Improving Predictions of Pluto Occultations at Lowell Observatory
Rebecca M. Jensen-Clem
1 Abstract
Using the astrograph at Lowell Observatory, Dora Gao and I observed Pluto
occultation candidate stars and a Kuiper Belt Object occultation star using the
method of CCD strip scan imaging. We learned how to use the astrograph and its
positioning and time-keeping systems as well as the camera control system used
at Lowell Observatory in order to carry out our observing plan. Our
observations resulted in nearly 150 strips that will contribute towards
improving the predicted occultation paths of Pluto and the KBO 55636.
2 Introduction
An occultation occurs when a nearby object passes in front of a comparatively
distant star from the point of view of an observer on Earth. Occultations
provide the most efficient means of probing the atmospheres of distant bodies
in the solar system by providing astronomers with information concerning their
atmospheric temperature gradients and compositions. These characteristics
cannot be observed from Earthbound telescopes under normal conditions due to
the small volume of these atmospheres, but by precisely measuring the position
of the two occulting objects, astronomers can predict when the atmosphere alone
crosses in front of the star, providing us with the means to study these
characteristics.
Observations of Pluto occultations have proved particularly useful since the
first observed event of this kind in 1988. This occultation led to the
discovery of a thin atmosphere with sharp temperature variations that require
more occultation observations to be fully understood. Our work this summer will
allow for greater precision in predicting when and where these events will
occur over the next ten years.
With Lowell Observatory's astrograph, we used the method of CCD strip scan
imaging to take data that would later be used for astrometric predictions. With
this method, we fixed the declination of the CCD camera while still allowing it
to track with the earth's rotation in right ascension. Because any object's
right ascension varies directly with time, this configuration makes it easier
to compare star positions to published astrometric data. We took long exposures
with this setup, thus obtaining long "strips" of stellar images. Later,
astronomers at PAL correlated the stars in the frame with stars in previously
published catalogues and measured the differences between our observed
positions and the published positions, thus improving the accuracy with which
we can specify the locations of the stars in the strips. The new star positions
are compared with the predicted path of Pluto (information available from the
Jet Propulsion Laboratory's ephemeris data) in order to find which stars will
be occulted by the planet and which parts of the planet will cover the star at
which times.
3 Methods
Because we were primarily observers, most of our time was dedicated to using the
astrograph rather than reducing the data; hence, this section will focus on the
nightly procedures we used to obtain our images.
The astrograph's CCD is cooled by liquid Nitrogen (LN2), which needs to be
re-filled every night before observing. We filled the camera using a small
10-liter dewar, which we refilled every other night in Lowell's lab using a
much larger LN2 tank. If we did not refill the dewar more than twelve hours
before observing, the LN2 did not have time to pressurize, leaving us with no
way to transfer LN2 from the dewar to the camera. Under these circumstances, we
used an alternative transfer method in which a
nalgene\textsuperscript{\textregistered} bottle took the place of the dewar.
After cooling the CCD to about -100 degrees Celsius, we focused the CCD. We
plugged the focuser into the camera, and then used the camera control system,
LOIS, to specify the star to focus on, the length of the test exposure, the
first focus value, the increments of the focus change, and the number of focus
values to try. We visually examined each test image, and then compared our
opinion of which was the best focus value to what LOIS calculated. Often these
values matched, and we would move the camera to this final position. If our
visual inspection did not agree with LOIS, we took a few more test images
before making our final decision. Finally, we unplugged the focus control from
the CCD, as this was found to reduce noise in the images.
The astrograph has a range of only 4 degrees in declination, so before slewing
to a given field more than 4 degrees away from our previous position, we needed
to unclamp the telescope's declination arm and reposition it manually. While
the dec arm is unclamped, the telescope control software, Move, looses the
equatorial position of the telescope, so it needs to be realigned on a star
from one of Move's stellar catalogues. We used the planetarium software
Stellarium to choose a star visible to the naked eye that was within 4 degrees
in declination from our desired field, and then manually centered that star in
the finder scope attached to the side of the astrograph. After re-clamping the
declination arm, we used the camera control system LOIS and image viewing
program DS9 to further center the star on the CCD chip. After centering, we
inputted the equatorial coordinates of the star into Move, which we could then
use to slew to our final field. We repeated this process to center on a check
star in our target field.
After slewing to our field, cooling and focusing the CCD, and setting up the
proper file structures, we were ready to take strips. A strip is preceded by
three stare frames to further center the image. We viewed each of these images
separately, and checked that LOIS was centering the correct star on the correct
coordinates. If this was the first strip taken of a given occultation field, we
would choose a calibration star that appeared towards the beginning of the
strip and note its coordinates in the frame.
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This process was repeated around 2:00am almost every morning in order to observe
the field for the KBO 55636 occultation.
For each new field that we observed, we assembled information about the setup
frame and the strip in a binder to use for future reference. In each section,
we included an information sheet, a setup field image created by USNO, a setup
field image from our CCD, and the portion of the strip that contained the
reference star (Figure 1). In the information sheet, we included the J2000
coordinates of the setup field, the x,y location of the setup star and strip
reference star, and the number of milliseconds per row, number of rows, and
time length of the strip.
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Figure 1
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4 Conclusions
The data that we took with Lowell Observatory's astrograph will be used to
improve predictions of the occultation path of the KBO 55636, which will take
place on October 9th, 2009, allowing them to better place their observers for
both the KBO occultation and Pluto occultations to come. Figure 2 shows a
summary of our data.
5 Acknowledgments
I would like to thank Amanda Bosh, Carlos Zuluaga, Jim Elliot, and Len Bright
for their generous time and support this summer.
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Figure 2
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