Humans, with our improved optical, mechanical and electronic vision now peer out through the entirety of the electomagnetic spectrum back toward 13.7 billion years of history and the view is improving, to the point that the data to come, will be simply overwhelming.
Both the 1997 and 2012 images were taken in visible light with Hubble’s Space Telescope Imaging Spectrograph in its coronagraphic imaging mode. A coronagraph blocks out the glare of the central star so that the disk can be seen.
The Hubble Space Telescope is a project of international cooperation between NASA and ESA (the European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington, D.C.
The photo at the bottom is the most detailed picture to date of a large, edge-on, gas-and-dust disk encircling the 20-million-year-old star Beta Pictoris. The new visible-light Hubble image traces the disk in closer to the star to within about 650 million miles of the star (which is inside the radius of Saturn’s orbit about the Sun). When comparing the latest images to Hubble images taken in 1997 (best), astronomers find that the disk’s dust distribution has barely changed over 15 years despite the fact that the entire structure is orbiting the star like a carousel. The Hubble Space Telescope photo has been artificially colored to bring out detail in the disk’s structure.
Less important than aperture, but still helpful to know, is the focal length of an instrument. The focal length is simply the effective distance from the lens or mirror to the focal point , where an eyepiece or camera would go. In refractors and Newtonian telescopes (described more below), the focal length is the actual distance from the lens or mirror to the focal point, as shown below.
In some telescopes, such as the popular Schmidt-Cassegrain telescopes (SCTs), the optical path is folded, bouncing the light off two mirrors. In this case, the distance from the mirror to the focal plane is about twice the length of the telescope tube. However, due to the curvature of the mirrors, the effective focal length is around 5 times the length of the tube. The diagram below shows how this occurs.
Visually, the focal ratio is not tremendously important. Only when the focal ratio is very small does it begin to significantly affect the image quality. This is not normally seen in commercial telescopes. All things being equal it is easier to make a telescope with a larger focal ratio, and a larger-focal-ratio telescope will have fewer inherent aberrations. In practice, this difference is normally not noticeable.
Focal ratio is very important photographically because it determines how quickly a picture can be taken. For this reason, smaller focal ratios are called faster, while larger focal ratios are termed slower. Thus an f/5 telescope is faster than an f/10 telescope. In fact, for photography, focal ratio is more important than aperture. This surprises most new photographers. Consider the image below, which was taken with a 3" telescope.
Refracting telescopes use lenses to gather light. The primary advantages of a refractor are that it is easy to maintain, easy to use, and is capable of terrestrial observing as well as stargazing. It also has the distinction of looking like a telescope. This seems trivial, but many a reluctant spouse has been sold on a telescope that looks nice sitting in the living room. Best Telescope Reviews 2015
There are two basic categories of refractors, achromatic and apochromatic Easier to remember might be inexpensive and expensive. Even inexpensive refractors cost more than reflecting telescopes of the same size, and do not generally provide better images. The primary advantage of an inexpensive refractor is the ability to view terrestrial objects. The image in a refractor is correct, while in a reflector it is upside-down. This allows nature-watching, birdwatching, spying on your neighbors, whatever you’re into.