Contents of poster presentation,
June 2001
F. Ghigo, D. Balser, G. Langston, R. Maddalena, M. McKinnon
| Summary of GBT Commissioning Activities |
|---|
|
HI Detection.
This shows the raw data from an early test observation using the new GBT
spectrometer in the 21 cm band. Even without the subtraction of
an "off" spectrum, the baseline is flat.
Hard at work in the GBT servo room during an early commissioning run.
[L to R: T. Minter, N. Sharp, W. Campbell]
Pointing Calibration at 2.0 GHz
All-sky pointing calibration determined from local pointing corrections (LPCs)
as a function of elevation. A seven parameter pointing model fits the
data with an rms of about 8.5 arcseconds.
The trend of azimuth error is entirely explained by a lateral tilt of the feed arm, and can be compensated with tracking of the subreflector in the Zs direction.
No significant azimuth-dependent pointing errors were found.
Focus tracking calibration at 2.0 GHz
The Gregorian subreflector has 6 degrees of freedom. Translations are along
the axes Xs, Ys, illustrated at the right, and Zs, perpendicular to Xs and Ys,
and pointing up from the paper.
For observing frequencies below 50 GHz or so, it is probably not necessary to
rotate the subreflector.
To calibrate the focus tracking, cross scans were done on calibration sources
over the full range of motion in Xs and Ys. The location giving the
maximum amplitude is the optimum reflector position. The observations were
repeated with the telescope at a range of elevation, with the results shown
in the graphs at right.
The RMS of the focus curves is 0.66 cm in Ys and 1.6 cm in Xs, sufficiently accurate for observing up to 15 GHz.
Gain is about 2.0 Kelvin per Jansky at 2.0 GHz. The decrease in gain
at low elevations is due to atmospheric attenuation.
The corresponding aperture efficiency is about 70%.
The increase of Tsys with lower elevation is explainable by atmospheric emission.
The gain, or G/T, is about 0.1 (per Jy).
Beam profile through Cygnus A at 2 GHz
Upper plot: the vertical scale is linear.
Lower Plot: the same data in units of db.
The near sidelobes are over 30 db down from the peak.
Scan rate is 3 arcminutes per second.
The horizontal extent of the plots is about 2.25 degrees.
The Galactic Center area at 2.0 GHz
800 MHz image of the Cygnus region
The Cygnus region using the prime focus 800 MHz receiver.
Cygnus A is seen at the right, the Cygnus X complex at the left.
(The image was processed using aips++).
Image of the Cygnus-X region at 2.0 GHz
