Tips in CCD Solar Imaging.
Gordon Garcia
ALPO Solar Section Ass't. Coordinator
(copyright 1999)

On April 10, 1993 at 18:59 UT, Gordon Garcia of ALPO's Solar Section and CCD camera designer R. Radhakrishnan took a CCD image of the Sun's west limb using Radhakrishnan's homemade camera with a Texas Instrument TC-211 chip. The image was significant as it marked the first time a CCD image of the Sun was to be sent to the ALPO Solar Section. Since that time there has been a proliferation of electronic images of the Sun by observers. During 2000 electronic images in the Solar Rotation Reports outnumbered both photographs and drawings. The attraction of electronic imaging is simple. Once the image is secured on a disk or hard drive there is no darkroom needed to develop and print the image. A computer and printer makes processing and printing an image much easier. Electronic images can also be sent across the Internet and copied a multitude of times without loss of image quality. The purpose of this section is to outline some information about imaging the Sun with a CCD camera. These are not the only means of electronic imaging. Virtually all video cameras and digital still cameras today are based on CCD technology. Images from these cameras can be transferred by various means into a computer. Prints and negatives can also be scanned and converted to electronic format for image processing or transmission on the Internet. The CCD or Charged Coupled Device is a small solid-state sensor consisting of silicon. Each CCD chip is made up of tiny photo sites called pixels. Each pixel reads the incoming photons of light and converts them into an electrical charge that is amplified and converted into a numeric or digital format that can be read and processed by a computer. As in lunar or planetary observing it is important to match the aperture and focal length of the telescope to the pixel size of the sensor. The formula for determining the correct pixel size for a particular focal length is: F (focal length in millimeters) = 206 x P (pixel size in microns)/S (arc seconds per pixel). To achieve one arc second per pixel resolution with a TI-255 sensor (10 microns) the focal length of the optical system must be at 2,060 mm. However, the Nyquist theorem requires that for an image to be properly sampled, the airy disc must occupy at least two pixels. This would require that each pixel sample 0.5 arc seconds, or a minimum focal length of 4,120 mm. Localized seeing conditions may not allow long focal lengths. To achieve higher resolutions longer focal lengths and possibly additional aperture must be employed. Another consideration is the actual size of the sensor being used. To achieve maximum resolution on large sunspot groups (larger than 1500/millionths of the solar hemisphere) and frame the entire sunspot group a large CCD sensor must be used. The TC-211 and TC- 255 chips contain relatively small imaging areas. When considering the purchase or construction of a camera one must consider the aperture, focal length and field of view to be achieved. It is possible to take a series of images with a small CCD chip and then digitally stitch them together to form a mosaic that displays the entire active region. The CCD sensor should be capable of some relatively fast exposures to freeze seeing and wind induced vibration. The fastest commercially produced astronomy cameras can transfer a full frame with an exposure of about 1/100 second. It would be better if an exposure of 1/1000 second could be achieved. An enterprising amateur could combine a mechanical shutter in front of the CCD chip to attain a quicker exposure. Newer cameras and digital SLR and fixed lens cameras have faster shutter speeds. To achieve the best results the camera exposure must be triggered remotely. Although at 1/100 second electronic noise is greatly reduced, it is still advisable to take a dark frame as well as a flat field frame and bias frame, especially if solar photometry work is the goal of the observer. A flat field frame can be achieved by removing the solar filter and imaging the blue sky away from the Sun. The camera position and focus must not be changed while taking a flat field frame. Some camera routines allow for a bias frame. Otherwise, a zero-second exposure can be taken for bias correction. Bias should also be removed from both the flat field frame and dark frames before calibration of the image frame. CCD cameras can be used to image the continuum and various wavelengths of the solar spectrum, the two most popular being Hydrogen-alpha (6563 Angstroms) and Calcium-K (3934 Angstroms). With narrow wavelengths of light interference fringe patterns can be set up between the CCD chip itself and the protective window over the chip. The best way to avoid this problem is to initially purchase a camera that will not produce this fringing. You can contact the camera's manufacturer to determine if this problem exists. If you have already purchased the camera, there are a couple of things that may eliminate or lessen the problem. The first thing to try is tilting the camera slightly in relationship to the filter. If you are unable to tilt the camera, inserting a linear polarizing filter between the filter and camera appears to help somewhat in H-alpha imaging. Lastly, you may try either to flat field the interference pattern out of the image, or take several images so the fringes are averaged across the entire image, then adjust the histogram to alleviate the darker fringe pattern. In order to capture and process a CCD image a computer is needed. Although older 386 and 486 machines are capable of capturing and processing a CCD image, more modern Pentium and Power Macintosh computers make the job much easier and faster. Whether you are using a notebook or desktop machine you will have to provide shade in order to see the monitor in the bright sunlight and some way of keeping the computer from overheating on hot days. If possible, the computer should be kept inside and connected to the camera and scope by remote