G oand Webcam Images

O Oc Once you become comfortable in the use of your CCD camera or webcam, it will be

Z O no time at all until you will have collected a number of beautiful images. There can be a great deal of personal satisfaction in taking and processing the perfect image as the artistic side of imaging is certainly part of the pleasure. But, if you want your images to be more than just pretty pictures, then you have to be a scientist

and make them mean something. Of course, that is the major focus of this book, to encourage the reader to study Jupiter and add something to our knowledge of the planet. Certainly, your images will become part of the historical record of the physical appearance of the planet; but there is more to be gleaned from these images. For your images to be truly worth something scientifically, they will need to be measured to determine the position of features seen on Jupiter's disk.

Previously we discussed the use of central meridian transit timings to determine the longitudinal position of features seen on Jupiter's disk. Measuring CCD and webcam images has the same goal, to keep track of features over time. The longitude of features can be scaled from the planet's disk, using the images from webcams and CCD cameras.

Using a fan shaped device described in Rogers [516] (Fig. 9.13), it is relatively easy to determine longitude of any feature seen on the disk if the longitude of the central meridian is known for Jupiter at the time the image was taken. Once again, it is important to record all the pertinent information regarding the image just like we do for a disk drawing or transit timing. The imager should record the date and time to the nearest minute in Universal Time, the telescope size and focal ratio, seeing conditions, transparency, and the location of the observer. Additionally, information should be recorded regarding image processing, including the camera used, filters if any, integration times, number of frames and frame speed, and any other information that will help the user of your images understand how they were produced. This data gives credibility to your image and makes it useful. For example, without the date and time the image is almost useless for science.

Fig. 9.13. A measuring device used to scale the longitude of features on a CCD or webcam image. (Credit: John Rogers, 1995).

Fig. 9.13. A measuring device used to scale the longitude of features on a CCD or webcam image. (Credit: John Rogers, 1995).

Knowing the central meridian of Jupiter in your image, Roger's device is used to scale the longitude of features seen on either side of the central meridian. The image can be examined on your computer screen or you can print it out onto paper. To use the device, place it onto Jupiter's image with the outer rays of the fan on the left and right limb of the planet at the latitude of the belt or zone containing the feature you wish to scale. With the disk of Jupiter oriented so south is at the top, place the fan over Jupiter so that the widest opening of the fan is up if measuring features in Jupiter's northern hemisphere, and place it with the widest part of the fan down if measuring Jupiter's southern hemisphere. Each division in the fan represents 10° of longitude. With Jupiter oriented so that South is up, longitude on Jupiter will increase from left to right, or from the preceding side to the following side. So, a feature that is to the left of Jupiter's central meridian will have a lower longitude than the central meridian, and so on. I have used this device with great success, achieving an accuracy of one to two degrees with my measurements. Measuring is not difficult; it just requires patience and attention to detail. Even old film photographs of Jupiter can be measured using the Rogers device as long as you know what the central meridian is for Jupiter on that photo.

The longitudinal positions of features should be recorded on the transit form previously discussed, in the same manner as a central meridian transit timing using the same nomenclature to describe the objects measured. We have already discussed the value of recording the longitudinal position of features over time. This is one of the most important contributions the amateur astronomer can make.

CCD and webcam images have a great advantage over central meridian transit timings in that the images are not subjective and the measurements are not prone to timing errors, as transit timings can be. Images can also be used as a record of color and intensity, but care must be taken not to over process or to increase the contrast too much during processing. Color can also be skewed if care is not taken in the application of filters and color processing. Imagers should strive for a finished image that shows great detail but that is as natural as possible. One imager that has excelled at this is Ed Grafton of Houston, Texas. Much can be learned from his work. It is exceptional. To keep myself honest in my own image processing, I make regular visual observations of Jupiter so I know how the planet is supposed to look. Besides, as exciting as imaging can be, still nothing compares to seeing the planet visually in the eyepiece of a telescope.

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