Characterization of a CCD：一种CCD表征.doc

Characterization of a CCDThe physical properties and function of a CCD camera are described by the terms read noise, dark current, signal-to-noise, bias, quantum efficiency, gain, dynamic range and a few others. The following will attempt to demystify these terms and give a better understanding of what they mean.Readout Noise (or Read Noise)Read noise comes from the electronics used to “read” the CCD. There are two components to read noise: 1) the conversion from voltage (an analog signal that is actually what is read from each pixel) to a digital number, and 2) passage of the signal through an amplifier and analog to digital converter. As an example of the conversion from voltage to digital numbers, lets say you are reading the same pixel twice, each time with an identical charge. You will get a slightly different digital number each time. This is because you can never measure anything exactly. There is a statistical distribution of possible answers centered on a mean value. Second, the electronics themselves will introduce spurious electrons into the entire process, generating unwanted fluctuations in the output. These two effects combine to produce an uncertainty in the final output value for each pixel. The average of this uncertainty is called the read noise. So if the read noise of your CCD is 20 electrons, each pixel will, on average, contain 20 extra electrons each time the CCD is read. GainThe output voltage from a given pixel is converted to a digital number during readout. The amount of voltage needed (which translates into the number of collected electrons or received photons) to produce 1 count (also called an analog-to-digital unit, ADU) is called the gain of the CCD. A typical gain might be 10 electrons/count, which means that for every 10 electrons collected within a photosite, that pixel will produce, on average, 1 count. Take two bias frames and two flat field frames with either the Ap7p or the ST-2000XM CCD. 1) Determine the mean pixel value within each bias and each flat field. 2) Now crop the two flat fields to remove the dark corners. Calculate the mean pixel values for the two cropped flat fields. ImageMean Pixel ValueFlat 1Flat 2Bias 1Bias 2Flat 1 croppedFlat 2 croppeda. What is the difference between the mean pixel values for each uncropped and cropped flat image?b. How do you explain this difference?3) Now create three difference images: (Flat 1 Flat 2, Bias 1 Bias 2, Flat 1c-Flat2c) . Measure the standard deviation in each of these image differences. ImageStandard DeviationFlat 1 Flat2Bias 1 Bias 2Flat 1c Flat 2c4) Determine the gain of the CCD using: Gain = (F1+F2)-(B1+B2) (2 (f1-f12) - 2(B1-B2) 5) Recalculate the gain using the cropped flat imagesImagesGainNormal flatsCropped flats6) How does using the cropped flats affect the gain? Which do you think is appropriate to use given the definition of gain?7) Now calculate the read-noise of the CCD using Read Noise = Gain * (B1-B2) Sqrt(2)8) Imagine you are observing the planet Jupiter with a CCD with the read noise and gain you determined above. Jupiter has an average signal of 100000 electrons per second per pixel. a. What, on average, would be the counts coming from each pixel exposed to Jupiter? b. You want to do a long exposure of Jupiter to bring out details in its atmosphere. Given what you know about read noise, do you think it is wise to do a single long exposure or co-add several shorter exposures? Why?9) Now you are observing a faint spiral galaxy in Virgo with an average signal of 100 electrons per second per pixel.a. What, on average, would be the counts coming from each pixel exposed to the galaxy?b. Do you think it is wise to do a single long exposure or co-add several shorter exposures? LinearityOne major advantage of a CCD is that it is linear in its response over a large range of data values. Linearity means that there is a simple linear relation between the input value (charge collected within each pixel) and the output value. The largest output number that a CCD can produce is set by the number of bits in the A/D converter. A bit can have the value of 0 or 1. The output is based on powers of two. With a 14 bit A/D converter, numbers 0-16383 (214) can be represented. With a 16 bit A/D converter (both of our CCDs are 16 bit), numbers 0-65,535 can be represented (216). At high count rates, the CCD can start to become non-linear. It is important, when using your observations for scientific analysis, to know when and how the CCD becomes non-linear.There are several factors that can limit the largest usable output pixel value in a CCD image: two types of saturation (A/D saturation and pixel saturation), and nonlinearity. The CCD is said to be “saturated” when the A/D converter can not output higher numbers, and also when each pixel cannot hold any more electrons without leaking into surrounding pixels. 1) The Ap7p has a full well capacity of 300,000 electrons. The ST-2000XM has a full well capacity of 45,000 electrons. Using the gain you calculated above for the CCDs, calculate how many photons each CCD can hold before the A/D converter saturates. A/D max * gain = input photons2) Is this more or less than the number of electrons a pixel can hold?Both types of saturation lead to noticeable effects, such as “bleeding” of signal into adjacent pixels. Far more dangerous is “nonlinearity”, which can occur without the observer being aware. This it is important to know the response of your CCD to higher and higher counts. 1) Set up the Ap7p on the 24 inch and/or the ST-2000XM on the Meade 10 inch (if not already done). 2) Find a field of stars with a range of brightnesses (Double cluster in Perseus might be good for the Meade, check on the ECU computer for a cluster for the