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Gilbert A. Clark
TIE Director
President, TIE International

Freeman Dyson

Mary Cragg
Administrative Manager

Dan Campion
President, U.S. TIE

Nicholas Paizis
TIE Technology Manager

Randy Norrick
Site Selection Director

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Norrick Peak, AZ - Operational

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The TIE User's Guide and Workbook for Scholastic Programs and Amateur Studies:


Introduction to CCD Astronomy

Prepared and Edited by Blake Bartosh and Barrett Duff

Using the TIE equipment is easy, when you understand the function of all the pieces. Perhaps the most important aspect of the TIE equipment is your ability to command a camera, mounted to a telescope, to take an electronic picture of the sky and then download that image right to your computer...and you don't even have to get cold, or eaten by mosquitoes!

This guide will help you learn some of the key features of electronic cameras so that when you come on line you will know what is happening when you command the camera to "take an exposure."

The electronic camera we use for the TIE program is based on CCD (Charge Coupled Device) technology, and is your link to distant worlds. As a start, we'll explain the basic characteristics common to all CCD imagers:
- the number of pixels in the CCD "array"
- the resolution of the analog to digital converter (ADC) specified in "bits" (shades of gray)
- the method of cooling the CCD
- the software for controlling and reading the CCD
- the support hardware required for operating the CCD (personal computer, mass storage, etc.)

Array Size and Image Resolution: The CCD, or Charge Coupled Device, is a microchip that contains an array of thousands of pixels arranged in a matrix on its surface. A pixel is the basic element of the CCD, a place where photons are converted into electric signals. Pixels are quite small, typically measured in microns. An entire array of tens of thousands of pixels is typically less than a quarter of an inch square. The number of pixels in the array determines the resolution of the ultimate image made by the CCD. The higher the number of pixels per millimeter, the higher the resolution, and the closer the image will be to real life. Remember dot matrix printers? That's what we would call low resolution, now that we have 300 and 600 dpi (dots per inch) high resolution laser printers. Although not up to photographic film resolution yet, the resolution of a typical CCD chip is better than 1900 dpi! However, the array's physical size limits the actual image that it can record. Much smaller than a 35 mm film frame, the scene which is to be captured by the CCD must be concentrated into the area of the CCD's small array. So optics must be put in between the CCD array and the scene to capture it effectively. This is a good job for a telescope!

Putting CCD Cameras to Work: The CCD is housed in a casing that includes a fitting like the barrel of an ocular (eyepiece). The CCD sits at the center of this fitting, usually recessed in a sealed compartment which has a clear window above it. The fitting is often a standard 1.25" or 2" barrel, and fits directly in place of the telescope's ocular. Because the CCD camera is used in place of the ocular, two consequences result. One consequence is that we can't directly see with our eyes what the telescope is seeing, since the CCD has replaced the eyepiece. The other result is the image cast on the CCD is completely dependent on the optics of the telescope. The field size of the image is fixed by the focal length of the telescope, and cannot be enlarged or reduced by the operator.

Let's say that we have a scene being projected onto the CCD array. Like a camera, the CCD is electronically commanded to "take a picture" by accumulating charge into its pixels which previously have been cleared of charge. This charge clearance is necessary for good imaging, and is similar to winding an unexposed frame of film in place behind the shutter of a camera prior to making a picture. The pixels of the CCD, now "exposed" for a specified length of time to the scene, convert the photons of light hitting their surface into electron charges. The charges accumulate over time, in relation to the intensity of the photons present. By the way, the clearance of the charge in the pixels is important, but two other procedures must be done to ensure a good image. Dark frame and flat field exposures are made to calibrate the CCD before the "real" exposure takes place. More on that later. While reading on, see if you can figure out what these calibration procedures are, and why they improve the accuracy of the CCD's image.

A Picture is Worth 90,750 Words: The CCD has electronic circuits integrated into the array which extract each individual pixel's charge and "clocks" the array's individual pixel charges out of the CCD to a device called an analog to digital converter (ADC, read "A to D"). The ADC's job is to convert the charge that has built up in each pixel into a voltage which is easily measured (it's rather difficult to count electrons). The measuring of the voltage is also performed by the ADC, which gives us its measure of a pixel's charge in terms of a digital value, literally a string of ones and zeros. The pixel charge is clocked out of the array, one pixel at a time, row by row, and sequentially into the ADC, until all pixel charges are converted by the ADC. Each pixel's charge is converted into a digital "word" by the ADC. If each pixel's charge is represented by a digital word, then the array's output is a string of words, with the number of words being equal to the number of pixels in the array.

What is All this Excitement About?: If you can convert the array's image into a list of digital words which can be sent to a computer and stored as files, then you can display a re-creation of the image on your monitor, manipulate these stored images with the help of software programs, and transmit image files to others.Now you can see the power of gathering images with the CCD. You are able to see on your monitor what the telescope is seeing because the image is electronic.You can then share these images with people in ways that will open up communication between astronomers and the world.

