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Practical Astronomy - what kind of telescope do we need?

Telescopes can be differentiated in two ways, firstly depending on the way in which they are mounted and secondly depending on the optical setup of the telescope. Altazimuth and equatorial mounts are the primary types of mounts used for telescopes. Equatorial mounts are (were) advantageous to altazimuth mounts as equatorial mounts, by construction, only require the motion of one of it's two axis to be able to track stars reliably, instead of two in the case of altazimuth. One is better than two simply because of intrinsic errors in tracking objects. Tracking stars or astronomical objects is the primary objective of most telescopes as several minutes of continuous observation are necessary to be able to study objects well enough. Now-a-days, altazimuth mounts are just as good as equatorial mounts given the complex feedback systems involved tracking.

Coming to the optical setup of telescope, they are classified depending on the position of the eye piece with respect to the primary and secondary mirrors. Newtonian, cassegrain, maksutov are the popular optical setups implemented in astronomy. One can refer to the full list here. Most large telescopes have a modified cassegrain optical setup in which the eye piece is in the middle of the primary mirror and instruments can be placed at this point for automated recording.

Various optical setups and mount designs are what are used to popularly distinguish between telescopes. The f-number of a telescope and the consequences it has on observations, on the other hand, is a less-popular topic among amateur astronomers. f-number of a telescope is dependent on it's optical design. Depending on the f-number of a telescope, it can either be labelled deep-field or wide-field telescope. Telescopes with f-number in the range of f-7 to f-10 can be classified as a deep-field telescope as such telescopes can only capture 10 arcsec by 10 arcsec portion of the sky at a moment. On the other hand, wide-field telescopes, with f-number in the range f-2 to f-5 can cover 10 arcmin by 10 arcmin portion of the sky at a moment. Depending on the type of objects that we would like to observe using the telescope, we need to decide on the f-number of the telescope. Wide-field telescopes can cover a larger portion of the sky than deep-field telescopes can in the same amount of time. They are also better at discovering transient events like supernovae and asteroids as they can observe the same patch of the sky multiple times in a week. On the other hand, deep-field telescopes can be used to observe fainter objects than can be observed using a wide-field telescopes. Deep-field telescopes can also be used to observe objects with small angular sizes, which might not be resolved in a wide-field telescope. The SDSS telescope is an example of a wide-field telescope and the Keck telescopes are an examples of deep-field telescopes.

PS - I apologize if I there are mistakes or corrections in what I've written. Do comment and correct me if I am mistaken or if there's anything interesting related to these topics that I haven't mentioned.


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