Edward S. Cheng

MEASUREMENT OF THE COSMIC MICROWAVE BACKGROUND SPECTRUM BY THE COBE FIRAS INSTRUMENT

The cosmic microwave background radiation (CMBR) has a blackbody spectrum within 3.4 x 10(exp -8) ergs/sq cm/s/sr cm over the frequency range from 2 to 20/cm (5-0.5 mm). These measurements, derived from the Far-Infrared Absolute Spectrophotomer (FIRAS) instrument on the Cosmic Background Explorer (COBE) satellite, imply stringent limits on energy release in the early universe after t approximately 1 year and redshift z approximately 3 x 10(exp 6). The deviations are less than 0.30% of the peak brightness, with an rms value of 0.01%, and the dimensionless cosmological distortion parameters are limited to the absolute value of y is less than 2.5 x 10(exp -5) and the absolute value of mu is less than 3.3 x 10(exp -4) (95% confidence level). The temperature of the CMBR is 2.726 +/- 0.010 K (95% confidence level systematic).

Dipole anisotropy in the COBE DMR first year sky maps

We present a determination of the cosmic microwave background dipole amplitude and direction from the COBE Differential Microwave Radiometers (DMR) first year of data. Data from the six DMR channels are consistent with a Doppler-shifted Planck function of dipole amplitude ΔT=3.365±0.027 mK toward direction (l II , b II )=(264°.4±0°.3, 48°.4±0°.5). The implied velocity of the Local Group with respect to the CMB rest frame is v LG =627±22 km s −1 toward (l II , b II )=(276°±3°, 30°±3°). DMR has also mapped the dipole anisotropy resulting from the Earths orbital motion about the Solar system barycenter, yielding a measurement of the monopole CMB temperature T 0 at 31.5, 53, and 90 GHz, T 0 =2.75±0.05 KWe present a determination of the cosmic microwave background dipole amplitude and direction from the COBE Differential Microwave Radiometers (DMR) first year of data. Data from the six DMR channels are consistent with a Doppler-shifted Planck function of dipole amplitude Delta T = 3.365 +/-0.027 mK toward direction (l,b) = (264.4 +/- 0.3 deg, 48.4 +/- 0.5 deg). The implied velocity of the Local Group with respect to the CMB rest frame is 627 +/- 22 km/s toward (l,b) = (276 +/- 3 deg, 30 +/- 3 deg). DMR has also mapped the dipole anisotropy resulting from the Earths orbital motion about the Solar system barycenter, yielding a measurement of the monopole CMB temperature at 31.5, 53, and 90 GHz, to be 2.75 +/- 0.05 K.

The COBE mission - its design and performance two years after launch

COBE, NASAs first space mission devoted primarily to cosmology, carries three scientific instruments to make precise measurements of the spectrum and anisotropy of the cosmic microwave background radiation on angular scales greater than 7° and to conduct a search for a diffuse cosmic infrared background radiation with 0°.7 angular resolution. The mission goal is to make these measurements to the limit imposed by the local astrophysical foregrounds. The COBE instruments cover the wavelength range from 1.2 μm to 1 cm. The instruments are calibrated periodically in orbit using internal calibrators and celestial standards

Interpretation of the cosmic microwave background radiation anisotropy detected by the COBE Differential Microwave Radiometer

The large-scale cosmic background anisotropy detected by the COBE Differential Microwave Radiometer (DMR) instrument is compared to the sensitive previous measurements on various angular scales, and to the predictions of a wide variety of models of structure formation driven by gravitational instability. The observed anisotropy is consistent with all previously measured upper limits and with a number of dynamical models of structure formation. For example, the data agree with an unbiased cold dark matter (CDM) model with H0 = 50 km/s Mpc and Delta-M/M = 1 in a 16 Mpc radius sphere. Other models, such as CDM plus massive neutrinos (hot dark matter (HDM)), or CDM with a nonzero cosmological constant are also consistent with the COBE detection and can provide the extra power seen on 5-10,000 km/s scales. 39 refs.

