Light gathering Power: Magnification with eyepiece:

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1 Telescopes

2 Light gathering Power: The amount of light that can be gathered by a telescope in a given amount of time: t 1 /t 2 = (D 2 /D 1 ) 2 The larger the diameter the smaller the amount of time. If you get to use a telescope that is 3 times the diameter, how much less time will it take to gather the same amount of flux? Magnification with eyepiece: Only useful for extended objects: mag = f scope /f eyepiece Typically 25 mm, 12.5 mm, and 6 mm. For a 1m focal length, magnifications are x40, x80, and x170 power.

3 Field of View: The angular area of the sky imaged on a CCD or viewed through the eyepiece The field of view is a function of the optics of the eyepiece itself and its magnification which is a function of the telescope focal length

4 F-ratio The ratio of the focal length to the aperture: f/ = f/d Telescopes with small focal lengths (e.g.,f/2) are considered fast while ones with large focal lengths (e.g., f/8) are considered slow. Image Scale: CCDs have individual photoreceptor elements with sizes typically as: 9-30 µ = mm at optical wavelengths image scale (arcsec/pixel) = ( µ)/f µ is the pixel scale in microns and f is the focal length in mm Pixels should be about two times smaller than the resolution delivered by the optics + atmosphere

5 Campus Telescope The campus telescope is 0.8m in diameter and has a focal ratio of f/6 The CCD on the telescope has 4096x4096 pixels that are 9 µm in size What is the image scale of the CCD? What is the field of view of the CCD?

6 Nyquist sampling thereom: Optimal sampling requires ~2 pixels per resolution element. Sampling at finer resolution than does not yield more information. Sampling more coarsely means you are losing information

7 Limit of Resolution Diffraction makes the image of a point source into a central maximum surrounded by circular rings. Resolution: sinθ = 1.22 (λ/d) The position of the first null in the Airy disk

8 Resolution The angular separation of 2 objects such that the maximum of the first falls on the first minimum of the second. The atmosphere sets a limit on resolution because the waves do not all follow the same path through the atmosphere. This limitation is called seeing Adaptive optics move segments of the telescope mirror independently and in real time to account for these travel differences through the atmosphere and to, therefore, Improve the image beyond the usual seeing

9 What is a CCD Detector? This is basically the equivalent of a digital camera that we put on the back of a telescope CCDs are the detection device generally used in optical and infrared astronomy We want to count the number of photons that fall in each bin on a grid to get a measurement of the brightness of objects in the sky The image of the sky is recorded as a pattern of electric charge Image area Metal,ceramic or plastic package Serial register Connection pins Gold bond wires Bond pads Silicon chip On-chip amplifier

10 How do we count photons in each bin? A photon is absorbed by the valence band of a silicon semiconductor moving the electron to the conduction band by the photoelectric effect Photons with wavelengths longer than ~1.2 µm don t have enough energy to push the electron beyond the band gap Impurities in the silicon make the holes migrate away from the positive gate when biased with a positive voltage The full well capacity is the max. number of electrons that can be stored before electrons and holes recombine Increasing energy Conduction Band Valence Band 1.26eV Hole Electron

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12 How do we count photons in each bin? To read the pixels the voltages are varied so that the electrons are shifted by columns and rows into a readout register The number of electrons are then converted to voltages plus a small bias offset (amplification) This voltage is sent to an A/D converter (analog-to-digital) which computes that voltage to a number, the maximum value of which is defined by the number of bits for the device (12, 14, or 16 bits give 4096, 16384, and respectively)

13 Photoelectric effect causes electrons to move from one band to the other in the silicon crystal Increasing energy Conduction Band Valence Band 1.26eV Hole Electron

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23 ccd1/ccd_anim.gif

24 The image area is positioned at the focal plane of the telescope

25 Every third electrode is connected together making a pixel. Wires running down the edge of the chip make the connection. Channel stops keep charge from moving between columns.

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27 CCD Characteristics CCDspec/RNvsGain.html