Book Review – Observational Astronomy

An optical telescope gathers and focuses light mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct visual inspection, to make a photograph, or to collect data through electronic image sensors.

Sources of noise in optical telescopes can be shot noise (associated with the particle nature of light), and thermal noise (from dark current and electronics).

The key is to increase signal-to-noise. This can be by using bigger mirrors, longer exposure times, stacked exposures, cooling to reduce dark current in sensors, use of low-noise electronics, use of narrow band filters, reduce light pollution, better mirror and lens coating, good baffling, clean optics, blackened interiors, pixel size optimisation, and using better sensors and post-processing.

I’m guessing it must be possible to use “bracketing” (or multiple exposure times or short and long integrations). This can be taking multiple exposures of the same target with different settings. High Dynamic Range (HDR) astronomy, e.g. exposure bracketing to avoid saturation, bracketing across filters (broadband-narrowband), and optimising the sensors to take low gain (high dynamic range) and high gain (better faint sensitivity). 

Bigger mirrors means bigger telescopes, which means more light, means seeing fainter galaxies, more distant quasars, and more early-universe objects. But bigger telescope means also better angular resolution, which means better separation between stars, bright stars (better contrast), seeing more structure, and better position measurements.

I guess that the limit on ground-based optical astronomy is air turbulence, points start to look fuzzy. Not sure which is more important on the ground, is the limit on angular resolution set by the telescope diameter or by atmospheric turbulence? I suspect that a high-altitude installation reduced turbulence (e.g. stable airflow) and the stable, colder, drier conditions are an advantage (e.g. reduced cloud cover, clear nights, low humidity), with reduced light pollution a great bonus. Obviously, a space-based telescope will be an order of magnitude better for everything (except maintenance).

The level of detail (maybe the theoretical limit) is set by the aperture of telescope (assuming perfect lens, conditions, etc.). An infinitely large aperture would bring all the light captured to a single focal point, where every part of the wavefront would contribute, perfect constructive interference would occur only at that one point, and the resultant focus would be infinitely sharp. But, if I understand things correctly, a mirror or lens of finite size means the wave is abruptly stopped at the edge. The wavefront cannot “end smoothly”, and the edge acts like a source of diffracted wavelets. These wavelets are slightly off-centre because they will have slightly different phases and path lengths (e.g. partial cancellation occurs). Instead of a perfect focal point, the focal plane intensity will be a central bright spot (Airy disk), and some faint surrounding rings. The single “perfect” focal point can’t be infinitely narrow, it will be a diffraction-limited spot.

Christiaan Huygens was best known for his wave theory of light, and Augustin-Jean Fresnel adapted Huygens’s principle to give a complete explanation of the rectilinear propagation and diffraction effects of light in 1821.

Refracting telescope were the earliest type of optical telescope. Refracting telescopes typically have a lens at the front, then a long tube, then an eyepiece at the rear, where the telescope view comes to focus. The next major step in the evolution of refracting telescopes was the invention of the achromatic lens, a lens with multiple elements that helped solve problems with chromatic aberration and allowed shorter focal lengths. Chromatic aberration is a failure of a lens to focus all colours at the same point (it produces “fringes” of colour along boundaries).

A reflecting telescope uses a single or a combination of curved mirrors that reflect light and form an image. Although reflecting telescopes produce other types of optical aberrations, it is a design that removed chromatic aberration and allowed for very large diameter objectives and shorter focal lengths.

and scattering and absorption (leading to changes in signal intensity, phase, or polarisation).