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Practical astronomy - types of instruments

As mentioned earlier, depending on the science that we would like to extract from the telescope, we will have to decide on the instrument.

The most basic science expected from a telescope is regarding the position of stars i.e astrometry. Precise and accurate mount design will help us track astronomical objects better and a CCD with large number of thin pixels will help us image the object's spatial distribution or position precisely. The Hipparcos and Tycho satellites were pioneering in this regard, resulting in some of the most extensive and accurate maps of the milky way galaxy. Data sets from the Hipparcos, Tycho and a combined data set have paved the way for future work, inspiring Gaia.

Equipped with precise positions of stars, one can begin a photometric study to quantify the temperature, luminosity, variability and other such properties of stars.  As mentioned yesterday, photometric studies can be used to construct the HR diagram, a primer to understanding stellar evolution and it's dependence on luminosity or mass of a star. A photometric survey of astronomical objects is also used to distinguish between stars and galaxies. This can be done by looking at the color-color space, derived from the brightness of stars in multiple filters.

From photometric observations, once galaxies candidates are identified, they can be studied using spectroscopy. Just to be clear, i'm not implying that stars are not objects of spectroscopic interest! Spectroscopic study of a galaxy can yield the rotation curve of the galaxy i.e differential rotation observed in a galaxies. Study of the galactic rotation curves led to the discovery of dark matter as the observed curves did not match the theoretically predicted curves.Spectroscopic studies also help understand the composition of stars and galaxies by looking for specific emission or absorption lines. Emission and absorption lines are unique finger prints that atoms and molecules leave behind. Spectroscopy is also used to confirm the discoveries of the furthest known galaxies, by calculating the redshift of the galaxy.

Polarimetry and Interferometry are niches in astronomy, each used to understand it's own branch of astronomy. Polarimetry is used to study the amount of polarization of light we receive, from stars, galaxies and every object there is. It has been used to estimate the amount of dust and particulate matter in the galaxy and understand the galaxy's magnetic as a whole.

Polarimetry and Interferometry are quite significant in radio astronomy i.e the study of astronomical objects in the radio frequency portion of the EM spectrum. The diffraction limit poses a huge hurdle while observing astronomical objects in the radio spectrum and in attempting to pinpoint their positions. This figure can further give you an idea of the size of a primary mirror needed to study objects in various parts of the spectrum and image them to the diffraction limit. Interferometry is a consequence of the limit posed by the size of a primary mirror that we can construct and operate. And given the nature of telescopes in radio astronomy, the study of polarization of sources is trivial and doesn't require the use of a special polarimeter, as is the case with optical and infrared astronomy. This is a consequence of the fact that optical detectors cannot, as of now, register the phase of an incoming photon and information of it's phase is destroyed after capturing it, using a CCD. On the other hand, a radio detector captures the waves emitted by sources and therefore, preserves the phase information conveyed by the radiation.

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