NOISE, NOISE, NOISE ROBERTO BARTALI

ABSTRACT

Charge Coupled Devices (CCD) have many advantages, as image devices, over photographic plates, but, like all other electronic devices, temperature, electric noise and exposure to cosmic radiation, reduce their effectiveness. The aim of this project is the description of problems raised due to noise and how they can be fixed or reduced in order to obtain the best from them. Professional and scientific grade CCD are very different from commercial and amateur ones, but they have in common the same type of problems generated by noise, the difference consist in the amount of the figure of that noise and I will compare both type. Specifically I will describe: read noise, dark current, spectral sensitivity response, flat fields, charge transfer efficiency, saturation and blooming, cosmetic defects and cosmic ray effect. For each topic, whenever possible, I will present pictures, curves or graphic representation, this way the reader can understand better the topic; I will also, for each one, compare professional and amateur CCD.

Some kind of problems will be reduced or, until some degree, fixed electronically or thermally, but other ones need software specific routines to do that, so I will conclude that noise reduction is a very complex thing and time consuming.

Processing a picture, from raw data until obtain from it, scientifically valuable data, can take much more time than that spent for the exposition. INTRODUCTION

The Charge Coupled Device (CCD) is an electronic component sensitive to light, when photons interact with the silicon, electrons are released and collected into wells, after some time, the electrons in the well are shifted until they reach the output amplifier and they are transferred to the output pin for storage and further processing into a computer. It seems simple, but a lot of things may happens until the light captured by the CCD is converted to a real image full of

scientific data. Figure 1 Professional CCD Kodak KAF16802CE, 4K x 4K pixels

Problems are due to internal generated or external induced noise, temperature, manufacturing defect, cosmic rays, spectral sensitivity response, data storage and processing, and, of course human errors.

Figure 2 Amateur CCD Texas Instruments TC211, 192x165 pixels

In the following sections I will explain which are the sources of noise and how can be reduced, but not always corrected at all, so let first define the word noise. Noise is a signal that carry no useful information, mixed to the real important signal, that must be eliminated, otherwise the information is corrupted.

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After the exposure to the light, the CCD data must be stored and processed in order to reduce the effect of the various noise sources, basically, there are three steps to follows:

a) subtract the dark frame, b) subtract the bias frame, c) divide by the flat field frame

The dark frame eliminate the thermal generated noise. The bias frame eliminate the

charge transfer noise, amplifier noise, reset noise, flicker noise and all electrically generated noise. The flat field frame correct data for not uniformity of sensitivity, quantum efficiency, cosmetic defects, cosmic rays, transfer charge noise, etc

Other sources of noise are due to the amplifier electroluminescence and blooming. The analysis of each one, whenever possible, will be made for professional and

amateur CCD sensors, comparison between both are presented in tabular form. DARK CURRENT and Temperature

Matter is made of atoms, basically, atoms are formed by

protons, neutrons and electrons. Protons and neutrons are tight bounded together, but electrons are moving particles and can leave easily their rest place, This phenomenon take place always and do not care about the type of material (conductor, dielectric or semiconductor) at any temperature above the absolute zero K. Higher the temperature, higher the mobility of the electrons. If electrons move across the material we get an electric current (figure 3). In a CCD this is a very big problem, because the only electrons we want out of their place in the atoms are those that are

exited by the incoming photons. It is very important to observe the non linear growing of the dark current with the temperature as clearly shows the graphic in figure 3.

Figure 3 Dark current vstemperature

The final image captured by the CCD is just an array of numbers, each cell of the array contains a number that is proportional to the electron exited and displaced into wells by the photons that strike the silicon sensitive surface, plus all others due to noise. CCD temperature must be reduced as low as possible, with certain limits, because some properties of materials change dramatically at lower temperatures. Ambient temperature is also an important factor to be taken into account, because it sum to the CCD temperature. Noise thermally generated is not only a function of the chip and ambient temperature, but is proportional to the exposition time. If the CCD is exposed for a long time, the temperature has a cumulative effect or integrate in time all the electrons, so noise is proportional to the exposure time. The main reason for the thermal noise is because there are imperfections or impurities in the bulk silicon or in the silicon dioxide interface that acts as steps between the valence and the conduction bands, so thermal electrons are easily added to signal electrons.