Advanced camera for the Hubble Space Telescope

The Advanced Camera for the Hubble Space Telescope has three cameras. The first, the Wide Field Camera, will be a high- throughput, wide field, 4096 X 4096 pixel CCD optical and I-band camera that is half-critically sampled at 500 nm. The second, the High Resolution Camera (HRC), is a 1024 X 1024 pixel CCD camera that is critically sampled at 500 nm. The HRC has a 26 inch X 29 inch field of view and 29 percent throughput at 250 nm. The HRC optical path includes a coronagraph that will improve the HST contrast near bright objects by a factor of approximately 10 at 900 nm. The third camera, the solar-blind camera, is a far-UV, pulse-counting array that has a relatively high throughput over a 26 inch X 29 inch field of view. The advanced camera for surveys will increase HSTs capability for surveys and discovery by a factor of approximately 10 at 800 nm.

COBE differential Microwave Radiometers : preliminary systematic error analysis

The Differential Microwave Radiometers (DMR) instrument aboard the Cosmic Background Explorer (COBE) maps the full microwave sky in order to measure the large-angular-scale anisotropy of the cosmic microwave background radiation. Solar system foreground sources, instrumental effects, as well as data recovery and processing, can combine to create statistically significant artifacts in the analyzed data. We discuss the techniques available for the identification and subtraction of these effects from the DMR data and present preliminary limits on their magnitude in the DMR 1 yr maps (Smoot et al. 1992)

Detectors for the James Webb Space Telescope Near‐Infrared Spectrograph. I. Readout Mode, Noise Model, and Calibration Considerations

We describe how the James Webb Space Telescope (JWST) Near-Infrared Spectrographs (NIRSpec) detectors will be read out, and present a model of how noise scales with the number of multiple nondestructive reads sampling up the ramp. We believe that this noise model, which is validated using real and simulated test data, is applicable to most astronomical near-infrared instruments. We describe some nonideal behaviors that have been observed in engineering-grade NIRSpec detectors, and demonstrate that they are unlikely to affect NIRSpec sensitivity, operations, or calibration. These include a HAWAII-2RG reset anomaly and random telegraph noise (RTN). Using real test data, we show that the reset anomaly is (1) very nearly noiseless and (2) can be easily calibrated out. Likewise, we show that large-amplitude RTN affects only a small and fixed population of pixels. It can therefore be tracked using standard pixel operability maps.

A bolometric millimeter-wave system for observations of anisotropy in the cosmic microwave background radiation on medium angular scales

We report the performance of a bolometric system designed to measure the anisotropy of the cosmic microwave background (CMB) radiation on angular scales from 0 deg 3 min to 3 deg. The system represents a collaborative effort combining a low-background 1 m diameter balloon-borne telescope with new multimode feed optics, a beam modulation mechanism with high stability, and a four-channel bolometric receiver with passbands centered near frequencies of 3 (90), 6 (180), 9 (270), and 12 (360) cm(exp -1) (GHz). The telescope was flown three times with the bolometric receiver and has demonstrated detector noise limited performance capable of reaching sensitivity levels of Delta(T)/T(sub CMB) is approximately equal to 10(exp -5) with detectors operated at T = 0.3 K.

COBE differential microwave radiometers - Calibration techniques

The COBE spacecraft was launched November 18, 1989 UT carrying three scientific instruments into earth orbit for studies of cosmology. One of these instruments, the Differential Microwave Radiometer (DMR), is designed to measure the large-angular-scale temperature anisotropy of the cosmic microwave background radiation at three frequencies (31.5, 53, and 90 GHz). This paper presents three methods used to calibrate the DMR. First, the signal difference between beam-filling hot and cold targets observed on the ground provides a primary calibration that is transferred to space by noise sources internal to the instrument. Second, the moon is used in flight as an external calibration source. Third, the signal arising from the Doppler effect due to the earths motion around the barycenter of the solar system is used as an external calibration source. Preliminary analysis of the external source calibration techniques confirms the accuracy of the currently more precise ground-based calibration. Assuming the noise source behavior did not change from the ground-based calibration to flight, a 0.1-0.4 percent relative and 0.7-2.5 percent absolute calibration uncertainty is derived, depending on radiometer channel.

Large-scale anisotropy in the 2. 7 K radiation

We report the results of two baloon flights, covering half the sky, to measure the large-scale anisotropy of the 2.7 K cosmic radiation. Radiometers at three wavelengths give results which are well fitted by a dipole distribution with amplitude 2.99 +- 0.34 mK and direction R. A.=12./sup h/.3 +- 0/sup h/.4 and decl. =-1/sup 0/ +- 6/sup 0/. An upper limit of 2 mK (95% confidence) is found for the amplitudes of nonpolar components of a quadrupole distribution.