|The asteroid "Pratchett" (and a few more)
Here are the actual images that were being shot by the telescope in the time-exposure photograph on
the previous page. This movie loop spans 48 minutes of time on 6 September, 2010. Did you recognize that the stars
visible through the dome slit are the same ones as are visible in the photographs of Vesta at the top of the previous page?
So these images are also of Capricornus, but of a much smaller portion: this patch of sky is only about half the size of a
Now that you've practiced by finding Vesta, how many asteroids can you spot in this
image? In this movie loop there are 5 frames instead of just 2. The first frame is held for twice as long as the later ones.
To get you started, look in the exact center of the image and see if you can spot something moving diagonally down and
to the right. That's the asteroid "Pratchett". It starts out just to the right of a star.
At first glance, the challenge appears
to be spotting all the moving objects. But that's not really the challenge: there is lots of moving stuff
in the images above. The real challenge is ignoring the noise and seeing just the moving objects that are asteroids.
To discover something new, you have to have some sort of advantage over everyone else out there
also looking. One strategy is to look and look and look and eventually get lucky. That's how amateur comet hunters make their discoveries. (Of course, if your technique is bad you'll never discover anything no matter how long you look.) Or, you have
to build better equipment that lets you see things others can't see. That's how professional physicists usually make their
discoveries: think of the new particle accelerator in Europe. Or, you have to come up with a new idea on how to look, or a new place to look, or a new kind of thing to look for
that others haven't even thought about yet: that's how explorers and mathematicians make their discoveries.
Most often, though, all that's required is simply to push at the limits of what current instrumentation can
do with a thorough and diligent effort. That's the case here. And that means the images can't be pretty,
because if they're pretty you aren't pushing the data hard enough to discover much! The trick is to distinguish what's real
from all the noise. How can we do that?
Over such a short time interval and such a small patch
of sky an asteroid will move essentially in a straight line at a constant speed. So anything that does not move that way
can't be an asteroid! For example, the black blob near the upper right corner of the image is a bad spot on the camera
chip. Notice it doesn't move in a straight line: it dances around. Cosmic rays also can hit the camera chip during an exposure
and make a star-like dot in an image. But unless you are very unlucky, those will flash on and off, or at worst jump around:
they won't move in a straight line.
There is also "real noise":
something that really is in the sky, not a hallucination of the camera, but still is not interesting. For example, a manmade
satellite moves very very fast and crosses entirely through the image in one exposure. So it shows up as a line that blinks
on and off because it's only in one image. There is one in this set of images. Spotted it? Airplanes similarly leave
(wider!) trails through images. And because our observatory is in a swamp we sometimes get noise from fire flies in our
Use "Pratchett" as a guide to train your eyes on how an asteroid
should move, and use the dancing black blob to train your eyes as to how noise coming from the camera chip moves. The
vertical line just to the left of center is another bad area in the camera chip, and so it also moves exactly like
the black blob.
Now see how many asteroids you can spot! But don't go overboard! False data
is much more dangerous than overlooked data. Because, of course, the easiest way to make a "discovery" is to hallucinate