Introduction to comet astrometry
Your images can help pinpoint comets and trace their paths through the Solar System
Astrometry is the science of determining the precise positions and movements of astronomical bodies. This is especially important for small Solar System bodies such as comets where such measurements help refine the object’s orbit.
The high accuracy required for the measurements places certain demands on your images. The worldwide clearing house for submission of positional comet measurements is the International Astronomical Union’s Minor Planet Center (MPC), which maintains a detailed Guide to Minor Body Astrometry at www. minorplanetcenter. net/iau/info/ astrometry.html. Comet astrometry is normally done using a CCD camera and an image scale of two arcseconds per pixel. A scale of three arcseconds per pixel is considered the lower limit of quality. You can work out your setup’s image scale using this formula: Image scale (in arcseconds/pixel) = (206.3 x pixel size in microns) / focal length in millimetres. To carry out astrometry on a comet requires the reduction of an image containing the comet. This entails identifying the star field (a process known as ‘plate-solving’), determining the position of as many stars as possible from permitted catalogues and finally working out the position of the comet using a leastsquares plate-constants (LSPC) solution. This process must be done using software; manual reductions are not considered accurate enough.
Submissions to the MPC must follow strict formatting guidelines. Two submission forms are permitted: one for permanent observatories and one for roving observations. A permanent observatory will ultimately be given a three-digit MPC observatory code, while a roving observation will use the observatory code 247 along with extra data to precisely identify where the observation was made on Earth’s surface. There are full details on how to obtain a permanent observatory code from the MPC in the Guide to Minor Body Astrometry mentioned earlier.
The astrometry software listed under recommended software will produce the required MPC submission format automatically. Further information about permanent observatory submissions can be seen at minorplanetcenter.net/iau/ info/OpticalObs.html and for roving observations at minorplanetcenter.net/iau/ info/RovingObs.html.