Dark current is the amount of electrons in the well, and, in every other part of the signal path, due to the temperature, normally is not a linear function of the temperature.

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One important characteristic of the dark current is the doubling rate temperature; it is the amount of temperature increment that duplicate the noise electrons, serious manufacturer specify this figure in the data sheet. The following table shows the value of declared dark current by manufacturers.

CCD Model Manufacturer Dark Current at 20C Application TC255 Texas Instruments 200 pA/cm2 Amateur, commercial

KAF0401 Kodak 10 pA/cm2 Small observatory KAF4202 Kodak 10 pA/cm2 professional

SITe103xA SIT 2-10 pA/cm2 professional

From above data it is clear the difference between commercial or amateur application CCD and a professional one, the best has 20 times less dark current. Reducing the dark current

As we see, the CCD must work at low temperature. Basically there are two form to cooling as much as possible the CCD: Thermoelectrically (figure 4) and Liquid Nitrogen containers (figure 5). These methods are very different in performance, cost, installation, maintenance and operation. The first is best suitable for amateurs and small particular or college observatory; the second is best suited for professional astronomy. Thermoelectric, or Peltier module, only need a very good surface contact with the CCD chip and a DC power supply current, rated normally at 12 V and 4 to 5 A. When current flows through the

module, one side became cool and the other became hot. The temperature change proportionally and linearly with the voltage supplied. To prevent the burning of the module, a well suited heat sink, best if there is also a fan, must be placed in the hot side. The temperature must be very stable, maximum variation of +/- 0.3 to 0.5 C are easy to obtain.

Figure 4 Thermoelectric module

Figure 5 Schematic diagram of a liquid nitrogen container

A liquid nitrogen system, shown in figure 5, is very complicated, due to the much lower temperature, it need to work in the vacuum. Coolant gas, liquid nitrogen, must be enclosed in a

metal can, and must be refilled constantly. The liquid nitrogen temperature is much lower than the 100 or 110 degrees C below zero that we need for the CCD, so some kind of heater is also placed close to the sensor. Scientific grade CCD must

work at very constant temperature, normally +/- 0.1 C. Installation and maintenance is very expensive. The following table summarize the main characteristics of both cooling systems: Cooling method Temperature complexity maintenance cost Thermoelectric -10 to –50 C Very low null low Liquid nitrogen -100 to –120 C Very high high Very high

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Detecting and correcting the dark current

Detecting how many thermal electrons are present in the raw image data is not difficult. Before the observing session, with the CCD shutter closed, we “expose” the CCD for the same time as the real image exposure time, we call this as Dark frame (figure 6). The raw image obtained contains only electrons thermally generated.

It is a good practice to make several dark frames, normally 5 before and another 5 after the observing session, and then average them.

The first operation in the image processing is precisely the dark frame subtraction, it consist in subtracting the dark frame from the entire matrix raw data.

Figure 6 Typical dark frame

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Another method to know how many electrons are produced by temperature is to use the dark pixel information available normally in low to medium cost CCD (figure 7). The

entire pixel array is not fully under the transparent window, but some of that are under an opaque layer, there are a few (up to 16 in some cases) on each side (depending on the manufacturer) trailing and following light collecting pixels on each row. When

the system read all the array, it compare the value in the dark reference pixels and subtract it from the entire

Figure 7

Structure of a CCD row of pixel

matrix. ELECTRIC NOISE In this section I will describe noise and problems that suffer a CCD due to the internal electronic circuitry. Some are generated internally like the amplifier noise and others are generated by the external interface circuit. Normally all these type of noises are called together as “Read out noise”. Technically this noise is defined as the number of electrons introduced in the final signal when reading the device. Charge transfer noise This is a noise generated by the external CCD interface electronics. The transfer of the charges are controlled by the phase clock. Two types of noise appears.

The first is due to the read out speed or the transfer shift register clock, if we read the chip at high speed, there is a little change in the amplifier temperature, so the noise figure increment and the sensitivity are slightly reduced.

