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Jeff Christopher
Internship Report
Greene Internship Summer 2009
My internship this summer was working with Dr. Durig in the Observatory. Our
focus was variable stars and trying to obtain their light curves. I have always been
interested in Astronomy and this internship has given me some insight into the life and
work of an astronomer. As a math major, my post graduate experiences can range from
work in economics to work in the hard sciences, and everywhere in between. I have
always been interested in physics and more particularly astrophysics, and I believe this
internship has given me some experience to help me get into those particular fields. My
work was mainly geared toward figuring out a good process for observing and analyzing
variable stars, which Dr. During had only done a few times with his classes. He is now
going to use my results as teaching aides in his astronomy classes, and for any
presentations related to variable stars.
My work involved actual observations for data collection, so the first thing I had
to do was accustom myself to the sleep schedule of a full time astronomer. While
describing my internship to others I joked that I work just a regular nine to five job. Work
started at nine o’clock pm and if the weather permitted I would generally leave around
five in the morning. This meant that I had to change my sleep cycle to nearly the opposite
of a normal cycle. Another issue was the weather, observing was dependent on good
weather. There could not be clouds in your field of view, there could definitely be no
rain, and if the wind was too strong the images would be ruined. If the weather was an
issue there was always some other bit of work to do that night, or I would just have the
night off.
If the weather was good all night then I would have a long night of observing. We
started off thinking that if we timed it right we could observe maybe two stars a night, but
we soon found that their variations we not completely set in stone, so it was better to stick
with one variable star and image it all night, this would give us the best light curve. I was
using our biggest telescope, which was to be the best for photometry; the only problem
was that the computer controls did not communicate with the telescope. So to change
targets I had to go outside and do it manually, which was not really much of an
inconvenience. I mostly did my work on my own, with Dr. Durig assisting me if there
were any technical difficulties.
A typical night would start at nine pm; I would walk into the observatory, go out
on the roof, roll back the cover for the telescope, and turn on the telescope. While the
telescope was booting up I would go back inside check the weather forecast, finish up
any processing, or start looking for a target. I would then go back outside and start
aligning the telescope. The telescope figures out its position on earth with GPS and then
it wants to align itself with Polaris (the North Star). The problem with this is that the
location of the telescope on the roof makes it difficult to view Polaris. So I had to trick
the telescope into thinking that it was looking at Polaris and then manually point the
telescope at a known star that the telescope has in its catalogue and align it with that.
So I would detach the telescope from the motors that move its alignment, and tell
it to go to Polaris. The gears inside would spin as it tried to move the telescope to aim at
Polaris, but the telescope itself was not moving at all. It would then ask if it was aligned
to Polaris, and I would tell it that it was. Then it would ask to align with a known star, in
my case it was always Arcturus, in the constellation Boötis, I would tell it to do so,
meanwhile the telescope is still disconnected from the motors. The gears would spin
some more trying to move the telescope again when it stopped it would ask if it was
pointed at Arcturus. At that point I would manually point the target at Arcturus, lock it in
place, and confirm that it was pointed at Arcturus. The telescope would then be in proper
alignment connected to the gears again and I would be ready to find a target for viewing.
There were a couple factors in determining which star we wanted to image that
night. The star obviously had to be a variable star, which is a star whose brightness varies
for assorted reasons. We would use a data base of about 8000 variable stars of all kinds.
We also had to have one that would have a period of variation for preferably less than 6
hours, this way we could have a full light curve. Another factor was that we needed stars
that had brightness ranging from 10th to 16th magnitude. Furthermore, we needed stars
that were in a proper position in the sky. We needed to track the stars all night so we
didn’t want to pick a star that would set below the horizon before the Sun started coming
up. If the stars got below 30 degrees above the horizon there would be too much
atmospheric interference in our images, so we picked stars that at 9 or 10 pm would be
roughly 30 degrees above the horizon in the east, that way we could track them all night
and by the time it was sunrise the star would still be high enough over the western
horizon.
All the variable stars we observed were known, and the way that they were
labeled would involve which constellation they were in. So the easiest way to determine
which one you want to use is narrow your search to all the previous specifications and
determine which constellations would have the best position for our viewing. So the stars
that I observed were stars in the constellations; Cygnus, Ophiucus, Draco, Pegasus,
Hercules, or Boötis. Another factor in picking stars to observe was the type of variable,
Dr. Durig wanted a variety of different stars because the differing types of variables have
different light curves. I ended up observing 9 different types of variable stars however not
all of them gave the kind of light curves that I would want because of some external
problem with the weather or with the telescope.
After I picked a target I would figure out its exact position in the sky, none of the
stars were visible by the naked eye so I couldn’t just point the telescope with the aimer.
The coordinates that you would look up were its right ascension and declination; these
are coordinate directions on the celestial sphere, measured in degrees, minutes, and
seconds. Once I was aligned with the target I would go inside and take a 10 second
exposure to make sure I was looking in the right part of the sky, if I was then I would use
the computers to figure out the best star for the telescope to track. Once I had a guide star
I would tell the telescope to follow it, so every couple seconds the telescope would make
small changes in its position to follow the movement of the star across the sky. Once the
telescope was tracking, I would tell the camera on the telescope to take 2 minute
exposures. Once the telescope was tracking and the camera was imaging the only thing I
had to do was monitor everything and make sure the images were good and the guide star
didn’t get lost.
When I got about 50 images or so I would star processing them in a computer
program called Astrometrica. The program would determine the brightness of the stars by
subtracting out the background light from the image. Around 4:30 I would stop imaging
and finish up all the processing. After I shut down the telescope and covered it back up I
would start trying to get my light curve. To get the light curve I would use a different
computer program called Maxim DL. In the program I would open all the images up and
use its photometry tool; I would then pick a reference star and a check star and measure
the brightness of my target star relative to the brightness of the other stars. So I set the
reference star to a particular magnitude and relative to that the computer would compute
the magnitudes of all the other stars. I would then graph the magnitudes of each star to
each image, if the check star varied too much then either the reference star was a variable
or the check star was a variable. So it was important to have a non-varying reference star.
If the images and reference stars were good then there would be clear variation in the
graph from the target star, which is exactly what we were after.
Overall the experience of being a full time astronomer for a summer was a good
one. It was a good experience to have total control of the observatory on my own, and
being responsible for a lot of sensitive equipment. I got to learn a lot about variable stars,
I relearned some constellations, and at the same time I got to learn a good deal about
imaging and data collection of variable stars. Dr. Durig was a great advisor who was
always keeping things interesting and I believe that I got a good amount of work done for
him. It was a good job with fair pay, and I think it was very good experience for me if I
wanted to work in a related field. So the summer spent under the stars was a total success.