Asteroid Lightcurves 

Introduction

Currently, the primary work at Hunters Hill Observatory is to determine the lightcurve parameters of as many asteroids as possible. This gross survey means that any given asteroid is worked only until a reasonably accurate set of parameters (±0.005-0.01h and 0.02m) is determined for a given lunation.  Occasionally, more detailed work is performed at the request of other researches who are attempting to refine or remove ambiguity from previously established parameters.

The average main belt asteroid looks something like a potato so during each rotational period, it shows two elongated sides to us, and two shortened ends towards us.   During those times, the light curve will show two bright peaks, and two dim troughs.  This average asteroid will rotate at least once in an eight-hour period, allowing you to see most of the curve with one night's data.  Others, can be much more difficult.  Some of these asteroids will take several days or even months to rotate just once.  That means you need night after night of data to build the light curve.   Even worse, is when the asteroid’s rotational period is close to 24 hours.  Here the asteroid will show nearly the same face to you every evening making it difficult to complete the entire light curve.  In these cases, a collaborative effort with observatories spaced around the world is required.

Equipment

The 36cm LX-200GPS SCT using the Starlight Xpress MX716 with f/3.3 Focal Reducer with extension to provide a focal ratio of f/2.95 is used for all imaging runs.    The observatory was upgraded in 2005 with the assistance of the 2005 Gene Shoemaker NEO Grant (c/- The Planetary Society) with an SBIG ST-8E camera and CFW-8 Filter Wheel).  Occasionally runs are made with V, R and /or I filters to provide colour information and in the future, basic taxonomy calculations.  The camera is run at 30°c below ambient so I need to take darks for each 5°c drop or rise in ambient temperature and apply them to images taken within 2.5°c either side of them.  During winter, overnight temperatures consistently fall to below -5°c resulting in good photometric precision during a given session.

The telescope and camera are controlled by a 1GHz Athlon PC with 768mb RAM running Windows 2000 Professional. This PC is located in the observatory and connected to the house LAN 15 metres away via a 100mb/sec HUB.  Inside the residence I can control the observing session from my laptop computer.  

Exposures

Exposures with the 0.36m SCT range from 90s to 300s, depending on the brightness of the target asteroid and how fast it is moving.   For the 0.36m, when working asteroids m13 or brighter, an exposure of 40s to 60s is used.   Darks and flats are taken and images calibrated in post processing. 

Software

Custom software (MPO Canopus)  written at the Palmer Divide Observatory is used for photometric measurements and lightcurve determination.   Images are captured using MaxIm DL/CCD and ACP4.  Astrometric reduction is performed by Astrometrica while one off photometric results are run through AIP4WIN, specifically for my variable star observations.  Observation planning is performed by AstroPlanner and Telescope control is via ACP4. 

General Program Description

Asteroids are chosen by first determining which targets within range of the equipment are near opposition and favourably placed for lengthy overnight runs.  MPO's Asteroid Viewing Guide software is used for this.  In addition a list of targets are maintained on the CALL web site maintained by Brian Warner.  This initial list is then edited by eliminating asteroids for which there are well-known lightcurves. The source for this information is from Harris (1999).

 My LAN is then connected to the Web and a time signal sent to all PC's to synchronise them.  (Although the Desktop PC's do maintain good time, the Laptop can often be off by as much as 8 seconds a day).  Finally, the scope is turned to the target asteroid.   After confirming focus and pointing, the software is set to take images continuously.  This generally entails 120sec guided images that are spaced at roughly 125 sec centres (given the time to download the image from the camera and save it to file).   Periodic checks are made for the initial hours but, having to work during the day for a living, I eventually head to bed, letting the scope, camera, and software accumulate anywhere from 150-270 images of the asteroid overnight.  The objects set time on the observatory horizon is calculated and used to set my alarm to wake me to finish observing and shut down the observatory. 

 When I close up the observatory and I transfer the images from the observatory computer to the one with the photometry program (Laptop) so that they can be processed during my lunchbreak at work that same day. 

Data Reduction

Data reduction is done in Canopus, a program produced by Brian Warner at the Palmer Divide Observatory.   For each image, the following information is extracted and stored in a database:

• UT Date/Time of mid-exposure
• The instrumental magnitude of the asteroid and comparison stars
• The average magnitude of the comparison stars
• The Comparison - Asteroid magnitude

Differential photometry techniques are applied to the data reduction.  Several comparisons are used (two minimum, usually three to five) to provide additional stability to the average value of the comparisons and to assure that there will be at least one comparison, preferably five, that are not variable.  Canopus, since version 9, also has a new feature called StarBGone.  This feature allows the user to remove field stars that coincide with the moving target and allow the software to still measure the targets magnitude.

In addition, the distance of the asteroid from earth and its predicted magnitude are kept as part of a larger record associated with all the data for a given night's run.  These are used to determine the corrections required for phase angle differences and light-time corrections.  The mean value of all the averages for the comparisons is also stored.  This can be adjusted per session so that all data is eventually referenced to a common, but arbitrary, zero-point, i.e., the comparison value used for all data points, even over several nights is the same.

Period determination is accomplished using a routine based on a Fourier Analysis of the data, allowing different parameters such as number of harmonics, period, size of period steps, etc. to be held constant while others are varied.  This routine is included in the Canopus software.  Finally, a plot of the raw data or phased (all data merged into a single cycle from 0 to 100% of the derived period) is generated. This plot can be saved as a Windows BMP for reproduction and manipulation at a later time.

● Email Me: higginsdj at bigpond dot com ●