The second, much more dangerous, works in the same manner as described above, but it is generated, in the electronic interface, by the phase clock circuit, when it is not well filtered. Normally the rise and fall times of MOS clock circuits are very fast, this produce spikes that propagate through signal path and power supply lines. If not eliminated properly they introduce very high noise in the output amplifier.

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Correction of transfer noise Reducing the readout speed, often eliminate the noise, but for a very large professional CCD with 4 to 16 megapixels, the time needed to read the array could be too long, during this time the chip can suffer from temperature.

One manufacturer approach is to give two or more output channel to the chip, dividing the entire array in some smaller ones, letting the user to readout each one in parallel. Obviously the clock generator circuit must be well designed and implemented, otherwise the CCD performance falls down. The printer circuit board layout for the clock circuit play a fundamental role. Reset noise When charges from some pixel reach the output amplifier, they must be converted to a voltage. This conversion is made by a capacitor that transfer the applied voltage to a source follower, this way the impedance of the amplifier is very high and represent no charge to the capacitor and transport shift register. Before each conversion process, the sense (reference) capacitor must be discharged and biased to certain reference level. Because the field effect transistor (FET), acting as a switch, is not an ideal interrupter, is has a little resistance, so temperature can produce a little noise. This noise is an offset, positive or negative, for the main reference voltage, so, when the signal reach the output amplifier, is not the same as before. Correction of reset noise This noise is very low, only a tens of electrons. If manufacturers specify the amount of reset noise for their CCD, the user can be subtract its value from the value of each pixel for the entire picture. Amplifier noise Reset noise and charge transfer noise, act in the output amplifier, but the own amplifier produce also some noise. There are two main reason for this amplifier noise, one is, like the reset FET transistor, because it has a little resistance and the temperature can generate some current (noise), normally this is a white noise. The other noise source is called Flicker noise, it is inversely proportional to the readout frequency and proportional to the amplifier area. It is produced by electrons trapped in the silicon oxide interface of the MOS transistor that affect the current flow. Detecting and correcting the electronic noise When trying to measure these noises, the CCD must be in the “Dark”, the shutter

closed, because there must be no signal dependence on the light and the temperature. The CCD signal must be read out as fast as possible, no exposure time is needed, just read out all pixel information. This operation is called Bias Frame capture (figure 8) and, technically defined as a zero exposure time capture.

As for dark frame, it is a good practice to take several Bias frames and average them, depending on the experience of the astronomer and CCD

Figure 8 Typical bias frame

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quality, 10 frames are a good choice. When capturing the Bias frame, the CCD must be at the same temperature as it is

when is exposed to light, this is for eliminating a possible dark current difference. The second task, in image processing, is the subtraction of the Bias frame from the raw image. There are two ways to do so, one is the subtraction of the Bias frame from the matrix obtained in the first step, when the Dark frame was subtracted from the raw image; the other is to subtract the Bias frame directly from the raw image. There are no specific rule on this, so, as always, the experience is the best advisor. Bias frame is not dependent to the temperature, almost in theory as we see later, and it is constant in time, so, for a small CCD, like an amateur one, bias frame capture may be not a daily duty. This not true for a professional CCD, because even a very small difference in the response of each pixel can destroy scientific valuable information. It is possible, also, to subtract the Bias frame from the dark frame, this allows for a pure thermal information frame. As the CCD is only read out, thermal charges have no time to accumulate, so this frame contains the intrinsic thermal noise. It is the figure of how respond each pixel at the temperature of the CCD.

It is a good idea to capture this kind of frames at different CCD temperature, specially if the astronomer (amateur) is not planning to take bias frames every day, confident in the “invariability” of the internal generated electric noise, this is useful if the CCD is not properly cooled or the temperature control system is not as stable as it must be. The invariability of the bias frame is strongest dependant to the external electronic circuit connected to the CCD.

