Sponge: Draw the four types of reflectors.

Light from different directions focuses at different points, and an image is formed near the prime focus.

Light entering at great angles cannot be accurately focused. Off-center star images acquire “tails” (coma).

Coma worsens as the picture moves farther from the center of view.

A Schmidt telescope uses a correcting lens, which sharpens the final image of the entire field of view.

This image is focused on a specially shaped piece of photographic film (often called a “Schmidt Camera”)

Light can also be focused on a photometer. This measures the total light received on all or part of the image.

This light information can be split into its spectrum on a photographic plate or be stored on a computer.

A larger telescope can gather more light, so it is said to have a large collecting area.

Angular resolution is the ability to distinguish between two adjacent objects in the sky (with or without a telescope).

When the angular resolution is greater than the separation of the objects, all that can be seen is a fuzzy “blob”.

Even large telescopes have angular resolution problems because of “atmospheric blurring”.

Atmospheric blurring is the result of the different densities of the air deflecting the incoming light slightly.

This causes the image to move around on the detector (or on our retina).

This continual movement of the image causes stars to “twinkle”.

After a few minutes of exposure time, the image has been smeared out over a circular region. This is called the seeing disk.

The Hubble Space Telescope avoids these problems by placing the telescope above the atmosphere. It was launched into Earth orbit in 1990.

Hubble gives images that are up to 20 times sharper than those of ground- based telescopes.

Most newer telescopes use charge-coupled devices (CCD’s) rather than photographic film.

The tiny picture elements on CCD’s are called pixels.

CCD’s have great benefits: 1) gather more light, 2) gather light info faster, 3) images can be stored digitally, 4) “noise” can be eliminated.

Radio telescopes detect cosmic radio waves. They can be used 24 hours a day and weather has no effect on the waves.

Radio astronomy opens up a new picture of the universe. Objects that produce a great deal of visible light are not necessarily very high in radio wave emission.

Objects that emit detectable levels of radio waves may be completely undetectable in the visible region of the spectrum.

Emissions can also be detected in the other regions of the spectrum. Ultraviolet, infrared, microwaves, gamma rays, x-rays, etc.

Sponge: Why do stars twinkle? Why don’t planets twinkle?