioning of the platform main cables raised the platform considerably above its optimum position. It will remain there until the new tie-down system is connected in April. This moves the effective center frequency of the line feed up to about 438 MHz. Wider front-end filters centered on 435 MHz have been installed, and the exact frequency of maximum gain and state of the RFI environment are being evaluated prior to recommencing searching at this somewhat higher frequency.
Radio-frequency interference (RFI) is a significant problem for the searches. At the beginning of Dec 96, the Arecibo RFI team identified the source of a strong RFI in the 430-MHz band as a malfunctioning repeater of a local beeper company. Following communication with the company's engineer, the band was cleared of this source of interference. Early in Dec, observations were also interrupted by military exercises causing considerable RFI. Soon after this, members of the RFI team had discussions with the Spectrum Manager of the Atlantic Fleet and Weapons Training Facility at Roosevelt Roads. The existing agreement on RFI between the Navy and the Observatory was reconsidered and broadened to include both the mutual exchange of operational schedules, and a widening of the band over which the Observatory is considered to have an interest to 420 - 440 MHz. This coordination has worked well so far, and a protocol has been developed for communicating with the Navy Watch Supervisor when RFI is present.
Full Carriage House tracking and pointing are expected to be regained by mid-May to June. Until this occurs, the upgrade pulsar searches will continue with the capabilities provided by the new backends and detailed above.
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