The power supply is a very important and basically part of the system. If the voltage applied is unstable, the CCD signal follows that variations, because the output amplifier reference circuit is dependent to the supply voltage. The power supply must be very well filtered, electronic circuits produce a lot of electric noise and spikes at many frequencies, this unwanted noise can propagate and can be introduced into the CCD through the power supply lines. The clock generator itself produce noise at high frequency, it must be well filtered before the clock signal enter into the CCD circuit. MANUFACTURING PROBLEMS As I say in other section of this work, each CCD is unique and each pixel in it is not exactly as its neighborough. This is a problem and affect the final image captured, so is it considered a noise. Not uniformity of sensitivity

Each pixel is unique. Due to the production process, it is impossible to make 2 identical pixels. A very little difference in the physical size, sensitive area or silicon thickness, produce a difference in the charge produced by incoming photons.

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Quantum efficiency

Technically speaking, the quantum efficiency is the amount of electrons produced by incoming photons. The goal is one or more electrons per photon, but until now, the technology to do that is not yet developed, so we have a ratio of how many photons are needed to produce an electron.

Figure 9Absorption of photons by silicon

This is a quantity that depends on the wavelength of the photon and on the chip geometry. In the silicon, photons interact (they are absorbed) at different depth depending on the energy they carry, and this is a

function of their wavelength (figure 9). For the same quantity of photons, the number of electrons released change

depending on the photon wavelength, so the spectral response curve is dependant to the QE. Quantum efficiency depends on the thickness of the sensitive silicon area (figure 9

and 10).

Spectral response This is a graph that shows the response of a CCD to

different wavelength (figure 10), and it is dependant to the QE.

Figure 10 QE of front and back illuminated CCD

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CCD can be separated into two classes: front illuminated and back illuminated. The main difference is the spectral response, front illuminated are more sensitive to the red and IR part of the spectrum, back illuminated are more sensitive to the blue part.

Front illuminated CCD are thicker (figure 9 and 12), photons have much more possibility to being absorbed by silicon. The reason why they are poor blue detectors is that the electrode gates are placed in front of the sensitive area and there is an insulating layer in between. Blue, or short wavelength photons, are

mostly collected in the insulating layer and only a few can pass through this layer reaching the sensitive area.

Back illuminated CCD (figure 11), have the electrode gates on the opposite face, so the first thing a photon encounter is the sensitive layer. This way, all the short wavelength photons are collected, but the distance to the electrodes can not

be very large, so the sensitive part is thinned. The long wavelength red photons, can pass through the silicon freely, only

Figure 11

Back illuminated CC

a few are absorbed. The spectral response of a CCD is a very important graph, it is extremely useful

because normally a scientific CCD is monochromatic, but the objects we are imaging are polychromatic.

The exposure time is strongly related to the spectral response, if the CCD is very sensitive to the red part of the spectrum, needs less time, if not, the time must be increased and the only form to know how much, is observing the SR graphics.

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In figure 13 there are four spectral response graphics for four different CCD. Clearly the difference from each one jump to the eyes.

Figure 12

Front illuminated CCD

Figure 13A: Kodak KAF0261E, 512 x 512 pixels Figure 13B: Atmel-Thomson TH7887A, 1024 x 1024 pixels Figure 13C: Atmel AT71201M, 4096 x 4096 pixels Figure 13D:Texas Instruments TC255P, 336 x 244 pixels

Cosmetic defects

A cosmetic defect is a bad pixel or a defective shift register (transfer charge) zone. For a bad pixel we understand one that is poorly or not sensitive, it appear in the final image as a dark point; it may be also a pixel with some not changing charge inside, in this case the final image present a gray dot. Often in literature, cosmetic defects are called hot or dark spots.

If the problem is in the transfer circuit, the final image present a row or a column entirely black, because no information can be transported after the problematic zone.

A more technical definition of a defective pixel is one that respond to the incoming photons in a very different manner than the average of all others.

Manufacturers classify their products in Grades from scientific (the best) to commercial (the worst), the difference is precisely the amount of defective pixels or other

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type of problems, so a scientific grade CCD is nearly perfect (as we said in other sections of this work, it is impossible to make a perfect CCD).

