Engineering topics

Burrows, Christopher J.

Publications and source records attributed to Burrows, Christopher J..

9 records

NASA NTRS1998

HD 98800: A Unique Stellar System of Post-T Tauri Stars

HD 98800 is a system of four stars, and it has a large infrared excess that is thought to be due to a dust disk within the system. In this paper we present new astrometric observations made with Hipparcos, as well as photometry from Hubble Space Telescope WFPC2 images. Combining these observations and reanalyzing previous work allow us to estimate the age and masses of the stars in the system. Uncertainty in these ages and masses results from uncertainty in the temperatures of the stars and any reddening they may have. We find that HD 98800 is most probably about 10 Myr old, although it may be as young as 5 Myr or as old as 20 Myr old. The stars in HD 98800 appear to have metallicities that are about solar. An age of 10 Myr means that HD 98800 is a member of the post T Tauri class of objects, and we argue that the stars in HD 98800 can help us understand why post T Tauris have been so elusive, HD 98800 may have formed in the Centaurus star-forming region, but it is extraordinary in being so young and yet so far from where it was born.

Soderblom, David R.↗

NASA NTRS1996

Detection of the Tip of Red Giant Branc in NGC 5128

We present a color-magnitude diagram of more than 10,000 stars in the halo of galaxy NGC 5128 (Centaurus A), based on WFPC2 images through the V and I filters. The position of the red-giant branch stars is compared with the loci of the RGB in six well-studied globular clusters and in the dwarf elliptical galaxy NGC 185;...

NGC 185 galaxy NGC 5128 Centaurus A WFPC2↗

NASA NTRS1995

The performance and calibration of WFPC2 on the Huble Space Telescope

The Wide Field Planetary Camera 2 (WFPC2) was installed in the Hubble Space Telescope (HST) in 1993 December. Since then, the instrument has been providing high-quality images. A significant among of calibration data has been collected to aid in the understanding of the on-orbit performance of the instrument. Generally, the behavior of the camera is similar to its performance during the system-level thermal vacuum test at JPL in 1993 May. Surprises were a significant charge-transfer-efficiency (CTE) problem and a significant growth rate in hot pixels at the original operating temperature of the CCDs (-76 deg C). The operating temperature of the WFPC2 CCDs was changed to -88 deg C on 1994 April 23, and significant improvements in CTE and hot pixels are seen at this temperature. In this paper we describe the on-orbit performance of the WFPC2. We discuss the optical and thermal history, the instrument throughput and stability, the Point Spread Function (PSF), the effects of undersampling on photometry, the properties of cosmic rays observed on-orbit, and the geometric distortion in the camera. We present the best techniques for the reduction of WFPC2 data, and describe the construction of calibration products including superbiases, superdarks, and flat fields.

Holtzman, Jon↗

NASA NTRS1994

The on-orbit performance of WFPC2

The second Wide Field and Planetary Camera (WFPC2) was successfully installed in the Hubble Space Telescope during the STS-61 servicing mission in 1993 December. The primary objective of this new camera is to provide diffraction-limited photometric imaging over a wide field and a spectral range from 0.12 to 1.0 micrometer. Here we provide an overview of the characteristics of the new instrument and offer our perspectives based on the first 6 months of operations on-orbit.

Trauger, John T.↗

NASA NTRS1994

First Hubble Space Telescope observations of the brightest stars in the Virgo galaxy M100 = NGC 4321

As part of both the Early Release Observations from the Hubble Space Telescope (HST) and the Key Project on the Extragalactic Distance Scale, we have obtained multiwavelength BVR Wide Field/Planetary Camera-2 (WFPC2) images for the face-on Virgo cluster spiral galaxy M100 = NGC 4321. We report here preliminary results from those observations, in the form of a color-magnitude diagram for approximately 11,500 stars down to V approximately 27 mag and a luminosity function for the brightest blue stars which is found to have a slope of 0.7, in excellent agreement with previous results obtained for significantly nearer galaxies. With the increased resolution now available using WFPC2, the number of galaxies in which we can directly measure Population I stars and thereby quantify the recent evolution, as well as test stellar evolution theory, has dramatically increased by at least a factor of 100. Finally, we find that the stars are present in M100 at the colors and luminosities expected for the brightest Cepheid variables in galaxies.

Freedman, Wendy L.↗

NASA NTRS1994

Far-ultraviolet imaging of the globular cluster NGC 6681 with WFPC2

We have imagined the globular cluster NGC 6681 in the far-UV and visible with Wide Field/Planetary Camera-2 (WFPC2) on the Hubble Space Telescope (HST). Our far-UV images show a sparsely populated and fully resolved central region, and we detect 122 stars. The far-UV to visible color-magnitude diagram shows a well-defined horizontal branch with no evidence for hot, more evolved descendants. We find one hot horizontal-branch star significantly below the model zero-age horizontal branch, but the rest are consistent with evolutionary models within uncertainties in calibration, distance, and reddening. The center of the cluster harbors two luminous blue stragglers. Our far-UV images graphically confirm that there is no steep density gradient at small radii among the horizontal-branch stars of this post-core-collapse cluster and show no evidence for significant color gradients.

Watson, Alan M.↗

NASA NTRS1992

Optimal grazing incidence optics and its application to wide-field X-ray imaging

A class of high-resolution, efficient, and wide-field grazing incidence optics is discussed. Optical designs for searching efficiently for distant X-ray clusters are developed. It is shown that a rather general procedure exists for the design of an optical mission if well-defined scientific goals can be provided. Clusters of galaxies with an angular diameter of about 5 arcsec over a field of about 1 deg are resolved so that a comprehensive deep search is possible with a payload of 1/10 the linear dimensions of AXAF. By dropping the requirement for perfect on-axis imagery, searching within a suitably general class of telescope design, and optimizing a quantity directly related to the scientific requirement, it is shown that satisfactory designs do exist. The resulting telescope is shown to be no more difficult to fabricate than existing mirrors, and it can be nested.

Burrows, Christopher J.↗

NASA NTRS1991

The imaging performance of the Hubble Space Telescope

Problems with the HST instantaneous imaging performance and pointing performance are discussed. Optical tests have clearly demonstrated that the HST suffers from spherical aberration. The top level specification was that 70 percent of the energy be focused in a 0.1 in. radius, but the present, and close to optimum, focus setting gives only about 16 percent. The pointing control system also is having problems with the results that the spacecraft achieves a stability of about 0.007 rms in quiescent periods, falling short of specification which requires that such performance be maintained for 24 hr. The finite guidance sensors are not guiding well on faint stars. There is a loss of sky coverage at high Galactic latitude, especially for the Wide-Field/Planetary Camera. The consequences of these defects for the scientific program are examined.

Burrows, Christopher J.↗

NASA NTRS1990

On the feasibility of detecting extrasolar planets by reflected starlight using the Hubble Space Telescope

The best metrology data extant are presently used to estimate the center and wing point-spread function of the HST, in order to ascertain the implications of an observational criterion according to which a faint source's discovery can occur only when the signal recorded near its image's location is sufficiently larger than would be expected in its absence. After defining the maximum star-planet flux ratio, a figure of merit Q, defined as the contrast ratio between a 'best case' planet and the scattered starlight background, is introduced and shown in the HST's case to be unfavorable for extrasolar planet detection.

Brown, Robert A.↗