In many areas of science the analysis of images is an important laboratory technique. In astronomy this takes a major role as telescopes are the main way we learn about astronomical phenomena. The analysis of images has become much easier in recent years because of the advent of digital images. Because images can be stored in the computer, measuring, analyzing and altering them have all become relatively simple.In this lab we will use a software package for analyzing digital images to study some pictures of Saturn and some of Saturn’s moons. All images used in this lab have been taken with the Cassini spacecraft, a NASA mission that was in orbit around Saturn from 2004 to 2017. Thus these are the highest resolution close ups ever taken of the Saturnian system.
One of the most important issues in studying an image is determining the physical size of objects in the image. This is not easy because what the camera captures is the angular size of an object modified by the magnification of a telescope. When displayed on a screen the image can also be zoomed so that the apparent size of the image has little relation to its actual size. One can calculate the physical size of an image if you know the distance of the object, the magnification of the telescope and any zoom applied in displaying the image. Usually it is easier to just scale the image if you know the size of one thing in the image. In this lab we will scale all images by knowing the diameters of Saturn and her moons.
| Name | diameter |
| Saturn | 120,536km |
| Titan | 5150km |
| Thythis | 1060km |
| Enceladus | 499km |
Laboratory Tools
In order to analyze images of Saturn we will use ImageJ which can be downloaded to your computer or run in the browser if you are using Chrome or Firefox. I recommend you run it in the browser, you can easily download Chrome or Firefox if you don’t have either one. To run it in your browser just click on Run in Chrome or Firefox.
You will also need the images of Saturn and Saturn’s moon which you can download here.
The ImageJ tool will appear like the top of a window having a menubar and menu selections. The top row will have File, Edit, Image, Process, Analyze, Plugins, Process, Help. Below that are a set of menu buttons many of which are drawing tools. Below that will appear some text depending on what you are doing. To load an image use File->Open…. Once an image is loaded it is important to set the scale before making any measurements.
Set the Scale – To set the scale first choose the straight tool, its the 5th box down. Then make a line across the diameter of the object. You will see the pixel values and the length in pixels under the boxes. Then choose Analyze->Set Scale and a new window will pop up. In that window the first row, Distance in pixels, should be the same number that was in under the boxes earlier. In the second row, Known distance, enter the known diameter of the object. In the fourth row, Unit of length, enter km. Then hit Ok. Now when you measure something it will be correctly scaled in kilometers.
Measure a Length – The straight tool can be used to measure a length if you can place the cursor and end it at the correct location. If the intensity changes by a large amount the plot profile command can be much more accurate.
Line Profile – To get a line profile first use the straight tool to draw a line, and then choose Analyze-Plot Profile. A plot of intensity vrs distance will appear. You can use the line profile to measure a length where the intensity of the pixels change dramatically. Placing the cursor on the plot will tell you the value of X which since you set the scale is now in kilometers.
Adjust Brightness/Contrast – The contrast can be adjusted from Image->Adjust->Brightness/Contrast. This will bring up a new window that shows a histogram of the intensity values in the image. From the histogram you can see that not all of the range is being used. To enhance the contrast click on set which will open another window with the minimum and maximum displayed value. Change one of these values to give a smaller range that covers the histogram of values you see. You can keep adjusting.
Assignment
- saturn.jpg — Measure the width of the shadow Saturn’s rings cast on the planet using the line profile.
- green saturn.jpg — Measure the height of the shadow from Saturn’s rings in this image. Measure the width of the rings, the distance from the inside of the rings to the outside.
- rings.jpg — Note this image has no scale. Use line profile to determine the number of gaps in the rings.
- titan ring.jpg — This is an image of Titan behind Saturn’s rings. Set the scale with the diameter of Titan. Measure the thickness of Saturn’s rings from this image.
- titan lake.jpg — This is an image of Titan mostly in shadow but with one lake brightly reflecting light. Measure the diameter of the lake.
- tythis.jpg — This is an image of Tythis which has not been calibrated. That is why it has the clear artifact on the right side. Measure the diameters of the two very large craters on Tythis.
- enceladus.jpg — This is an image of Enceladus, mostly in shadow. The geysers can been seen on the top of the moon. Measure the height of the geysers. Now change the brightness using the Adjust Brightness/Contrast as described above. Remeasure the height of the geysers now that you can see them better.
Questions
- Why might the shadow made by Saturn’s rings be larger than the thickness of the rings themselves?
- If you know the lengths of two objects in the image, one smaller and one larger, which is better to use to determine the scale and why?
- Compare the height of Enceladus’s geyser to its diameter. What would be the height of an equivalent ratio geyser on Earth?
