How is the diffraction limited angular resolution of an optical instrument defined?
Diffraction results from the wave nature of light and is determined by the finite aperture of the optical elements. An optical instrument collects light through an aperture, so any point source of light will be imaged as a diffraction pattern, of which most is contained in the central spot. This limits the resolution of the instrument, that is, the ability to distinguish between closely spaced objects. For example, a telescope may not be able to distinguish two stars that are very close together, because their diffraction patterns overlap. Rayleigh’s criterion states that any two points with their angular separation less than theta=(1.22*wavelength)/d may not be resolved. Here d is the diameter of aperture. The factor 1.220 is derived from a calculation of the position of the first dark circular ring surrounding the central Airy disc of the diffraction pattern. Thus the bigger the aperture of a telescope, the better is its resolving power.
What is Martian hemispheric dichotomy?
The Martian dichotomy is the sharp contrast observed between the southern and northern hemisphere. The northern hemisphere is young, smoother and lower in elevation. Whereas southern hemisphere is old, cratered, rugged surface which is higher in elevation. Moreover the crustal thickness of southern hemisphere is about 58 km which is almost double the crustal thickness of northern hemisphere (32 km). The smoother northern surface with very less number of craters indicate a large event of resurfacing in the later stages of planetary evolution. This is explained by theories like a massive impact from meteorite or asteroid and Early plate tectonic activity like plate spreading.
Is the universe flat or does it have any curvature?
According to general relativity theory by Einstein, massive objects warp the space-time around them, and the effect this warp has on other objects is gravity. So, locally, space-time is curved around every object with mass. But when the large scale structure of universe is considered the curvature depends on the density of matter and energy of the whole universe. If this density is equal to critical density, then the universe has zero curvature and is considered flat and infinite. If it is greater than critical density then the universe will have a positive curvature like a sphere and will be closed i.e. at one point it will stop expanding and will start to contract. Whereas if the density value is less than critical density, then the universe will have a negative curvature like the surface of a saddle and will keep on expanding forever and hence called an open universe. Many scientists believe that the density value is just equal to critical density and hence a flat universe. Measurements from the Wilkinson Microwave Anisotropy Probe (WMAP) have shown the observable universe to have a density very close to the critical density. Of course the actual universe is much bigger than observable universe. But from the observable universe, it seems that the universe is flat and has zero curvature.