A simple, yet unusual, child's toy illustrates some basic features of the physics of resonance, waves and fluid dynamics. The acoustic modes of a corrugated tube open at both ends and rotating in a plane were examined as a function of rotational frequency and found to be similar but not identical to those of a stationary open-ended organ pipe. Measurements of the pressure difference at resonance across the tube ends as a function of rotational frequency agree well with a simple analysis based on Bernoulli's principle. A mechanism of sound production is proposed whereby the tube resonantly amplifies acoustic perturbations to the axial air flow, engendered by the corrugations, that occur at frequencies equal to the resonant frequencies of the tube. The mechanism is supported by direct measurements of the axial air speed as a function of rotational frequency.
We show that uniform time sampling of both the reference and the target channels in a continuous scanning Fourier transform spectrometer is a simple and versatile way of extending the Nyquist limit shorter than the wavelength of the reference channel. We also discuss the benefits of recording the reference channel when intensity calibrating the target data.
SIRTF requires detector arrays with extremely high sensitivity, limited only by the background irradiance. Especially critical is the near infrared spectral region around 3 µm, where the detector current due to the zodiacal background is a minimum. IRAC has two near infrared detector channels centered at 3.6 and 4.5 µm. We have developed InSb arrays for these channels that operate with dark currents of <0.2 e -/s and multiply-sampled noise of ~7 e -at 200s exposure. With these specifications the zodiacal background limited requirement has been easily met. In addition, the detective quantum efficiency (DQE) of the InSb devices exceeds 90% over the IRAC wavelength range, they are radiation hard, and they exhibit excellent photometric accuracy and stability. Residual images have been minimized. The Raytheon 256 x 256 InSb arrays incorporate a specially developed (for SIRTF) multiplexer and high-grade InSb material.
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