High Precision Astrometry of Occultation Asteroids Allan Alonzo-Ault Stephanie Toole Dr. William M. Owen, Jr. - Mentor Jet Propulsion Laboratory Table Mountain Observatory Purpose ● Image Main Belt and Trojan Asteroids ● Update Ephemeris ● Predict Occultations ● Characterize Asteroids Ephemeris A table or data file giving the calculated positions of a celestial object at regular intervals throughout a period. Source: Oxford Dictionaries Occultation The interruption of the light from a celestial body or of the signals from a spacecraft by the intervention of a celestial body; especially : an eclipse of a star or planet by the moon. (Asteroids in our case) Source: Merriam-Webster Dictionary Occultation Event ● Antiope, a binary asteroid in the outer main belt! ● Each colored line is a separate observation. ● Image consists of approximately 50 different observations of event. ● Helps determine shape and size of asteroid. On Location: Table Mountain Observatory ● Wrightwood, CA ● Elevation 7500 ft. ● Est. 1925 by Smithsonian Inst. ● JPL arrived 1961 ● Atmospheric Monitoring ● Optical Communication ● Asteroid Tracking Every Telescope Needs A Home ● TM-12 ● Temperature Controlled ● Remote Access is Possible from JPL Eyes on the Prize ● Astro ● f/16 Mechanics ● German ● 0.6 m Equatorial ● Ritchey - Mount Chrétien ● 9500mm reflector Focal Length ● Installed in 1966 Your Humble Observers A Picture Is Worth 16.8 Million Pixels ● ProLine PL16803 ● 4096 x 4096 Pixels ● 9µm Pixel Size ● 36.8 x 36.8 mm Sensor ● Monochrome ● Thermo-Electric Cooling to 55℃ below ambient Predicting our Targets ● ● Prior to arrival at TMO ● Trajectory Geometry Program (TGP) ● GhostView ● Three updates per night to pinpoint target location near local meridian. ● Criteria: ○ Declination angle > -30° ○ The more stars the better! Our Workspace ● Programs to control the dome, telescope and camera. Focus On The Task Ahead ● Use the Position Client to choose a star. ● Astrometric Catalog lists stars near the Zenith to minimize atmospheric distortion. ● Choose the faintest star on the list. FOCUS! ● 10 second exposures of the star. ● Goal is to obtain the sharpest light profile of the star. ● Once satisfied with focus, center the star and calibrate telescope. Controlling The Telescope ● Position Client ● Telescope Control Program ● Paddle Client ● RA / Dec of ● Moves Telescope to the ● Manually Target Desired Position Adjust for Calibration Imaging ● 2 to 3 images of each target, depending on star field. ● Offset in different directions to capture different background stars. ● 3 minute exposures. Calibration Field ● M11: The Wild Duck Cluster. ● Done each night of imaging. ● 5 different offsets, one centered, one towards each corner. ● Used in data reduction to calibrate position of images. Captain’s Log ● Point file keeps track of RA and Dec of every image taken. ● Temp file keeps track of Temperature, Pressure and Humidity at the time each target is imaged. ● TMO Weather Station Data. Challenges to Making Good Observations ● Clouds ● Turbulent Atmosphere ● High Humidity ● Telescope Out of Focus ● Smoke!!! Mistakes Were Made ● Telescope cover closed. ● Targets observed on the wrong night. ● Pointing errors. Data Reduction We image ~50 targets per night, but only publish one line of data per target. Least Squares Method We solve directly for changes in RA and Dec of the telescope and the stars. We also solve for a rotation angle, the focal length and the aspect ratio of the pixels. Then we add in the 2nd through 5th order terms using products of Legendre polynomials. Why Legendre? They have the nice property that if the stars are uniformly distributed, the solution for each one will not be statistically correlated with the others. Put another way, they're orthonormal: if you integrate the product of two of them over [-1, +1] you get zero if they're of different order. Why Reduce? ● it turns the pixels into RA and declination in the sky ● Gives us more information about the image ● Catches errors Mostly Autonomous Data Reduction ● The linux based command scripts that Dr. Owen wrote for the data reduction are extremely powerful ● They can almost autonomously reduce the data ● We step in to fix our own errors, help with centroiding for faint targets, and delete the “bad stuff” Two (or Three) is Better Than One For our data reduction technique to work, we need at least two different images of the target. Why? ● By taking two or three images of our target with different pointing, you get information about the telescope distortions by seeing how star patterns change in each image Calibration Field We used the A calibration M11, Wild field tells us Duck Cluster, how the as our telescope and calibration field camera are because it behaving that contains about night. 2900 stars. The point file records where the telescope is pointed for each image. The temp file records the temperature, barometric pressure, and humidity. doit This script calls on four other scripts: Prepare Amptemp Centroid Reduce reduce TGP (trajectory geometry program): provides coordinates for predicted stars or the target asteroid AMP: list of the observations with identified targets and with positions for all the targets, catalogued and uncatalogued stars found AOPG and ADAP: calculates expected (x,y) coordinates of images which are subtracted from observed (x,y) coordinates, which lead to the residuals. Calculates partial derivatives of the coordinates using the variables for determining (x,y). ADAP processes this and makes summary files, doing mathematical calculations. Check ● Checks to see if the number of targets and pictures equals the number of targets located in the images Xrover ● If a target is not found we use a program called Xrover to manually locate the target findbad ● Supplies a list of images with “bad residuals” ● A bad residual is anything bigger than one pixel ADAP ● We delete the stars with bad residuals by commenting them out in their input file ● The input file is read by ADAP (Astrometric Data Analysis Program) Reduce, rinse, repeat We continue with the same steps until arriving here. deliver This script condenses the summary files as produced by the adap script. A file is created in emacs that we edit to credit observers and measurers. This file gets sent to the Minor Planet Center, to a few people in the occultation community and to a few people at JPL whose job is to update the orbits of the planets and their satellites. cleanup Problems and Issues ● Xrover does not like number lock, and we do not know why ● Adap files do not like being wider than 80 characters, because that is how many characters can fit on an IBM punch card ● You must be in the correct directory ● Emacs is the text editor we use, if you search for a file in the wrong directory it will not tell you that you are wrong Our Results ● 678 images taken. ● 145 unique objects imaged. ● 519 positions obtained. ● 7 successful deliveries to the Minor Planets Center. Not Just Asteroids ● Updated the positions of 6 of Saturn’s satellites. ● Titan, Rhea, Tethys, Hyperion, Dione and Iapetus. ● Failed to do the same for Mars’ satellites, Phobos and Deimos. ● Attempted on 3 different nights near opposition. Failed Occultation Event ● Asteroid 1015 Christa ● 60% probability of TMO being in the occultation path. ● No success this night. ● Friday July 13, 2018! Acknowledgements This project was completed at NASA’s Jet Propulsion Laboratory in Pasadena, CA and Table Mountain Observatory in Wrightwood, CA. This work was supported by NSF grant #AST-1460538 to Los Angeles City College. We would like to thank our mentor at JPL, Dr. William M. Owen Jr., CURE Coordinators Paul McCudden and Derrick Kiley at Los Angeles City College, JPL Program Coordinator Roslyn Soto, Heath Rhoades and the entire support staff at Table Mountain Observatory..