prak astro

Information about prak astro

Published on November 15, 2007

Author: Elena

Source: authorstream.com

Content

1 The CCD detector:  1 The CCD detector 1.1 Introduction 1.2 History of the CCD 1.3 How does a CCD work ? 1.4 Advantages of CCDs 1.5 The CCD as a 3 dimensional detector 1.6 Observations with a CCD 1The CCD detector:  1The CCD detector 1.1 Introduction It seems that Uranus was the first celestial object to be photographed by a CCD in 1975 by astronomers at the JPL and University of Arizona. This image has been obtained by the 61 inche telescopes located at Santa Catalina mountains near Tucson (Arizona). It has been made at the 8900 Å wavelenght in the near Infrared. The dark region in the image correspond to an absorption region with some Methane bands close to the southern pole of Uranus. 1 The CCD detector; 1.2 History :  1 The CCD detector; 1.2 History Determining the brillance distribution of an astronomical object (star, planet, galaxie, a martian spacecraft ?) with the help of a CCD is pretty much similar to the measurments of the quantity of infalling rain on a farm. As soon as the rain stops, collecting buckets are displaced horizontally on conveyor belts. Then the water content of the buckets is collected in other buckets on a vertical conveyor belt. The overall content is sent onto a weighting system. 1 The CCD detector:  1 The CCD detector 1.3 How does a CCD work ? The way a CCD works is illustrated by means of a simplified CCD made of 9 pixels, an output register and an amplifier. Each pixel is divided in 3 regions (electrodes who serve to create a potential well). (a) when an exposure is made, the central electrode of each pixel is maintained at a higher potentiel (yellow) than the others ( green) and the charges collecting process takes place. (b) At the end of the exposure, the electrodes potentials are changed and charges transfered from one electrode to the other. To Output amplification Output register Pixel Electrodes Electrons (a) (b) 1 The CCD detector:  1 The CCD detector 1.3 How does a CCD work ? By changing in a synchronized way the potential of the electrodes, electrons are transfered from pixel to pixel. Charges on the right are guided to the output register (b) The horizontal transfer of charges is then stopped and charge packages at the output register are transfered vertically, one by one, to an output amplifier and then read one by one. The cycle starts again until all the charges have been read (reading time of about 1 minute for a large CCD). (b) (a) Impurity (doping) 1 The CCD detector:  1 The CCD detector 1.4 Advantages of CCDs 1) Good spatial resolution 2) Very high quantum efficiency 3) Large spectral window 4) Very low noise 5) Large variations in the signal strengh allowed 6) High photometric precision 7) Very good linearity 8) A reliable rigidity 1 The CCD detector:  1 The CCD detector 1.4 Advantages of CCDs Mosaïc of 4 CCDs, containing each 2040 x 2048 pixels. This composite detector is about 6 cm large and contains a total of 16 millions pixels (Kitt Peak National Observatory, Arizona). 1 The CCD detector:  1 The CCD detector 1.4 Advantages of CCDs Quantum efficiency curves of different types of CCDs as a function of the wavelenght compared to the one of other detectors. We can see on this plot the large domain of wavelenghts for the spectral response of CCDs. 1 The CCD detector:  1 The CCD detector 1.4 Advantages of CCDs Very thin CCDs are characterized by a very large spectral reponse. 