MEETING OF THE GBT SCIENTIFIC WORKING GROUP October 26, 1993 4 PM EDT Call-in (913)749-0048, ID# 108N AGENDA 1) Update on construction progress and schedule (Bob Hall) 2) Notes from the GBT Advisory Committee Meeting (Jay Lockman) 3) Monitor and Control News (Mark Clark) 4) Spectrometer Developments (Ray Escoffier; Rich Lacasse) 5) Receiver Priorities for 1995 (Jay Lockman; Roger Norrod) Status of planned receivers The Q-band system Review of priorities Multibeam receiver Very low frequency receiver --------------------------------------------------------------------- GBT RECEIVER STATUS October 25, 1993 GREGORIAN RECEIVERS Receiver Freq (GHz) Dual Feed Rank Status (completion date) ------- ----------- --------- ---- ------------------------------- 1 1.15-1.73 No 1 Under construction (July 94) 2 1.73-2.60 No 4 3 2.60-3.95 No 3 4 3.95-5.85 No 2 Under development (March 94) 5 5.85-8.20 No 3 6 8.00-10.0 No 2 Testing (December 1993) 7 10.0-12.4 No 3 8 12.0-15.4 Yes 1 Testing (December 1993) 9 15.4-18.0 No 4 10 18.0-22.0 Yes 2 Done -- testing 11 22.0-26.5 Yes 1 Done -- testing 12 26.5-33.0 Yes 3 13 33.0-40.0 Yes 3 14 40.0-52.0 Yes 1 Under Design; Ordering Parts (1994) PRIME FOCUS RECEIVERS Rx No. BOX NO. Freq (MHz) Feed Type Polarizer ------- -------- ----------- --------------------- ------------------ PF-1 1,2 290-395 Coaxial Fed Crossed Dipoles Cavity-Backed Dipoles or Backfire Antenna PF-2 1,2 385-520 Coaxial Fed Crossed Dipoles Cavity-Backed Dipoles or Backfire Antenna PF-3 1,2 510-690 Coaxial Fed Crossed Dipoles Cavity-Backed Dipoles or Backfire Antenna PF-4 1 680-920 Waveguide Fed Waveguide OMT Corrugated Horn PF-5 2 910-1230 Waveguide Fed Waveguide OMT Corrugated Horn All prime focus receivers are two-channel, accepting linear polarization. Box 1 is under construction; its 680-920 MHz channel will be finished by late 1993. Box 2 will be built in 1994. * * * * * * * * * PROPOSED MILTIBEAM RECEIVER We propose that a 7-beam 12--15.4 GHz receiver be built for the GBT. It would have the same filled-hexagon beam pattern as the 7-beam prime-focus 4.85 GHz receiver that has been used on the 300-foot telescope, the 140-foot telescope, and the Parkes 210-foot telescope to map most of the sky. Larger feeds are needed to provide the narrow beams to illuminate the GBT subreflector properly, so the LOWEST frequency band at which such a receiver/feed package can fit in a single dewar is 12--15.4 GHz (Roger Norrod, private communication). This is the standard GBT Ku-band, so the development costs for the polarizer and thermal transition/vacuum window have already been paid, making the Ku-band receiver relatively economical. The 12--15.4 GHz band is also good for several scientific reasons. It is the HIGHEST frequency at which continuum observations can easily be conducted through the atmosphere, yielding a GBT resolution of slightly under 1 arcmin FWHM. This is sufficient for mapping fairly large low-surface-brightness sources such as nearby spiral galaxies. The rms confusion from extragalactic sources will be only about 25 microJy, so deep continuum surveys of small areas can be made. This receiver could also cover large areas (~1 sr in a week) at a frequency 3X higher than existing surveys. It would be ideal for finding exotic radio stars in the Galactic plane by their variability, using the Galactic Patrol technique pioneered by Gregory & Taylor (1986, AJ, 92, 371) to discover radio stars such as the low-mass X-ray binary GT2318+620 (Taylor et al. 1991, Nature, 351, 547). Finally, this receiver could be used for studying fluctuations in the cosmic microwave background, both on scales of several arcmin (when mounted on the GBT) and tens of degrees (when off the telescope and pointing up at the sky). --- Jim Condon and Juan Uson