cable. There are many software packages that are provided by commercial vendors. Unfortunately, it is often necessary to purchase several programs in order to achieve the desired results. Software programs that are produced solely to process astronomical images are often the best choice. However, a second program geared toward the graphic arts industry is needed to add text, composite more than one image on a page, etc. Internet astronomy discussion groups are often an excellent place to hear from other astronomers the merits and faults of various software products. When imaging in the field, the first thing is to consider is how to organize the raw data on your computer. One easy way is to create a new folder for each imaging session by date (i.e., 991130, year, day, month). The most important thing in the field is to get as many images as possible. Dark frames and bias frames can be taken ahead of time if you can control the shutter speed and temperature of the camera. It is a good idea to run the camera at a cooler temperature than the ambient air to avoid thermal noise. Be careful to bring down the temperature slowly or dew can form on the CCD chip's protective window. The first thing is to orient your camera so north, south, east and west is squared onto the CCD chip position. Cameras usually have a decal or marking on the back of the camera to delineate orientation. Also, inspect the chip with a magnifying glass and make sure the CCD is free of dust particles. Dust can be removed with a hand held air bulb and some Q-tips. One way to avoid dust is to keep a minus-UV type filter over the CCD chip. Make sure your computer clock & calendar are accurately set using a WWV time source. There are also software programs available that set the computer's clock in reference to the U.S.N.O. master clock. In order to find the area of interest on the Sun either a flip-mirror previewer or separate guide/finder scope with safe solar filtration is very helpful. Otherwise, you will first need to locate the active region with an eyepiece and center it in the field of view. Once you have located the area to be imaged, you can place the camera on the telescope. Most cameras will have a focus mode where you can image only a portion of the sunspot and the image will be refreshed on your monitor rapidly so you can tweak the final focus. In order to start at a point close to focus an eyepiece parfocal with the camera can be used to achieve an approximate focus. A final focus can then be achieved by watching the image on the computer. Focusing screens (off-axis dual or triple hole masks placed in front of the solar filter) are also helpful. Once your camera is in focus proceed to take your reference flat field images. Be sure to remove a dark frame and bias frame for each flat field frame taken. If you change the orientation of the camera (or move it at all) you will need to take a new flat field frame. Be sure to title these reference images so you can identify them later (Flatf1, Dark1, etc.). You can then begin your actual imaging of the Sun. The key is to take as many images as possible during your observing session. Don't worry about processing the images until you are done. Be sure to check the histogram as you experiment with different exposures. Make sure that pixels are not being saturated. The properly exposed image should have a peak approximately 75% of saturation. When processing the images the best rule is that the best image is the one that requires the least amount of processing. First begin by examining each image from the ones you will ultimately process. Select those images that appear the sharpest and are properly exposed. Start by calibrating the images. Be sure to save both your raw images and calibrated, unprocessed ones as they may be needed at a later date if a request is received by the professional community. Also be careful where the zero-point is set on the image. Some photometry programs create a wrap-around problem whereby umbrae values may end up being higher than the surrounding photosphere. The first step in processing is to perform a histogram stretch. The next step is to run an unsharp mask routine. Most image processing programs have at least one unsharp mask. Start with a pixel radius of at least three pixels. Experiment and keep notes. After you are satisfied with the image adjust brightness & contrast and final "tweaking" such as a slight sharpening routine. The final image should have more detail, appear sharper, but not look over processed. There are no hard, fast rules for processing solar CCD images. Experiment and see what works and what doesn't. The final step is to add the necessary information to the image. With the great amount of digital images being received by the Solar Section they are easily separated if the identifying information is not embedded in the image. The necessary information can be added as text above or below the image or embedded in the FITS header. The extension to the file name should clearly indicate the format the image is in (TIF, JPG, FIT, etc.). The minimum information should include the date and time the image was taken. The time should be indicated in Universal Time. The name, address and E-Mail address of the observer must also be included. Other information that is needed with each image is: the telescope used (150 mm f/8 reflector, etc.), the effective aperture, type of solar filter used, seeing (in arc seconds), Carrington Rotation Number, Central Meridian, Active Region Number (NOAA SEC)(if applicable) and approximate heliographic coordinates. Two cardinal points should also be indicated (North - up, West - right, etc.). When sending your images to the ALPO Solar Section we strongly recommend that you send compressed JPEG images, with the file name as the date of the image (year, month, day). An example would be 010517a.JPG, 010517b.JPG, etc. This is due to size constraints of our Internet server and speed in uploading files on the Recent ALPOSS Observations.

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