Telescopes & Light: Part 3All About Observing

Detectors

• CCDs (charge-coupled devices), much like what is found in your digital cameras, are used to produce images with telescopes (CCD is placed where the eyepiece would normally go).

• CCDs replaced photographic plates as the standard imaging detector.

• CCD data is manipulated using various computer software packages.

Image Processing

• Images taken on research quality CCDs are always monochromatic - the colors are added in after the fact.

• Say you take data through a blue filter (which blocks out all but blue light) and then through a red and green filter. You can assign each image its color and then add the three images to produce a “true color” image. (True color means that’s how the object imaged would look if you could view it yourself up close.)

Light-Gathering Power

• A telescope’s light-gathering power increases as the square of its diameter.

• The more light you can gather, the shorter your exposure times. So you use a larger telescope (larger mirror diameter) to observe fainter objects.

Resolving Power

• Resolution - ability to form distinct separate images of objects lying close together in the field of view.

• The better the resolution, the more detail you can see.

• In astronomy, we talk about the separation of objects on the sky, or the angular resolution.

Atmospheric Blurring• Atmospheric turbulence can blur the light from a star (or galaxy,

etc.) as the light passes through the atmosphere.• Astronomers use the term seeing to describe this blurring effect.

The smaller the seeing, the crisper your image.• Good seeing means the atmosphere is fairly stable.

(Unfortunately, the best seeing often comes along with clouds!)

New Telescope Design• Two techniques - both used to improve seeing.

– Active optics - changes the set-up (mirror temperature, airflow, etc.). See below left.

– Adaptive optics - changes the shape of the mirror (mirror resembles a “honeycomb” shape with many small mirrors making up the primary mirror - each mirror can be moved independently to achieve the best focus). Usually focus telescope using a laser. See below right.

The Value of Radio Astronomy

• Sun is a weak radio source, so radio observations can cover nearly the entire sky.

• Observations can be made during daytime to within a few degrees of the sun.

• While visible light can be blocked by gas and dust between us and the object, radio waves usually pass through unaffected.

• One drawback - usually has poor angular resolution.

Interferometry• Two or more telescopes are

used in tandem to observe the same object at the same time (telescopes combined in such a way are called an interferometer).

• The effective diameter is equivalent to the distance between the outermost dishes. The larger diameter results in much better angular resolution.

Infrared and Ultraviolet Astronomy

• Infrared telescopes are often optical telescopes used with detectors sensitive to longer wavelengths. There are only a few windows (wavelengths) where IR radiation is not absorbed by the atmosphere.

• Ultraviolet observations have to be done from space since UV radiation is mostly blocked by Earth’s atmosphere.

High Energy Astronomy• X-ray telescopes are space based.• Currently, we have the Chandra X-ray Observatory.• Gamma ray telescopes simply count photons received - no

image is produced.Image of supernova remnant.

Full Spectrum Coverage• Full-spectrum coverage is the complete picture, imaging an

object at all possible wavelengths.