Using Image J to Measure the Brightness of Stars (Written by Do H. Kim) What is Image J? Image J is Java-based image processing program developed at the National Institutes of Health. Image J runs on everywhere, including Windows, Mac, Linux, etc. Image J enables users to automate tasks and create custom tools using macros. Moreover, the Image J is the world s fastest pure Java image processing program. The program itself with more than 200 macros and 300 plugins are available freely at http://rsb.info.nih.gov/ij/ Getting Ready 1. First of all, open an image by File > Open. Open Recent will show last 15 files opened. Open Next will open the next image in the folder that the active image is stored. Or if you do not have any images, you can just use samples by using Open Samples. [Call what you have just opened Image 1] 2. Open another image of same sky, but taken with some other band (V Band, R Band, B Band, U Band, or I Band). [Call this Image 2] * In this case, I chose V Band for Image 1 and B Band for Image 2.
3. Locate a star you want to measure in Image 1 by zooming in and out by Image > Zoom > In or Out. Or simply use the Magnifying glass on the menu bar. Select the Magnifying glass and click on the star several times to get a better view of the star. *You can only zoom in with the Magnifying glass. To zoom out, Image > Zoom > Out. 4. To find a faint star, adjust brightness and contrast by Image > Adjust > Brightness/Contrast. You can adjust manually by changing the Minimum, Maximum, and Contrast control. Easily, clicking on Auto will find majority of stars. If you are not satisfied with what you have done with Brightness/Contrast, simply click on Reset to restore the original brightness and contrast. *You have to highlight the image you want to adjust (Image 1 or 2)
5. Now, you have two images on the screen and you need to install the Circle Tool by Plugins > Macros > Install. Then browse the circle tool from you folder. It will create a new icon on your menu bar. * Macros are usually formatted in.txt or.ijm Measuring a Star in Image 1 6. Now, you know which star s brightness you want to measure from Image 1 and have it zoomed in to the star. Double-click on the Circle Tool you created in Step 5. You will have this new window. Set the radius and click on the star you want to measure. It will show a yellow circle around the star. Use arrow keys on your keyboard to move the circle so that the star will be on the center of the circle. 7. If the circle is too big or small, change the radius by clicking on the Circle Tool again. The Circle Tool window will show the current radius. Set the radius accordingly so that the circle
perfectly encloses the star, but does not include any other star. * When measuring several stars, set radius for the biggest star so that all stars are measured with the same radius. * In crowded field, it may not be possible to enclose only one star in a circle. Ignore the overlapping stars. 8. If you are done with enclosing a star with the Circle Tool, you are ready to measure the brightness of the star. Go to Analyze > Set Measurements, then check on Min & Max Gray Value, Integrated Density, and Centroid and click OK. (Uncheck everything else) 9. Go to Analyze > Measure to get the measurement. 10. Integrated Density of Result 1 tells you the number of counts that are captured from the yellow circle in Image 1. (In this case, 948598) *the number of counts is a function of the number of photons that hit the CCD
11. Now, move the yellow circle to blank black sky in Image 1. 12. Now, measure again. Analyze > Measure. 13. Integrate Density of Result 2 is the number of counts that are captured from the sky itself. (75904) 14. Subtract the Integrated Density of Result 2 from Result 1. (948598 75904 = 872694) That is the actual number of counts from the star. *You can do this simply with Excel or manually use a calculator. Measuring a Star in Image 2 15. What you need to do now is to find the same star on Image 2 that you opened in Step 2. It might seem difficult to find a star from hundreds of stars in the image. However, your Results Window shows X and Y value, telling where the star is. ( X= 1209, Y= 347) It is easier to find the exact point if you zoom in the image. When you find the star, use Circle Tool to enclose it. The Radius Window will show last-used radius so just click OK.
* The X and Y values may be slightly different if the telescope moved between two exposures. A little error on X and Y values is ok as long as you can locate the same star. 16. Measure again. Analyze > Measure. 17. Integrated Density of Result 3 tells you the number of counts that are coming from the yellow circle in Image 2. (236903) 18. Now, you need to repeat what you did in Step 11. Move the yellow circle to blank black sky in Image 2. Here, you do not have to locate the same spot you used in Step 11. Different spots on blank sky emit similar number of counts. 19. Measure. Analyze > Measure.
20. Again, the number of counts from yellow circle the number of counts from blank sky = actual number of counts from the star. (236903 19056 = 217847) Analyzing 21. After you finish measuring, be sure to save the results by File > Save As in Results Window. * It will automatically save the results to Microsoft Excel Worksheet format. 22. In this example, Result 1 Result 2 tells you the number of counts from a star taken with V Band and Result 3 Result 4 tells you the number of counts from the same star taken with B Band. If you use B Band and R Band images, more counts in B Band image means that the stars hotter and more counts in R Band image means that the star is cooler. Other Tools in Image J - To copy a part of your image, select the area you want to copy by choosing rectangular selection, elliptical selection, polygon selection, freehand selection, straight line selection, or point selection. Then Edit > Copy or Edit > Cut. - Edit > Clear will erase the part you selected. Clear Outside will erase everything outside the selected area. - Edit > Invert will create a reversed colored image of an entire image or selected part. - Image > Type lets you change the type of an image to 8-bit, 16-bit, 32-bit, 8-bit color, or RGB color. - Image > Type > RGB Stack will create 3-slice (red, green, blue) stacks of an image. - Image > Type > HSB Stack will create 3-slice (hue, saturation, brightness) stacks of an image.
- Image > Color > RGB Split will create 3 windows with same image but different colors (red, green, blue). Image > Color > RGB Merge will put the 3 windows together into one image again. - Image > Stacks > Add slice will add a slice next to the active slice. Image > Stacks > Delete Slice will delete the current slice. - Image > Stacks > Convert Images to Stack will put images in different windows into one window with many slices (stacks). Convert Stack to Images will do the opposite. - You can change the size of an image by Image > Adjust > Size. If you uncheck the Constrain Aspect Ratio, you can change the length and width of the image however you want, disregarding the original ratio. - You can change the size the canvas without changing the actual image by Image > Adjust > Canvas Size. - Image > Stacks > 3D Project rotates the image onto a plane with any angle and axis you want. - Image > Stacks > Start Animation repeatedly displays stacks of the active image in sequence. Use Stop Animation to stop the animation. - Image > Crop cuts a selected part and zoom it. Image > Duplicate does the same thing, but in a new window. - Process > Smooth will blur the active image or selection. Process > Sharpen will do the opposite. - Process > Find Edges highlights sharp changes in intensity in the active image or selection. - Process > Add Noise adds random noise to the image or selection. Process > Add More Noise to intensify this effect. - Process > Shadow produces a shadow effect to the active image. You can choose which way you want the shadow to be produced. - Process > Filters > Gaussian Blur lets you blur the image with your desired radius. Larger the radius, more blur the image. - Process > Filters > Median reduces noise level in the active image. Larger the radius, less the noise. Process > Filters > Mean does the similar thing.