Handbook of X-ray astronomy

"It may be obvious why visible astronomy utilizes images, but it is illustrative to consider the value of focusing to X-ray astronomy. A list of advantages offered by the best possible two-dimensional angular resolution would include: (i) Resolving sources with small angular separation and distinguishing different regions of the same source. (ii) Using the image morphology to apply intuition in choosing specific models for quantitative fits to the data. (iii) Using as a "collector" to gather photons. This is necessary because X-ray-source fluxes are so low that individual X-ray photons are detected; the weakest sources give less than one photon per day. (iv) Using as a "concentrator," so that the photons from individual sources interact in such a small region of the detector that residual non-X-ray background counts are negligible. (v) Measuring sources of interest and simultaneously determining the contaminating background using other regions of the detector. (vi) Using with dispersive spectrometers such as transmission or reflection gratings to provide high spectral resolution. The Earth's atmosphere completely absorbs cosmic X-rays. Consequently, X-ray observatories must be launched into space; so size, weight, and cost are always important constraints on the design. In practice this leads to a trade-off between the best possible angular resolution and the largest possible collecting area. Realizing an X-ray telescope involves two key issues: reflection of X-rays, and formation of an image"--