Exercise 6.
The steps you must follow to blink images are outlined below.
1. Retrieve the data from the
SDSS Retrieve Images tool.
You want run 125, rerun 7, camcol 5, field 178, objid 55.
Enter these numbers in the textbox, separating each
value by a space. For the data type, deselect Atlas Images and select
Corrected Frames.
Select the r and g bands (they are on opposite ends of the camera, so
the most time passes between the r and g bands, making it easier to
find asteroids). Return the files as a .zip file. Unzip
the files into the default directory you specified earlier.
Rename the files so they are easy to remember. Be sure the
filename ends with the .fts extension.
2. Align the fields:
Since some time passes between the two exposures, the fields are not
exactly aligned. To directly compare the images, we need to shift
their alignments. Open
the green picture first. Adjust the max and min values to make the
images clear. Select any easily identified bright
star in the field. Draw a box around it with the mouse.
Right click on the box and select PSF (this stands for Point Spread
Function). Record the x and y coordinate of the center.
Open the red picture and repeat the process to find the center of
the same object.
Now, subtract the x-coordinate of
the green image from the x-coordinate of the red image. Subtract
the y-coordinate of the green image from the y-coordinate of the red
image. Load the green image. Click on Geometry and select
Shift. You will see a box as shown below:
Enter the offsets you calculated. Be sure not to drop the
signs! Click OK when you have the images aligned, then save both files.
3. You are now ready to blink the images! Click on the
 in the toolbar. This button
will bring up a small command box. Type the blink command.
The blink command has the format:
blink image#1 image#2 delay
For image#1 and image#2, type the filenames for the red and green
images (no .fts extension on the end, and be sure they are in your
default directory). "Delay" is how long the images stay on the
screen before switching, given in milliseconds. Try 200 - 300 for the
delay (this will give you a 0.2 to 0.3 second delay). You can change the delay
later if you like. To stop the
images from blinking, type the command blinkoff.
All of the objects in the images should remain stationary, except for the
asteroids. Find the asteroid(s)! Some objects may get
brighter or dimmer but not move. That is all right. Some
objects are brighter in the green or red filter. |
