Telescopes

Looking at distant objects

A telescope is designed to observe distant objects. Unlike a microscope, it usually views objects so far away that incoming rays are nearly parallel. A telescope's main jobs are to collect light, improve angular resolution, and magnify angular size.

Astronomical telescopes are judged less by magnification alone and more by aperture, resolution, sensitivity, and image quality.

Refracting telescope

A refracting telescope uses lenses. The objective lens has a long focal length and forms a real image of a distant object near its focal plane. The eyepiece then magnifies this image for the eye.

For an astronomical refracting telescope adjusted for relaxed viewing, the angular magnification is approximately

M=-rac{f_o}{f_e}

where $f_o$ is objective focal length and $f_e$ is eyepiece focal length. The negative sign indicates the image is inverted.

Reflecting telescope

A reflecting telescope uses a curved mirror as the objective. A concave primary mirror collects light and focuses it. Secondary mirrors redirect the light to a convenient viewing or detector location.

Reflectors are widely used in astronomy because large mirrors can be supported from behind, avoiding some difficulties of large lenses. Mirrors also avoid chromatic aberration because reflection does not depend strongly on wavelength in the same way refraction does.

Light-gathering power

The amount of light a telescope collects is proportional to the area of its aperture:

ight)^2$$ where $D$ is aperture diameter. A larger aperture gathers more photons, allowing dimmer objects to be seen. Compared with a pupil diameter $d_p$, light-gathering power scales as $$left(rac{D}{d_p} ight)^2$$ This is why large telescopes reveal faint galaxies and nebulae. ## Angular resolution Diffraction limits the smallest angular separation a telescope can resolve. For a circular aperture, the Rayleigh criterion gives approximately $$ heta_{min}=1.22rac{lambda}{D}$$ Larger aperture improves resolution. Shorter wavelength also improves resolution. Atmospheric turbulence often limits ground-based visible telescopes. Adaptive optics can partially correct for this by deforming mirrors in real time. ## Magnification limits High magnification spreads light over a larger image and does not create detail beyond the resolution limit. Empty magnification makes an image larger but not sharper. Useful magnification depends on aperture, optical quality, atmospheric seeing, and detector resolution. This is why small inexpensive telescopes advertising extreme magnification can be misleading. ## Radio telescopes and interferometry Radio wavelengths are much longer than visible wavelengths, so a single radio dish has poorer angular resolution for the same diameter. Radio astronomers use interferometry, combining signals from widely separated antennas. The effective baseline can be much larger than any single dish. Interferometry also works in optical and infrared astronomy, though with greater technical difficulty. ## The big idea Telescopes collect and focus light from distant objects. Refractors use lenses; reflectors use mirrors. Aperture controls light-gathering power and diffraction-limited resolution, while magnification changes angular size. The best telescope is not simply the one with the highest magnification, but the one with the best light collection, resolution, and optical quality for the task.