1 The CCD detector:  1 The CCD detector 1.4 Advantages of CCDs CCDs are extremely linear detectors, i.e the recieved signal increase linearly With the exposure time. The CCD thus enables the simultaneous detection of very faint objects and bright objects. In contrast photographic plates have a very limited linear regime. First of all there is a minimum exposure time below which no image of the object forms. At some higher degree of exposure, the image gets quicly saturated 1 The CCD detector:  1 The CCD detector 1.4 Advantages of CCDs (a) (b) (c) “flat field” technique (see text below) 1 The CCD detector:  1 The CCD detector 1.5 The CCD as a 3 dimensional detector 1.6 Observations with a CCD As can be seen from this serie of 4 exposures ( figures above + next page ) of 1, 10, 100 and 1000 sec, of the M100 galaxy, obtained with a 11 inches Celestron telescope, the signal to noise ratio changes in a crucial way as a function of the exposure time 1 The CCD detector:  1 The CCD detector 1.6 Observations with a CCD Additionnaly to the improvement of the S/B ratio as a function of the exposure time , we can also clearly see the change in the regime of the noise, mainly caused by the readout noise of the CCD in the shorter exposure, and to the photons noise in the sky for the longest exposure. 1 The CCD detector:  1 The CCD detector 1.6 Observations with a CCD 1.6.1 Substraction of the bias Raw image ... …Processed image 1 The CCD detecttor :  1 The CCD detecttor 1.6 Observations with a CCD 1.6.2 The darks Sn(t) = Rn0 2(T - T0) / T t. (1.6.2.1) 1 The CCD detector:  1 The CCD detector 1.6 Observations with a CCD 1.6.2 The darks ST = N S et BT2 = (N B2), (1.6.2.2) ST / BT = (S / B) N . (1.6.2.3) S = Sa - ST et B = (Ba2 + BT2), (1.6.2.4) S / B = (Sa - ST) / (Ba2 + BT2). (1.6.2.5) 1 The CCD detector:  1 The CCD detector 1.6 Observations with a CCD 1.6.3 The flat field technique S = So / Sf, (1.6.3.1) (S/B) = 1 / [(Bo/So)2 + (Bf/Sf)2]. (1.6.3.2) 1 The CCD detector:  1 The CCD detector 1.6 Observations with a CCD 1.6.3 The flat field technique Raw image (left) from which we substract the Bias image (right) ... and the dark image (below) (see also next page). 1 The CCD detector:  1 The CCD detector 1.6 Observations with a CCD 1.6.3 The flat field technique We then divide the obtained result by the flat field image (above) and obtain The final image (right). 1 The CCD detector:  1 The CCD detector 1.6 Observations with a CCD 1.6.4 Cosmic rays The impact of many cosmic rays are visible on this dark image 1 The CCD detector:  1 The CCD detector 1.6 Observations with a CCD 1.6.5 Improving the S/B ratio of astronomical observations B = B12 + B22 + B32 + .... (1.6.5.1) S = So + Sn + Sc, (1.6.5.2) B2 = Bo2 + Bn2 + y2 + Bc2, (1.6.5.3) S/B = (So + Sn + Sc) /  Bo2 + Bn2 + y2 + Bc2. (1.6.5.4) 1 The CCD detector:  1 The CCD detector 1.6 Observations with a CCD 1.6.5 Improving the S/B ratio of astronomical observations S/B = (So + Sn + Sc) /  So + Sn + Sc + y2. (1.6.5.5) S/B = Co / 1 + Cc / Co + n y2 / Co. . (1.6.5.6) S/B = Co. (1.6.5.7) 1 The CCD detector:  1 The CCD detector 1.6 Observations with a CCD 1.6.5 Improving the S/B ratio of astronomical observations S1 = Si = N Si, B1 = (Si) = (N Si), S1/B1 = (N Si) (1.6.5.8) S2 = N Si, B2 = S2, S2/B2 = (N Si). (1.6.5.9) 1 The CCD detector:  1 The CCD detector 1.6 Observations with a CCD 1.6.5 Improving the S/B ratio of astronomical observations S1 = Si = N Si, B1 = ((Si + y2))  (N y2), S1/B1 = (N Si)(Si /y) (1.6.5.10) S2 = N Si, B2 = S2, S2/B2 = (N Si). (1.6.5.11) S1/B1 = S2/B2 (Si / y)  S2/B2. (1.6.5.12) 1 The CCD detector:  1 The CCD detector 1.6 Observations with a CCD 1.6.6 Determination of the gain and the read out noise of a CCD g  Nmax / 216. (1.6.6.1) B2 = So + Sn + Sc + y2, (1.6.6.2) B2ADU = SADU / g + BDL2. (1.6.6.3) 1 The CCD detector:  1 The CCD detector 1.6 Observations with a CCD 1.6.6 Determination of the gain and read out noise of a CCD Noise (ADU) as a function of the signal (ADU) 1 The CCD detector:  1 The CCD detector 1.6 Observations with a CCD 1.6.6 Determination of the gain and read out noise of a CCD (f1 / f2) / f1/f2 2 = 1 / (f1/f1)2 + (f2/f2)2  1 /  2(f/f)2, (1.6.6.4) f2 = (f2 / 2) (f1/f2)2. (1.6.6.5) 1 The CCD detector:  1 The CCD detector Further issues

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