by Joe Heafner
Catawba Valley Community College, Hickory, NC
Finding asteroids in SDSS images can be tedious when done by simple
visual inspection of the images. If you've done the Asteroids
project, you've looked through lots of images to find asteroids like the one
shown to the right, and you've seen how hard you have to look. A much more efficient way
it to use computer software to help find asteroids.
Traditionally, this involves
taking an image of the same part of the sky at no fewer than two
different instants and then using computer software to "stack" the
images, which simply means to overlay the two images so that
corresponding stars are aligned. The computer software then rapidly
displays the individual images in succession, a technique called
"blinking". The stars, being fixed objects, do not wiggle but
asteroids, being moving targets, will appear to wiggle from image to
SDSS images allow us to use a slight variation on this technique
that relies on the fact that an image of a given part of the sky is
taken with different filters at slightly different times. Unlike
optical images, in which asteroids show up with the same color from
image to image, SDSS images show asteroids in two different colors. An
SDSS image is also a composite of three different colored images, and
each separate component image is taken at a slightly different time
from the other images. Think of an SDSS image as three different images
that have been digitally "stacked" as described above. If we could
somehow "unstack" an SDSS image and then "blink" the individual
component images, we might just be able to detect asteroids! In
practice, this technique works and here is how to do it.
The first step is to obtain a computer program that will manipulate the
images for us. An excellent choice is ImageJ,
developed by the National Institutes of Health. ImageJ is a Java program
that runs under Linux, Mac OS 9, and Mac OS X, and Windows. It is freely
available for download at the ImageJ
website. Your computer must have the ability to run Java programs. Your
instructor or teacher can give you more information about making sure your
computer can run Java programs - some additional software may be necessary
if not already present.
ImageJ has many advanced features needed for different kinds
of image analysis, including astronomical work and medical work. If you
have not done so, or if your instructor has not yet done so, you will
need to download ImageJ and install
it on your computer. Instructions are provided in the download archive.
The first step, assuming ImageJ is now properly installed on your
computer, is to get an SDSS image. You can use the
Get Fields tool to retrieve an image given its run, camcol, and field
numbers. If you don't have a special run, camcol, and field that you want to work
with, you can just choose these numbers at random. Be sure to get the
largest image you can, 1984x1361 pixels. This will make seeing the
asteroids visually easier.
Once you have an image, right-click on it to save the image to your
desktop. We recommend that you rename the image according to its run,
camcol, and field. The example image we are using has been named
r752c1f373.jpg, indicating Run 752, Camcol 1, Field 373. Note that the
image has been saved in JPG format, which is fine for our purposes.
Run 752, Camcol 1, Field 373 (click to open in a new window)
The second step is to load the image into ImageJ for analysis. Your
instructor or teacher will give you instructions on how to start the
ImageJ program. Once it has been started, select File -> Open from the
program's menu and load your image into ImageJ. The image will then be
displayed in a window on your screen. The screenshot to the right shows what
a sample image looks like opened in ImageJ.
The third step is to separate the composite image into its three
component images. The composite image is called an RGB image, meaning a
combination of red, green, and blue images. Actually, these are just
names given to the three component images and they might not
necessarily correspond to actual red, green, or blue as we usually
think of them. This is not important for our purposes however.
To do this in ImageJ, select Image -> Color -> RGB Split. You will see the
original image replaced by three separate images. Note that each one
will be labeled with its component color. A screenshot of the split
image is shown to the left.
The fourth step is to create an image "stack" with the three component
images. To do this, select Image -> Stacks -> Convert Images to Stack. The
screenshot of the stacked image is shown to the right.
We now have one single image again, or so it appears. In a way, we are
just undoing the separation we just performed but that is not exactly
what we are doing. We are instructing ImageJ to put the component
images together in a way that allows us to animate them, blinking them
in quick succession.
The fifth step is to animate the stack to see what wiggles. To do that,
select Image -> Stacks -> Start Animation. By default, ImageJ will
cycle through the three component images at one second intervals, but
this interval can be adjusted if desired. Depending on your computer's
speed and memory capacity, blinking may slow down your computer and it
may be slow to respond to key and mouse movements. A better option
might be to use the > and < keys to manually cycle through the
component images so you can start and stop as you wish. Stars and
galaxies in the images will remain stationary with respect to each
other. Their brightnesses may change, but their positions will not
change. If you see a dot that moves as ImageJ blinks the images, you
have most likely found an asteroid! The screenshots below show
two blinking images. The image below show the location of an asteroid
(marked in yellow).
You can get the location of your find in the image by moving the mouse
cursor to the dot that moves. The coordinates of this dot, in pixels,
will be displayed in the main ImageJ window, the one with the tool buttons.
In our sample image, a moving object is seen at coordinates X=894, Y=225.
You should always look at the same coordinates in the original image,
which you can reload into ImageJ using File -> Open again, to confirm
whether or not there is actually something there. An asteroid on the
original image will show up as two close but separate dots, one usually
green and the other usually blue.
In accordance with good scientific practice, your asteroid discoveries
should always be followed up by visual inspection of the original
image. When you record your findings, be sure to include the
coordinates of each asteroid in your records so that other astronomers
can verify your findings and use them for future work and for
reference. Upon looking at the original image, we surely do see a pair
of blue and green dots at very close to the location above. You can
always zoom the image to get more precise coordinate readings.