Calculating operators of continuously moving objects presents some unique challenges, especially when the operators involve aggregation or the concept of congestion, which happens when the number of moving objects in a changing or dynamic query space exceeds some threshold value. This paper presents the following six d-dimensional moving object operators: (1) MaxCount (or MinCount), which finds the Maximum (or Minimum) number of moving objects simultaneously present in the dynamic query space at any time during the query time interval. (2) CountRange, which finds a count of point objects whose trajectories intersect the dynamic query space during the query time interval. (3) ThresholdRange, which finds the set of time intervals during which the dynamic query space is congested. (4) ThresholdSum, which finds the total length of all the time intervals during which the dynamic query space is congested. (5) ThresholdCount, which finds the number of disjoint time intervals during which the dynamic query space is congested. And (6) ThresholdAverage, which finds the average length of time of all the time intervals when the dynamic query space is congested. For these operators separate algorithms are given to find only estimate or only precise values. Experimental results from more than 7,500 queries indicate that the estimation algorithms produce fast, efficient results with error under 5%.

No abstract

During its first 6 years of operation, the cold (-60°C) optical blocking filter of the Advanced CCD Imaging Spectrometer (ACIS), on board the Chandra X-ray Observatory, has accumulated a contaminating layer that attenuates the low-energy x rays. To assist in assessing the likelihood of successfully baking off the contaminant, members of the Chandra team developed contamination-migration simulation software. The simulation follows deposition onto and (temperature-dependent) vaporization from surfaces comprising a geometric model of the Observatory. A separate thermal analysis, augmented by on-board temperature monitoring, provides temperatures for each surface of a similar geometric model. This paper describes the physical basis for the simulations, the methodologies, and the predicted migration of the contaminant for various bake-out scenarios and assumptions.

No abstract

Bidirectional Reflectance Distribution Function (BRDF) measurements that characterize the optical scattering properties of surfaces are extremely important to the design of space telescopes and instruments. Sophisticated stray light analysis codes such as APART /PADE require a characterization of the scattering function of optical black surfaces at several angles of incidence and a wide range of detector angles. This paper presents BRDF measurements taken at 5 and 30 degrees incidence of both space qualified black surfaces and some new black surfaces designed for future space instrument and telescope use. These surfaces represent the optical black surfaces most commonly specified for space telescope and instrument baffle and vane surfaces. The scatter measurements were made on a newly built visible and infrared scatterometer designed for accurate measurements of the scatter function of black surfaces.