012.4 Telescope Limitations

Light gathering power is proportional to the area of the main mirror. The 200 inch Mt. Palomar Hale telescope has 4 times the light gathering power of the 100 inch Hooker telescope. A picture that requires a 15 minute exposure on the Hale would need a one-hour exposure on the Hooker.

Resolving Power is limited by the size of the mirror and the wavelength of light. When a telescope is pointed at a light source and turns a bit away from it, the light intensity that is sent to its focus does not drop off to zero immediately. The telescope must turn until its outer edge has moved through about one wavelength of the incoming light before the intensity drops. That one-wavelength turning angle is called the diffraction limit of the telescope's resolving power.

overcome the diffraction limit by using several telescopes placed some distance apart. Combining the light from these different telescopes means that the turning angle that drops the intensity from a distant source to zero becomes that of a mirror whose diameter is the distance between the telescopes. Such an arrangement is called an interferometer.

Earthbound telescopes such as the Hale at Mt. Palomar are not diffraction limited because they have a worse problem: Turbulence in the Earth's atmosphere. It makes stars 'twinkle' at night and causes the telescope image of a star to jump around. The image jumps too quickly for a photographic film to keep up with but not too quickly for computer controlled actuators to react. Using the actuators to distort the mirror to compensate for this jumping image problem is called adaptive optics.

Find a bright star and distort the mirror quickly enough to keep the image of that one "guide star" fixed in place. Other stars near the guide star will then be stabilized as well.

Create your own "guide star" by shining a powerful laser beam into the sky. Some light will be scattered out of the beam in all directions by dust and other things. Think of what a searchlight looks like. Use the light that is scattered straight back at the telescope as an "artificial guide star".

Avoid the atmosphere entirely by putting the telescope into orbit.

Because of adaptive optics, earthbound telescopes are now catching up to the HST. Future space telescopes will need much larger mirrors to compete. There is no limit to the possible sizes of orbiting mirrors because their support structures do not need to carry weight.