Why it is desirable for telescopes to have a large diameter objective lens or mirror in terms of both sensitivity
and resolution.
A
telescope is a device used to magnify distant objects or images so that they stretch over your retina. (Your retina is too small to collect more light from the image and collect a brighter image) The parts that make this possible are
The two types of
telescopes:
*Refractor telescope which uses a glass lens
*A reflector telescope which uses mirrors instead of the lens.
*An eyepiece lens takes the bright light form the focus of the objective lens or primary mirror and magnifies it to take up a large portion of the retina. This is much like a magnifying glass.
It takes a small image and spreads it out over the retina of your eye so that it looks big. Both components combined create a telescope. The objective of a telescope is to collect lots of light to form a bright image inside the telescope and then magnify the bright image so that it takes up a lot of space on your retina.
The aperture. The telescope’s ability to collect light is directly related to the diameter of the objective lens or primary mirror. Generally the bigger the lens or mirror, the more light the telescope collects and brings to focus, and the brighter the final image.
The ability to see fine detail in an image depends on the telescope’s capacity to separate or resolve two close objects. Resolution depends on the aperture of the telescope, the quality of the optics and the observing conditions (such as dark, dry, humid, light)
Sensitivity: It’s ability to pick up faint objects for the observation, or it’s light gathering power. This depends on the collecting area of the lens and mirror and how big the diameter (the bigger the better).
Interferometry:
Is a technique used to combine the data form several elements of an antenna array in order to achieve a higher resolution.
The Very Large Array is an interferometer which combines data from each element of the array to form an interference pattern. Computers are used to mathematically analyse these patterns to reveal information about the structure of the radio source.
Interferometry has been used to correct blurred images from large optical telescopes. Speckle interferometry.
Active optics:
Uses a slow feedback system to correct sagging or other deformities in the primary mirror of large modern reflector. 8-10m telescopes Recent telescopes, unlike the older versions, have a thin mirror approximately 20cm thick. These mirrors will change shape as the telescope changes direction or heats up or cools down. However, the back of the mirror is fitted with many actuators that can push the mirror back into shape.
When light leaves the primary mirror and before it reaches the final lens, it is slowly sampled by a wavefront sensor. This type of interferometer can detect how the incoming light has been altered. By sampling slowly, the effect of the atmospheric turbulence is eliminated and any remaining effect is then due to deformities in the primary mirror. A computer calculates the required shape adjustments and then loves to the actuators as required every few minutes.
Adaptive Optics:
Use a fast feedback system to attempt to correct for effects of atmospheric turbulence.
A wavefront sensor is still used between the primary mirror and the lens but rapid computer corrections are fed to one or two secondary mirrors that straighten out the light. These corrections are made at up to 1000 times per second, and this speed is the major difference between adaptive and active optics.
One of the possible secondary mirrors is called a tip-tilt mirror, which is able to adjust for slight changes in the position of the light. The other is a deliberately deformable mirror to adjust deformities in the light. This type of image correction still needs some development before it can be applied to larger telescopes.