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Radio Emission from Stellar Objects
Published in Ronald L. Snell, Stanley E. Kurtz, Jonathan M. Marr, Fundamentals of Radio Astronomy, 2019
Ronald L. Snell, Stanley E. Kurtz, Jonathan M. Marr
Can we measure the sizes of stars like our Sun at radio wavelengths? The closest Sun-like star, Alpha Centauri A, has an angular diameter (see Section 1.2.4) of only 3.5×10−8 radians or 7.2 mas (milli-arcsecond). Alpha Centauri This would require a resolution just beyond the capabilities of current radio interferometers, such as the JVLA and ALMA. Higher angular resolution can be achieved with very long baseline interferometry. However, to obtain detections, such techniques require sources with much higher brightness temperatures than the thermal emission from the photospheres of stars. Thus, the angular sizes of stars like our Sun cannot currently be measured at radio wavelengths. More massive main sequence stars have larger angular diameters; however these stars are generally much more distant, so they have even smaller angular sizes, and are also impossible to measure. Main sequence stars
Fundamentals of Ocean Optics
Published in Victor Raizer, Optical Remote Sensing of Ocean Hydrodynamics, 2019
The angular resolution of an optical system is severely limited by atmospheric turbulence (aerosol impact is not considered). Instead diffraction-limited angular resolution αopt ~ λ/D, it will be αatm ~ λ/r0. The reduction is by factor D/r0 which, e.g., for IKONOS imagery is D/r0 ~ 0.7/(0.1 ÷ 0.2) ~ 3 ÷ 7 (note that for regular optical telescope D/r0 ~ 25). For example, in the case of an atmosphere/sensor imaging system with the ratio of D/r0 = 5, the speckle (glitters) patterns will be spread over an area approximately five times larger than the diffraction-limited PSF. The reason of data degradation is optical turbulence which exists even at excellent seeing conditions. It means, in particular, that ocean surface features registered in the optical (IKONOS) images always will comprise some geometrical distortions (blurring effects) due to turbulence-induced changes in pixel resolution. In some situations, this can limit capabilities of thematic data analysis and the detection capacity.
Atmospheric Sounding
Published in Iain H. Woodhouse, Introduction to Microwave Remote Sensing, 2006
Microwave radiometers will therefore tend to have relatively poor angular resolution. The other factor that influences the angular resolution is the size of the antenna. In Section 4.4 we saw that the angular resolution of an antenna is directly proportional to the wavelength, so that as the measurement frequency gets lower, the angular resolution gets proportionately larger. For atmospheric sounding, this is not a drastic problem — a footprint in the order of tens of kilometres or more in diameter is quite appropriate for mapping variations in atmospheric properties such as temperature and chemical composition since these properties change slowly over horizontal scales. For atmospheric studies it is usually more important to achieve high measurement accuracies, with high temporal frequency (< 1 day) and with a high vertical resolution for profile measurements (ideally down to kilometres).
The key performance indicators of projection-based light field visualization
Published in Journal of Information Display, 2019
Peter A. Kara, Roopak R. Tamboli, Oleksii Doronin, Aron Cserkaszky, Attila Barsi, Zsolt Nagy, Maria G. Martini, Aniko Simon
There is a linear relationship between these parameters. This means that if the angular resolution is doubled while the pixel size remains the same, the depth budget becomes twice as big. The same is true if the pixel size is doubled while the value of the angular resolution is unchanged. The only condition that must apply for this linear relationship to be true is that the pixel size needs to be significantly smaller (at least one order) than the projector period. The projector period is technically the physical distance between the adjacent projectors in the array (see Figure 1). If this condition is not fulfilled by the display system, the depth budget will increase faster than linearly. Although this may result in a fairly large depth budget, it also means that it will become challenging to perceptually distinguish the different depth levels of visualization.