Nothing is Black and White: Let's discuss a subtle issue concerning ADC resolution. Remember that the ADC converts pixel charge to a digital word. What is the range of values that the word can have? If the word were simply a single digital bit, either on or off, then the resolution would be "one bit." Therefore, this one bit resolution can take one of two levels, completely white (bright) or completely black. This would be like printing a picture with black dots and white spacing, which is how a newspaper photo is printed. With enough dots you can make a pretty good picture. But the resulting picture is by no means high quality. And the image that the picture is trying to represent is certainly made up of more than simple black and white components. Now suppose the ADC can measure the value of the pixel's charge more accurately. Then we should be able to recreate the image more accurately. The CCD can record not just black and white, but shades of gray as well. If you have a computer with a high resolution monitor, change the settings from "Grays" to "Black and White" and you'll see the difference, especially when viewing a photo image. Also, if you can play with the resolution (number of colors or levels of gray) you'll also get a feeling for how the resolution of an ADC affects the image.

Folks who do a lot of image processing consider 256 shades of gray, that afforded by an 8 bit ADC, the absolute minimum for decent image reproduction. In fact, many CCD camera manufacturers use higher resolution ADCs, such as 12 bit (4,096 shades of gray) and even 16 bit (65,536 shades of gray).

Our CCD Camera is Really Cool: The camera used in the TIE equipment is thermoelectrically cooled, which means the chamber that the CCD is mounted in is cooled by a device that only requires electric current to provide heat transfer (typically this is a Peltier cooler, a device developed for aerospace imaging systems). The cooling is done in two stages, which brings the camera temperature down to about 45C below the ambient temperature.

Why is the CCD camera cooled? All CCD pixels will "fill up" with unwanted charge as a result of any heat near the CCD array. This unwanted charge is not related to the photon energy gathered by the telescope and focused on the CCD chip. This unwanted charge tends to reduce the quality of the image, plus we want to be able to expose the CCD chip to the faint light that the telescope gathers for many seconds (up to 40). If the CCD chip is cooled, then thermal charge buildup is reduced, the exposure times can be longer, and the final image will be of high quality.

Software is the Key to Control: You will use software to point the telescope to the desired object, command the temperature, the exposure time, and image resolution.

Back to Dark Frames ...: By the way, now that you've had some time to think about dark frames and flat fields, let's discuss what these mean. As just mentioned, one characteristic of the CCD is that its pixels can fill up with charge due to both the light of the desired image, and undesired noise, a result of thermal conditions near the CCD chip. This undesired noise, or dark current, is what we wish to "filter out." Fortunately, dark current noise is repeatable because it is due to ambient (outside) temperature. We can take advantage of the digital nature of the CCD's image and literally subtract the effect of dark current from the image we get from a CCD exposure. All we need to do is keep the CCD pixels covered while we "expose" the CCD and take a picture of complete darkness (kind of like forgetting to take the lens cap off of your camera before making an exposure). One condition is that this "dark frame" exposure last as long as the image exposure that you wish to correct. Once you have the raw CCD image and the dark frame image files, software included with the CCD camera will allow you to literally subtract the dark frame image data from the raw CCD image. The result is an image that has been corrected for the CCD's dark current noise. SkyPro automatically takes a dark frame prior to taking an exposure so you can be sure your images are the most accurate possible.

...and Flat Fields: The other calibration method that is required for good CCD imaging is that of the flat field exposure. This is similar to a dark current, except that instead of calibrating for dark current noise, you are calibrating for pixel sensitivity to light. This procedure requires imaging a homogeneous image, such as a clear twilight sky or a photographic "gray card" (a card used in photography to calibrate light meters, is gray in color, and reflects 18% of the light falling on it). Theoretically, each pixel of the CCD should see the same light intensity and same color, and hence respond with the same amount of electrons at the end of any exposure duration. The output data for such an image should be a file containing words of the same value. In practice, this is not the case. Variations in chip manufacturing processes, pixel to pixel variations, optical aberrations, and dust on the optical surfaces will all contribute to variation of the flat field response. As with dark current, this effect is repeatable, until focus is changed or a different filter is used (filters are discussed in the Projects later in this guide). The calibration using the flat field exposure is the same technique as the dark current calibration. The image file created by the flat field exposure is a sensitivity measurement, or gain adjustment, of the raw image file. For most image work, the flat field exposure is not nearly as important as the dark frame calibration. However, for science projects involving photometric measurements, the flat field calibration is essential. Images taken by the camera are not automatically flat field corrected, but provisions will be made available for taking flat fields where necessary for science projects. Flat field correction may be applied to any image by selecting Combine, Flat Field.

What is CCD Autoguiding?: Some CCD camera systems can be used as an autoguider for conventional astrophotography or CCD imaging. In this application, a CCD camera is allowed to image on a guide star, and special software supplied with the CCD camera forces the telescope to "track" the guide star. This tracking is accomplished because the software can control the R.A. and Declination axes motion of telescope. Of course, the CCD hardware requires an electrical connection to the slow motion control of the telescope mount. The CCD software acts like a very dedicated observer, detecting if the guide star's light is moving off of a particular pixel, and if it is, sending commands to the telescope drive motors to correct the movement and bring the guide star back over the proper pixel.