The cost is proportional to the grade, a scientific grade CCD can cost 10 to 50 times more than the same CCD in commercial grade.

Non linearity

It is the percent of deviation from the ideal linear response. A perfect linear response is a straight line from the saturation to the dark signal.

The photon collecting (absorption) process in the silicon is considered as nearly perfect linear, but geometric imperfection and impurities, reduce the linearity a little. A good CCD have an averaged linear response of 99% or better.

When the CCD is exposed to very low light intensity, the linearity falls well down from the mentioned value, because there is a low limit of the amount of charges, due to the threshold and trapping phenomenon, the linearity deviate from the ideal straight line also if the CCD is exposed to very intense illumination, because it can be saturated easily.

Dynamic range limitations

Dynamic range is the ratio of the saturated signal to the dark-biased signal, normally expressed in decibels (dB) or in bits. In other words in the fully useful signal.

Theoretically, the useful signal may be divided into as many integer parts as the resolution of the system analog to digital (ADC) converter circuit. But this is not true, because there are some small deviations in the reference capacitor and in the output amplifier (see above). If we read many times the same quantity of charge from the same pixel, the result is not the same every time.

This limitation depends, also to the sensitivity of the output amplifier, thermal current can produce a very small, but measurable, offset to the incoming signal, so sometimes it is above and sometimes it is below the average value, if this value is less than the bit threshold, the output value is a bit more or a bit less than the real value.

Dynamic range minimum and maximum values can be retrieved directly from the CCD, because some manufacturers place on each side of every row of sensitive pixels, some others called dark reference (figure 7) and some called white reference pixels.

White reference pixels are not exposed to light, but as the dark ones, they are covered by an opaque metallic layer. All these pixels are biased to the saturation value. The dynamic range is then just the dark reference value (all dark pixels averaged for a better result) subtracted from the white pixel value (again is better if all white references are averaged).

In a very low cost CCD or commercial camera, obtain the dynamic range immediately and without complex processes is cheaper and faster.

Cosmic rays. These are very high energetic particles traveling at near the speed of light. When they hit a photosite (sensitive area) of the CCD, are able to release a great quantity of electrons. A cosmic ray can hit the silicon in two forms, straight way or side way. If it travel normally to the CCD the effect is a single pixel affected, the image have a high intensity white spot, if it arrive at some angle, the effect is a white streak. The probability to get a cosmic ray affected image is proportional to the altitude above the sea level, to the surface area of the CCD and to its thickness. The first is because

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the atmosphere is thinner and can absorb or scatter less particles, the second is obvious, larger the area, greater the possibility, is the silicon is relatively thick, as for front illuminated CCD, there is more chance for the cosmic ray to be absorbed. Detecting and correcting these problems

All the problems described above can be fixed by expose the CCD to a uniform illuminated field. This is not easy, because in theory this picture must be very uniform, monochromatic and featureless, best if it is white or light gray. In practice there are no way to do that, so, normally, astronomers use the sky at twilight, because is almost uniform, there is no necessity to take away the CCD from the telescope and the environment temperature is not too hot. This image is called Flat Field Frame (figure 14).

Another useful way to take flat fields is to point the telescope to a screen inside the observatory and point a source of light to it,

Figure 14 Typical Flat FieldFrame

then expose the CCD.

The flat field frame must be exposed on objects more luminous than those we need to observe. As for the dark and bias frames, we need to average a certain quantity to ensure the best results. Many astronomers take flat field frames before and after the observing session. Flat field frame is very important because with it we can make the correction of many problems and generally cancel the effect of the cosmic ray, dust on the CCD surface, dark and hot spots. If the frame is really uniform and monochromatic, the value obtained for each pixel reflect the QE, spectral response, linearity and dynamic range. Make the correction is easy, the image array, previously correct for dark and bias, is divided, on pixel basis, by the flat field. At this point the image is much better than the raw data, many times the raw data contains no image at all. Many more image processing tasks are needed in order to obtain the most scientifically useful data from the image, but for an advanced amateur or a low cost CCD camera, this method guarantee a good result. CHARGE TRANSFER PROBLEMS The charges stored in the wells, must be shifted (transported) from one well to the other (serially) in the best efficient way, but due to the physical construction of the CCD, temperature and clock signals, this is not 100% reliable. This means that some of the charges are not transported until the output amplifier and then, the image information is corrupted. Depending on the manufacturers, two definition of this error are issued. When professional CCD are exposed to the lowest possible and practical temperature, around 100 degree C below zero, dark current is extremely low, but a transfer problem is issued because the mobility of the charges are reduced. The mobility is proportional to temperature, this fact increase the charge transfer inefficiency. Charge transfer efficiency (CTE) This is the fraction of charges that are effectively transferred in a CCD cycle. As the CCD is a two dimensional array, charges are transferred both horizontally and vertically, so the transfer charge efficiency depend on the total cycles needed to transport charges until

