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Exoplanet Transits Kyle Meyers
Science, Discovery, and the Universe
Explore the Universe, UMD Observatory
Introduction
Methods
Results
1. Cool CCD down to -15°C. (-5°C in summer) 2. Allow CCD to reach thermal equilibrium; frost dissipates
from the field of view 3. Decide on target and prepare sky charts 4. Open Maxim-DL software 5. Connect camera to telescope 6. Take flat field images (dome or sky) 7. Go to star nearby target and synchronize on it 8. Input right ascension and declination of target 9. Find the target in the frame 10. Take test images to check suitable exposure 11. Prepare a sequence, which includes the dark and light
frames, needed for calibration 12. Ensure telescope never loses target 13. Import all images into AstroImageJ software. 14. Create master bias, dark and flat field frames to be
applied to each image to calibrate them. 15. Use AstroImageJ to select multiple apertures for target
and reference stars. 16. Plot the results of the multiple aperture photometry
technique. 17. Stack target graph with reference, relative flux versus
time.
For my capstone I did the Explore the Universe project through the UMD Observatory. I worked together with one other UMD student, and a high school senior to observe exoplanets, which are planets that orbit other stars. We conducted research on these stars using a technique called differential photometry so that we could experimentally prove the existence of the exoplanets, by the measuring the dip of the light intensity as the exoplanet transits between us on Earth and the star of interest. In the past, the Explore the Universe was structured as a service learning program in which we helped a high school student along with a science fair project. However, it was restructured this year as a research/internship experience. As opposed to past years, we had full control of the telescopes during our observation sessions, and learned all of the required steps to taking astronomy images. We also had the freedom to select our subject of interest, ours being exoplanet transits. I will elaborate on some of the techniques because they are not intuitive. While we collected the raw light frames (images of the target star), we had to complete some noise reduction techniques in order to make our data more reliable. This involved taking bias frames, which are “zero second” exposures, and dark frames, which don’t expose the aperture of the camera to light and reveal thermal inconsistencies across the CCD chip. The final noise cancelling image used is a flat-field, in which a uniform subject is photographed. The pixels of the noise image can be subtracted from the raw image. This is done for the flat as well. The pixel value for the noise reduced flat is divided out of the raw frame, normalizing it. There are examples of bias and flat field frames to the right. A master flat, bias, and dark are made from a collection of each, and applied as follows:
𝐶𝑎𝑙𝑖𝑏𝑟𝑎𝑡𝑒𝑑 𝐼𝑚𝑎𝑔𝑒 =(𝑟𝑎𝑤 − 𝑏𝑖𝑎𝑠 − 𝑑𝑎𝑟𝑘)
(𝑟𝑎𝑤 𝑓𝑙𝑎𝑡 − 𝑏𝑖𝑎𝑠 − 𝑑𝑎𝑟𝑘 𝑓𝑙𝑎𝑡)
The light curve below runs from the beginning of an exoplanet transit of the star HAT-P-20 through the end, with the back end of the curves occurring after the exoplanet completely passed the plane of Earth and the target star. The blue curve is for HAT-P-20, while the rest of the curves are reference stars. To create this graph, we took all of the light frames into AstroImageJ. To process the data, we applied the noise reduction techniques and then took these calibrated light frames and plotted them against time. The software then prompted us to select an aperture for the target star and reference stars. The apertures measure all the photometry data (light) and the program tracks the stars through the entirety of the imported image sequence. AstroImageJ allows us to graph the relative flux of the star over time with respect to the other reference stars. In practice, it seems that selecting more apparently visible stars as references in the frame helps get a clear curve for the target star with its exoplanet. Each star does show minor variation over time, with the blue curve for HAT-P-20 having a strong dip that corresponds to the exoplanet blocking the light of the star. In addition to HAT-P-20, we observed other exoplanet transits, like those orbiting COROT-7 and HAT-P-24. Further research would continue to improve, as taking flat-fields, obtaining the right field of view, and processing the data correctly are all skills that are developed over time.
I would like to thank Elizabeth Warner, the UMD Observatory coordinator and our mentor, as well as the SDU staff for giving feedback on the project along the way and keeping us at a steady pace during the capstone.
Acknowledgments
(Above) Flat Field Image (Below) Bias
Light Curve for HAT-P-20
Photos of the UMD Observatory
Aperture
Calibrated Image for P-HAT-20
