Transcript
PHYS0220 Lab 3 Introduction to CCD Imaging
Image of M104, the Sombrero Galaxy Taken by a PHYS0220 student Claudia Schhwarz, Spring 2009
As the introduction of photography about a century ago revolutionized observational astronomy by allowing scientists to get images of celestial objects too faint to be seen visually even with the biggest telescopes, so did the introduction of „charge-coupled devices‟ (or CCDs) two decades ago. A CCD is a small electronic chip divided into a large number of little squares, so-called „pixels‟. When light hits one of these pixels, the material of the CCD emits electrons which are trapped within that pixel, so that charge accumulates on each pixel proportional to the light intensity. When such a chip is attached to an astronomical telescope and exposed for a certain period of time, the pattern of the charges on it represents an image of the region of the sky the telescope is point towards. That pattern can be read out by a computer, so that on its screen a picture of the corresponding celestial object will be displayed. The big advantage of taking CCD images over photography is the much higher sensitivity of a CCD. Whereas exposure times of 10 or 15 minutes are necessary with standard film material in order to get an image of a star cluster or a nebula, the CCD has to be exposed for only 10 to 20 seconds to get the same result. Therefore all of the modern research telescopes are equipped with CCD cameras, which have greatly pushed out the limits of detecting faint celestial objects, similarly to the introduction of photography 100 years ago. Another advantage of the CCD is that the image is available in a format allowing computer aided processing which can bring out more and fainter details.
The chip in our CCD camera holds about 1 million pixels in 1024 columns by 1024 rows. Each pixel has a physical size of 24 m (1 m – 1/1000 mm). The information on the charge (and therefore the intensity) on each pixel is transformed into a scale of integer numbers from 0 to 65535 before it is transmitted to the computer operating the CCD. 0 stands for zero charge, and 65535 for the highest possible value. This is usually set to be above the saturation charge on a pixel, which is the maximum charge that can be collected.
The CCD is similar to the detector that‟s inside your digital camera (or your video phone), but a little more expensive because it‟s been designed to give a very uniform signal and be sensitive to very faint objects.
Part 1, Taking Images
Procedure When you arrive fir your lab, the camera will already be set up on the telescope, and the TA will have initialized both the telescope and the camera. In particular, the TA will have set the temperature on the camera to the ambient temperature, minus 30 degrees Celsius, and verified that the camera is recording the correct date and time of the images in the image headers. Focus and Synchronize
Poorly focused image
Nicely focused image
The camera should be close to the correct focus position. However, you should verify this; your lab grade depends on proper focus. Select a bright star near the object you plan to image (preferably within 10 degrees). Slew the telescope to that star, and make sure that the star is centered in the image (the guide telescope attached to the main telescope is accurately aligned, so if it‟s centered in that telescope you will be o.k. If it is not centered, slowly move the telescope until it is centered and then perform the “sync on object” operation within Starry Nights. Once you have the star centered, use the CCD control software to set the exposure to “focus”. You will be prompted to enter values for the exposure time (and other values). You should set the exposure time to around 1 second if you have picked a bright star. Use the defaults for other values.
The camera will begin taking images of the sky continuously. Adjust the focus slowly, observing whether the star image becomes bigger or smaller as you change the focus. You will note that as you focus the telescope, the image of the star may appear to move away from the center of the view. This movement, called “image shift”, is caused by the focusing action which moves the primary mirror of the telescope back and forth. Once you have determined the focus setting that yields the smallest images, you may need to recenter on the star, and again perform the “sync on object” operation within Starry Nights. You are now ready to take your images. It is critical to take the time to assure that the best focus has been achieved and to periodically check the focus throughout the night. Telescopes can experience a change in focus due to large-scale scope movement and changes in ambient temperature. Select the object you wish to image, and slew the telescope to it. Field Rotation Before taking your first image, turn on the field derotation device. Field rotation is the slow rotation of the image orientation as stars rise and set. It can cause some stars in the image to seem perfectly tracked while others may appear elongated in a slight arc. Our telescope incorporates an image plane derotation mechanism to prevent field rotation for accurate integrations longer than a minute or two. Tracking Tests Drive gear error, vibrations, and optical shifting can ruin your images. If you notice that stars in your image appear elongated or tailed, it may be due to any one or combination of problems listed above. Drive gear errors tend to appear intermittent, and this is known as “periodic error”, which is caused by bad spots in the drive system of the telescope. Try taking several images, doubling the exposure of each successive image to determine what the longest possible exposure before any problems become apparent. If you should experience periodic error during the exposure, you can either try taking a shorter exposure when taking the next image, or simply wait a few minutes for the tracking to improve. Image Taking If imaging a faint deep-sky object, take MANY MINUTES of total exposures so that a very deep image (also referred to as a good signal-to-noise-ratio) can be processed later. Determine the maximum exposure duration usable for the tacking and pixel saturation limit. Very bright stars will produce a “bloom” of light along a column of pixels if you exceed the saturation limit. This should be minimized as much as possible, but without sacrificing the usable exposure duration needed for acceptable signal-to-noise-ratio (SNR). If limited to very short exposures (less than 30 seconds), a good SNR can still be achieved by acquiring and stacking numerous exposures. Color Images To create a color image with our monochrome camera, you will take successive images using a set of color filters, and then later combine the separate „filtered” images to create a color composite. The filters are mounted in a filter wheel mounted inside the camera housing. Using the camera control software, you can select the filter you want, and it will rotate into the light path of the camera. To create a color composite image, four filtered images will be required – a “luminance”; “red”, “green” and “blue”. After each exposure, be sure to save the image in FITS (.fit) format, giving the image a unique name (a suggestion is to name it by the object, filter, day & time – i.e objectname_B_2010_02_15_20_30.fit). Once you collected all of your images, you are done with the observations. The files will be transferred to room 217, and, in a separate lab session, you will combine the filtered images to create a final color composite.
Part 2: Image Processing Use astronomical image-processing software to calibrate, stack, and otherwise process your raw CCD images to bring out the objects of interest (nebulae, star clusters, etc.) so that they can be seen in detail with appropriate brightness, contrast, range of grayscale, and (if applicable) color balance. The following is a flow chart of instructions that you will use in “MaxIM DL” image processing software during the indoor portion of this lab, in room 217. Your TA will explain more on how to do this. Open images in MaxIm DL (more than 1 image of each filter) File Combine Select all Luminance images Align Mode – Auto Star Matching Bicubic resample Median Overlay all images OK Process Histogram Specification Lognormal OK (These steps sometimes bring out subtle detail, however, it might not improve the image, in which case select “undo”) Save as Master Image Now repeat above steps for each of the colors images, RGB. Then combine master images for color composite, using following steps. Color Combine Color In the blank fields, choose your L,R,G,B files (master images) Conversion type should be LRGB Align Overlay all images OK Process Histogram Specification Lognormal OK (Brings out subtle detail) Optional: FFT Filters Low pass Filter Hardness Mild Save your final image as a JPG and select “auto-stretching”. E-mail a copy of your image to yourself and your instructor.