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the output amplifier. The x CTE not necessarily is the same as the y CTE. The number of cycles depends on the number of phases that has the CCD, so a four phase CCD has four cycles per transfer, obviously, CTE is proportional to the number of phases. Charge transfer inefficiency (CTI) This is the fraction of charges that are not transferred in a CCD cycle, in other words, charges that are left in the well collecting pixel area or in the transfer shift register. As for CTE, the inefficiency in the row transfer may be different of that of the y transfer. The total CTI depends on the total cycles transfer are needed to reach the output amplifier. The number of cycles depends on the number of phases that has the CCD, so a four phase CCD has four cycles per transfer, obviously, CTI is proportional to the number of phases. Measuring charge efficiency This is a very difficult task, only at manufacturing process time can be done. A laboratory test may be realized but the result obtained can be different from the manufacturer specifications. It consist of illuminate the full CCD sensitive area with a very precise, uniform, monochromatic light with the shortest possible exposure time (something like a flat field frame), then read out the array and compare it with a predicted values, the difference shows the inefficiency. Correction of charge efficiency If the CCD is exposed to a very low light intensity, it is possible that charges can not be transferred from a well to the other. This is because the silicon contains some impurities or even the gate voltage is not uniform, due, perhaps to gate imperfections or power supply problems. To solve this problem we can expose the CCD to a flash light for a very short time, this technique is normally used also in normal photography. The flash introduce a bias, so the minimum quantity of charges in each pixel is above the limit. This method is called Fat Zero. The problem, doing this, is that we are introducing an offet on the signal and more noise. OTHER TYPE OF NOISE AND PROBLEMS Amplifier electroluminescence

Sometimes the output amplifier introduce a very bright spot in a corner of the image which intensity decrease with distance (figure 15). This phenomenon is not commonly observed (it happened to me just a couple of times in more than 20 hours of observation) and not predictable. The guilty of this effect, called electroluminescence, is the output amplifier, that sometimes emit light for its own. The reason can be a weak residual radioactivity of the silicon doping

material in the amplifier chip.

Figure 15 Electroluminescence effect on the image

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Correction of electroluminescence When this happened, nothing can be done, the full image information is lost. The amplifier power supply must be shut off and after a minute or so turned on again. Blooming

This is another problem that is impossible to fix. The well capacity of each pixel in the CCD is limited to its physical size, so only certain number of electrons can be filled in it. If the light (number of photons) arriving from a star or other bright object like a planet, release a number of electrons greater than the maximum allowed, they falls in adjacent wells. The information is then well corrupted. This effect is called Blooming (figure 16). The final image present a strike in the charge transfer direction which intensity

decrease with distance. Figure 16 Blooming effect: Júpiter and its satellites

This unwanted effect reduce the possibility of taking pictures with bright and faint objects in the same field of view. A possible form to reduce the effect is lowering the exposure time, but this way we lost faintest objects.

Correction of Blooming It is not possible to correct the blooming effect, if we do not want it, we have to purchase a CCD with an antiblooming gate. This gate, trap all the electrons in excess. There are, however, two drawbacks using the antiblooming CCD. First, the sensitivity decrease and in some value also the linearity; second, spatial resolution change, because there are more distance between pixel centers on one axis because of the antiblooming gate. Even when the antiblooming can absorb as high as 100 times more electrons than those for the saturation level, the problem is not completely eliminated because of the charge diffusion effect. Depending on the application, it is better to use a normal CCD, as, for example, in photometry. CONCLUSIONS CCD imaging is easy if we know exactly how to do that. Like photography, where each film is different, an electronic picture depends on the specific CCD we are using. There are no common rules or empirical methods to follows, if something works well for the CCD x, it do not work for the CCD y.

