Telescopes and their configurations. Quick review at the GO level

Transcription

1 Telescopes and their configurations Quick review at the GO level

2 Refraction & Reflection Light travels slower in denser material Speed depends on wavelength

3 Image Formation real Focal Length (f) : Distance from lens to focus Focal Ratio (f/#) : f / lens diameter (D) virtual real virtual A real image can be projected onto a screen, film or detector

4 Galileo Telescope Strong lens Weak lens Long focal length This telescope makes a magnified virtual image Retina: detector array of 25 million photoreceptors

5 Galileo Telescope Strong lens Weak lens Long focal length This telescope makes a magnified virtual image Where would the detector go?

6 Kepler telescope Telescope evolution

7 Lick 1-meter telescope Why so long?

8 Problems with lenses?

9 Reflecting Telescopes Who came up with this idea?

10 Telescopes with long focal lengths in a shorter telescope LBT, Magellan, VATT, Giant Magellan Cassegrain Parabolic primary and ellipsoid secondary Parabolic/hyperbolic primary and hyperbolic secondary Gemini

11 Focii Primary Cassegrain Nasmyth

12 What are the advantages of each of these focii?

13 Examples Nasmyth Subaru f/2, f/12.2, f/12.6 LBT

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15 Mounts one rotational axis parallel to the Earth's axis of rotation equatorial Alt-azimuth

16 Angular Magnification For look through telescopes

17 For a Detector Array Put the detector at the focus (orange) and consider the angle that subtends 1 mm For a telescope where there is no eye piece the angular magnification is specified by the Image scale or plate scale, θ: the number of arcseconds imaged onto 1 mm of the detector

18 Field of View Field of view depends on the linear scale, L, of the detector The larger the focal length the larger the angular magnification The smaller the focal length the larger the field of view.

19 Summary The configuration of the telescope determines the angular magnification, the field of view, the amount of light gathered, and the mounting possibilities for the instruments Next section: diffraction

20 Diffraction 1803, Thomas Young: Light behaves like waves

21 Interference Young s sketch to Royal Society (1803) What happened in 1961 and 1974?

22 What happens if you use electrons instead of photons?

23 What happens when you fire electrons at the 2 slits one at a time?

24 Jonsson at Tubingen: eform defraction patterns (1961) Merli at Bologna: e- one at a time make defraction patterns (1974)

25 Interference Young s sketch to Royal Society (1803) Jonsson at Tubingen: e- form defraction patterns (1961) Merli at Bologna: e- one at a time make defraction patterns (1974) But if you measure which slit the e- go through -> no diffraction pattern

26 Diffraction Limit limiting angular separation of two point sources in the sky

27 Question What is the size of the pixels that we need to resolve a diffraction limited image on the detector which sits at the focal plane?

28 Question What is the size of the pixels that we need to resolve a diffraction limited image on the detector which sits at the focal plane? θ=x/f Diffraction limit angle Angular extent of a pixel size x

29 Diffraction Limit The f-stop = f/# = f/d The focal length/lens diameter Considering the viewing of a uniformly lit field, the brightness of the projected image (illuminance) relative to the brightness of the scene in the lens's field of view (luminance) decreases with the square of the f-number Tells you the optimal pixel size for a telescope, which depends on D and f (f/#) and wavelength range.

30 Diffraction Limit My new Nikon 300 Middle f-stop: f/8 Pixel size of CCD is 5.3 um Is this consistent with the optics Consider our eyes Middle f-stop: f/5 Max density of cones 70,000/mm 2 Spacing is 2.5 um Is this a reasonable design? Tells you the optimal pixel size for a telescope, which depends on D and f (f/#) and wavelength range.

31 Diffraction Limit Tells you the optimal pixel size for a telescope, which depends on D and f (f/#) and wavelength range. For my Nikon 300 Middle f-stop: f/8 X = 1.22 * 8 * 0.42 um = 4.1 um Real pixel size is 5.3 um! Not bad For our eyes Middle f-stop: f/5 X = 1.22 * 5.7 * 0.42 um = 2.9 um Max density of cones 70,000/mm 2 Spacing is 2.5 um Nice design!

32 Atmospheric effects

33 Atmospheric Effects Seeing FWHM of optical image of a point source The Strehl ratio: the ratio of the peak aberrated image intensity from a point source compared to the maximum attainable intensity using an ideal optical system limited only by diffraction over the system's aperture. Alternatively one uses not of the peak intensity but the intensity at the image center (intersection of the optical axis with the focal plane) due to an on-axis source

34 Point spread function: Rayeigh s criterion Angular Resolution degraded by atmospheric turbulence to arcsecs (called seeing )

35 Adaptive Optics Object & Guide star AO slides from Claire Max

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41 Note PSF varies with time

42 Summary Telescope configurations define some observational limitations The diameter establishes: the diffration-limit spatial resolution the light bucket size The focal length affects the angular magnification the photon flux in the detector combined w. array size gives FOV The focal number indicates the power of the lens The mount can affect time-sequenced observations AO capabilities increases spatial resolution in IR measurements mess up photometric measurements (variable PSF)

43 Telescopes

44 Field of View L/2 Field of view depends on the linear scale, L, of the detector

45 Adaptive Optics Object & Guide star AO slides from Claire Max