phys1902v-lecture10-09022022.pdf - Lecture 10 Chapter 5:...

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Lecture 10Chapter 5: Telescopes5.2: Telescope Size5.3: Images and Detectors5.4: High-Resolution Astronomy5.5: Radio Astronomy5.6: Interferometry5.7: Space-Based Astronomy5.8: Full-Spectrum CoverageTelescope Size5.2Magnification is NOT the most important aspect of telescopesbig misconceptioncan easily be changed by changing the focal length of the eyepiece lensa blurry or faint image will still appear blurry or faint when magnifiedfor example, 225x magnification of cover boxes.Magnificationdoesn’tmean good quality of imagesA better consideration is Telescope Sizethe bigger diameter, the betterLight-Gathering Power: Allows fainter objects to be seenResolving Power: Allows smaller details to be seenLight-Gathering PowerHow big the light bucket is (bucket analogy from last lecture)Larger telescopes have more light-gathering powerThe larger the primary mirror, the more photons can be gathered from the source in agiven amount of timeObserved brightness is proportional to the area of the mirror, which is proportional to thesquare of its diameter:𝐴 = 𝜋?2→ ? =?2𝐴 = 𝜋 (?2)2
𝐴 =𝜋4?2Where d is the diameter, r is the radius, and A is the area.Example. Suppose you have an 8-inch (20 cm) diameter telescope.How many times more light-gathering power does the Hubble Space Telescope (HST) (diameter 2.4 m) have?𝑌????????????:?2=(0.2?)2= 0.04?2𝐻??:?2=(2.4?)2= 5.76?2?𝑎?𝑖???𝐴??𝑎?:5.76?20.04?2= 144HST has 144 times more light gathering power than your 8-inch telescopeLight-Gathering PowerLarger diameter telescopes have more light-gathering power, so they can see fainterdetails better than a smaller telescopePhoto (b) was taken with a telescope twice the diameter of the telescope that took photo(a), with the same exposure time (Andromeda galaxy, the nearest spiral galaxy to our own,milky way)Resolving PowerLarger telescopes have better resolving power: - ability of a telescope to:better distinguish objects that are closer together in the field of viewsee smaller detailsWhen a telescope’s angular resolution is poor, two objects that are close together appearas a single fuzzy “blob”
When a telescope’s angularresolution is good, the two sources are discernible as separateobjectsResolving power is limited by diffraction.Larger diameter telescopes experience less diffraction than smaller diametertelescopesShorter wavelengths diffract less than longer wavelengthsTo improve resolving power (less diffraction):Use a larger diameter telescopeObserve shorter wavelength radiationAngular resolution is directly proportional to wavelength and inversely proportional totelescope’s diameter:angularresolution(”)=0.25wavelength(μm)diameter(m)We want small angular resolution to distinguish objects that are close together, or to seesmall detailsDetail becomes clearer in the Andromeda Galaxy as the telescope’s angular resolutionimproves (gets smaller)when the angular resolution is 10, it is very blurry. If youdecrease it to 1’’, the image becomes shaper and clearer.

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Term
Winter
Professor
Staff
Tags
Astronomy, Telescopes, Wavelength, Hubble Space Telescope, Radio astronomy

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