The first step is then, understand as perfect as possible the characteristics of the CCD we intend to use. After that, we have to take many pictures with it, for understand how it works and which are the limitations.

But, first of all, we have to select the right CCD, depending on which objects we are trying to observe, there is not an universal CCD, if we want to do a scientific work.

Knowing all the problems we can encounter in the process of imaging an astronomical picture, we will find the correct solution for most of them, this way our picture is no longer just a pretty snapshot of the sky, but it is a picture full of interesting data.

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To obtain a good picture with a CCD is not only important the CCD type or manufacturer, but all the environment where the CCD must work, all the associated electronics must be as good as the CCD, the same is for the processing software. In the reference list there are some accessible and easy to use links to software developed for astronomical purposes.

The two example that follows show how a raw data frame change after a little processing. The processed picture is corrected for all the noise and problems described above with dark, bias and flat field frames, the original Fit data contains much more detail than the JPG image presented here. Both images are taken with a Takahashi Mewlon 300 telescope and a 300 second exposure on a DreamMachine 1024x1024 pixel CCD cooled at 12 degree C below zero.

Figure 17 M13 raw frame

Figure 18M13 processed

Figure 20M51 processed

Figure 19M51 raw frame

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REFERENCES (1): Covington M, ASTROPHOTOGRAPHY FOR THE AMATEUR, Cambridge, 1999 (2): Buil C, CCD ASTRONOMY, Willmann Bell, 1991 (3): Kitchin C, ASTROPHYSICAL TECHNIQUES, IOP, 1998 (4): Howell S, HANDBOOK OF CCD ASTRONOMY, Cambridge, 2000 (5): Berry R. et al, THE CCD CAMERA COOKBOOK, willmann Bell, 1994 (6): Area array Image sensing products, Texas Instruments, 1994 (7): CCD Image sensor noise sources, Kodak, 2003 (8): Charge Coupled Devices CCD Image sensors, Kodak, 2001 (9): Solid State Image Sensors Terminology, Kodak, 1994 (10): Kodak CCD Primer, Kodak INTERNET REFERENCES (11): www.kodak.com Kodak CCD technical literature (12): www.ti.com Texas Instruments CCD technical literature (13): www.atmel.com Atmel CCD technical literature (14): www.st.com Thomson CCD technical literature (15): www.sbig.com CCD camera (16): www.photonics.com CCD technology (17): www.site-inc.com professional CCD (18): http://www.tetech.com/modules/ Thermoelectric modules (19): http://www.zts.com/thermion/home.htm Thermoelectric modules IMAGE PROCESSING SOFTWARE (20): DS9: http://hea-www.harvard.edu/RD/ds9/index.html (21): CADET: http://www.terra.es/personal2/oscarcj/introeng.htm (22): IRIS: http://www.astrosurf.com/buil/us/iris/iris.htm (23): ASTRA: http://www.phasespace.com.au IMAGE CREDITS Figure 1: Kodak, KAF16802 CCD data sheet Figure 2: Richard Berry, The CCD Camera Cookbook Figure 3, 5, 6, 8, 9, 14: Steve B. Howell, Handbook of CCD Astronomy Figure 4: http://www.tetech.com/modules/ Figure 7: Atmel, TH7899M CCD data sheet Figure 10: www.site-inc.com Figure 11, 12, 15: Christian Buil, CCD Astronomy Figure 13A: Kodak, KAF0261E data sheet Figure 13B: Atmel, TH7887A data sheet Figure 13C: Atmel AT71201M data sheet Figure 13D: Texas Instruments TC255P data sheet Figure 16, 17, 19: Roberto Bartali Figure 18, 20: Arnie Rosner, www.arnierosner.com