Optical telescopes: Reflectors and refractors E O Refractors use lenses E: eyepiece O: objective
Reflectors use mirrors: Newtonian Cassegrain
Objective (O) = Main light-gathering surface (lens or mirror) Aperture= a = Diameter of objective ("size" of telescope) Focal length= f = Distance from objective to image ("length" of telescope) Eyepiece (E) = Lens used to inspect image from objective
1) LIGHT GATHERING POWER (how faint you can see) • LGP ~ a2 (Bigger = better) • 2) MAGNIFYING POWER (how large image is) • MP = f(O) / f(E) (Bigger = better) • So can change this with eyepice choice, not a fundamental property of telescope • Practical limit 50 x O (in inches) • 3) RESOLVING POWER (size of detail, sharpness of focus) • Size of detail ~ 1/a (Smaller = better) • i.e., larger ‘a’ allows smaller details to be seen
Spitzer Space Telescope An infrared telescope in orbit around the Sun!
How to improve resolution? Radio interferometry The Very Large Array, Socorro, New Mexico
X-ray telescope design (Chandra X-ray Observatory) It is very difficult to focus (redirect) high energy photons, because they are so readily absorbed by matter. Thus it is difficult to tell where they came from, and to reconstruct the image.
Chandra image of X-rays emitted from hot gas surrounding galaxy Cygnus-A
X-ray image Visible (optical) Hot stars give off a lot more Xrays than cooler stars do.
Observing gamma rays Space-based telescopes (Compton Gamma ray Observatory