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by Rich Lohman
Introduction: On March 4, 2005,
telescopes from two different observatories
in the United States took simultaneous images
of an asteroid called “1998wt”. The
two observatories were Gettysburg College Observatory
in Pennsylvania and Yerkes Observatory (24” telescope)
in southern Wisconsin. They are approximately
970 km apart as the crow flies. This was
not a new discovery of an asteroid, but the
images were taken to demonstrate the use of
parallax to determine the distance to a relatively
nearby object in the sky. When you view
these images you will discover a noticeable
difference in the location of the asteroid against
the background stars as you compare one set
of images to the other set. A measurement
of this “parallax shift” and the
knowledge of the distance between the telescopes
will allow you to calculate the distance to “1998wt”. - Image Processing software
Images: FITS files 1998wt-050304-0245g,
1998wt-050304-0250g, 1998wt-050304-0255g,
1998wt-050304-0245y, 1998wt-050304-0250y,
1998wt-050304-0255y. (Don’t use
those labeled “zoom”.) The
three images ending in “g” are
from the Gettysburg scope. Those ending
in “y” are
from the Yerkes scope. The “050304” indicates
the date, March 4, 2005. The “0245”,
etc. indicates the time the images were taken
in Universal Time (UT). As you can see
the images were taken simultaneously and with
5 minute intervals. There are two ways to
get thses images: on the GSS CD-ROM or download
the compressed archive file in the right column
on this page.
Procedure:
1. Using the IP software open the three Yerkes (y) images and place them side by side. Click on the “Brightness and Contrast” icon
and click “Auto” once or twice to see the images clearly. Try to locate the asteroid moving through
this field by comparing the 0245 and 0250 images. Often an asteroid will have something of an
elongated, sausage-like look. Once you
have found the asteroid then see if you can find it in the 0255 image. If you’re unsuccessful, or as a confirmation,
use the procedure in step #2 below to locate the asteroid.
2. Click on the “Images” menu. Click “Stacks”. Click “Images to Stack”.
3. Once again click the “Images” menu. Click “Stacks”. Click “Start Animation”. You will now see the asteroid in motion as it
moves across the image.You can stop the
animation by clicking anywhere on the image.
4. Move the slider on the bottom of the stack to the left so
you are viewing the image 0245y. Put
your cursor over the asteroid, and look up in the orange bar to see the x and y
coordinates of this point (pixel).
Record
those coordinates (x, y) = (_______, _______)
5. Close all 3 of the Yerkes images. Then open all 3 of the Gettysburg (g) images and place them side by
side.Adjust the Brightness and Contrast
as before.
6. You will notice
the obvious difference that these images have a much larger field of view, so
finding the asteroid by sight is quite difficult. Go through the same process as above by
creating a stack and starting the animation to locate the asteroid moving
across the images. You may find that
zooming the image will help you see it more easily.
7.Once you have
located the asteroid in the 0245g image identify the (x,y) coordinates and
record them:
(x, y) = (_______, _______).
8. Close all but the 0245g image. Then open the Yerkes 0245y image.
Place them side by side. You now have open the 2 images which were
taken at the same time from the two locations. Recall the location of
the asteroid
in each image.
Now you’re in a position to measure the parallax shift of the asteroid. If the two telescopes were identical and using the same cameras, then this process would be very simple. You would superimpose one of the “g” images onto one of the “y” images. Then you would measure the pixel shift and convert the pixels to an angle using the Plate Scale. It’s not possible here since the plate scales and image sizes are different. So another procedure is called for. Here are two methods:
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