During commissioning of the Green Bank Telescope (GBT) in the fall and winter of 2001/2 a background, 21-cm observing program is being run during the times when the telescope is not occupied with tests or calibration. This observing program does simple on-off spectral line measurements of global HI profiles of galaxies at redshifts out to about 10,000 km/s using integration times between about 10 and 60 minutes and typical bandwidths of 5 or 10 MHz, depending on the expected signal strength and line profile width. All observations so far have used the FFT spectrometer, which has 1024 channels on each of the two linear receiver polarizations. The system temperature is slightly under 20 K at high elevations, and the 100-meter aperture efficiency is roughly 70%. Lists of objects to be observed may be contributed by anyone with the understanding that the data will be immediately put in the public domain. This web site is the current avenue of access to the survey data.
Since the observations are being conducted during the commissioning phase of the GBT, the data are not necessarily up to the standards intended for routine service of observing proposals. In particular, the servo electronics at the telescope focus area is a source of RFI that produces narrow features in the spectra every couple of MHz. In many cases these interference defects may be edited out without much, if any, loss of information. Work is currently underway to suppress this RFI with new shielding and filtering. Reasonable care has been taken to verify the frequency and velocity scales of the spectra. The intensity calibration is tied to the NVSS flux density scale through observations of about five dozen continuum sources with flux densities between 2.2 and 6.0 Jy selected to avoid significant multi-source confusion with the nine-arcminute GBT beam. Details of the calibration results may be accessed through the table of contents above.
To expedite execution of this survey background task, the telescope control scripts were written specifically for this purpose. Data file formats and a few details of telescope control were still being modified while the survey was in progress so some of the data are not entirely compatible with the standard aips++ "filler." For the purpose of presenting reduced data on these web pages, special purpose, glish-language scripts were written to read the telescope data files directly and to incorporate patches for any data format changes that occurred during the survey. The telescope data files are available upon request, but it may require some time and effort to export the data to standard reduction packages or to your own software. The glish code written for the displayed data reduction is, of course, freely available on request, although it may not be extensively documented.
NRAO Green Bank Home Page
Rick Fisher's Home Page