Telescopes & Light: Part 3 All About Observing

1. Telescopes & Light: Part 3All About Observing

2. 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.

3. 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.)

4. 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.

5. 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.

6. 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!)

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. Full Spectrum Coverage • Full-spectrum coverage is the complete picture, imaging an object at all possible wavelengths.