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.

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.

Figure 1
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.
Figure 2