INTRODUCTION

This research was conducted under the guidance and mentorship of Dr. Alan E. E. Rogers during the 2000 REU program at Haystack Observatory. Haystack Observatory has developed a small radio telescope (SRT) capable of continuum and spectral line observations in the L-band (1.42 Ghz). Currently the telescope uses an ambient-temperature vane calibrator to obtain a measure of the system noise, background sky, atmosphere, and spillover from the feed. The electronic noise calibrator emits a signal created by a noise diode to obtain the same measurement. The benefits of the electronic noise calibrator include its reliability, durability and affordability.

THE CIRCUIT

Utilizing a NOISE/COM 302L noise diode (the blue thing in the center of the circuit board), the circuit creates an electronic Gaussian noise signal. (Link to circuit diagram.) The signal is emitted to the SRT receiver from the dipole antenna.

THE DIPOLE

The dipole antenna is located at the vertex of the SRT dish. The dipole is constructed to emit at the 21cm wavelength, l = 10.5 cm, h = 5.25 cm, or approximately 4 inches and 2 inches, respectively. Correct polarization alignment of the dipole and the receiver's antenna must be achieved to ensure maximum signal from the electronic noise calibrator. (Link to schematic for dipole/calibrator housing.)

CALIBRATION

The vane calibrator uses the ratio of the power measurement from the receiver with the vane enabled and blocking the feed divided by the power measurement from the receiver when the vane is retracted and the signal is from the sky.


The sky signal includes contributions from spillover.

The electronic noise calibrator uses the ratio of the power measurement from the receiver with the noise calibrator on divided by the power ratio when the noise calibrator is off.


These power measurements include contributions from spillover and the sky.

The temperature of the electronic noise calibrator is determined from comparisons between the vane calibrator and the noise calibrator. In this case, the average temperature was found to be 108.5 K. (Calibrator data used for comparisons: Vane | Electronic Noise.)

CONCLUSION

Calibrations with the electronic noise calibrator can occur frequently and quickly, thus improving the quality of the data. Weather which could alter the characteristics of the vane calibrator have no effect on the electronic noise calibrator. The errors introduced by the method of calculating the electronic noise calibrator's temperature could be substantially reduced by using a method of calibrating with the ground. This option exists with the new SRTs.

A detailed write up of the construction of the electronic calibrator can be found here.