ASTPLOT builds a plot that depicts the position of stars and asteroids within a defined region, typically the field-of-view of a telescope. The star positions derive from the USNO-A2.0 and PPM catalogs. The asteroid positions are supplied from the Lowell Observatory asteroid catalog and from orbits published in the Minor Planet Electronic Circulars.
Cookies are used to store default values for the user. Defaults are stored for the following parameters:
Users should not select dates outside the range 1890 to 2020. Asteroid positions are calculated using full planetary perturbations. All positions are ploted at equinox J2000.0.
The current UTC time is automatically supplied. You may change the time as you wish, but please use the same format as the default values. The location is specified as RA and Decl in sexagesimal. Shortcuts will not work. Please supply RA in "hh mm ss.s" format and Decl "[+/-]dd mm ss" format.
The limiting V magnitude is the faintest V magnitude your instrument can image. No asteroids or stars computed to be fainter than this limit will be presented in the plot. As the magnitudes of many asteroids are not well known, you should be aggressive with this parameter, say by adding as much as a magnitude to what you think your telescope can detect.
This selection parameter eliminates all asteroids whose probability of imaging is less than the specified value. The probability of imaging depends on your FOV and on the ephemeris uncertainty, expressed in standard deviations, or sigma, of the asteroid.
Sigma is the standard deviation described in all statistics texts. In the context of asteroid orbits, the position of the asteroid is uncertain because the orbit depends on astrometric measurements that contain errors. Thus, the orbit calculation, almost always a least-squares fit, will reflect some of the measurement errors. Almost all the uncertainty lies along the line of variation (LOV). the orbital arc of the asteroid as seen from Earth.
Because we know where the LOV crosses your FOV, we can use Gaussian statistics to calculate the probability that you will see the asteroid. For example, if we can see one sigma on either side of the nominal position of an asteroid, the probability of finding the asteroid is 68%. If we can see three sigma on either side of the asteroid, the probability becomes 99.7%
This selection criterion may produce unexpected results. Suppose an asteroid with a one-sigma ephemeris uncertainty of one arcmin and a horizontal LOV is just off the FOV in the RA direction. You have nearly a 50% chance of seeing the asteroid, but if you select an MPI of 0.5, the asteroid will just miss the cutoff and not be selected. Even more unexpected is when the nominal position of the asteroid is in the corner of the FOV and the LOV is at such an angle that only a small segment of the LOV is on the FOV. This asteroid will not be selected because it is more likely the asteroid will be off the image than on it.
In order to overcome these unusual selection problems, the user can set the value of the MPI to a small value, say 1.0%. Unfortunately, this action causes some other unexpected behavior. Specifically, asteroids with nominal positions well off the FOV can be selected. Their uncertainties and motion vectors are plotted on the graph but the nominal position and asteroid identification are not. To help with this problem, the software provides a list of all of asteroids selected, even if they are not plotted and labeled on the plot.
The software assumes a rectangular FOV aligned in the RA and Decl directions, and requests both a width and height of the FOV. Note that the units are in arcseconds. A circular FOV can be handled by entering the FOV diameter in both the width and height field. Then the user should ignore the corners of the plot.
The legend shows the dot size used to indicate magnitudes. If the legend is not important, it can be suppressed. If it is not suppressed, asteroids can be hidden by the legend box.
The motion vector can indicate either 24 hour motion or 1 hour motion. Typically, 24 hour motion is good for large-FOV instruments and 1 hour motion is good for small-FOV instruments and/or fast-moving asteroids. The motion vector can also be suppressed.
The ephemeris uncertainty can be displayed as either a 3-sigma or 1-sigma dashed line along the LOV. Most asteroids with uncertainties small enough to be encompassed in the FOV of a telescope will be found very near the LOV. The 1-sigma uncertainty indicates the region in which the asteroid is expected to be found 68% of the time. Three-sigma uncertainty indicates where the asteroid is expected to be found 99.7% of the time. The uncertainty indicator can be suppressed.
However, note the following:
If the ephemeris uncertainty is zero, the ephemeris was derived from orbital elements supplied by the Minor Planet center. We do not have the covariance matrices for these orbital elements, so we cannot calculate the ephemeris uncertainty. In these cases, however, you may be sure that the ephemeris uncertainty is small (less than tens of arcseconds).
All asteroid markers (asteroid identification, motion vector, ephemeris uncertainty indicator, circular asteroid locator) can be made black, transparent (light gray), or invisible. Usually transparent is the best choice because stars can be seen through the markers.
The plot is centered at the position (time and space) specified by the user. A small circle surrounds each asteroid's position (marked by a dot). A dashed line indicates the user-specified 1-sigma or 3-sigma uncertainty in the asteroid's position and an arrow represents the user-specified 1-day or 1-hour change in position of the asteroid (assumed linear). The asteroid's name is indicated near its nominal position. The observatory name, time of observation, and sky position are indicated at the top of the page. The radius of a plotted star/asteroid increases with brightness. All selected stars/asteroids fainter than 20th magnitude are plotted as 20th magnitude. North is plotted to the top of the plot and East is plotted to the left. Expect execution to take at least one minute; it may be longer if our server is busy.
Plots are produced in three formats: PostScript, compressed PostScript and JPG. The JPG plot is automatically displayed. The PostScript formats provide a higher level of detail than JPG but they require a PostScript viewer or printer. A PostScript viewer, ghostview, is available via ftp from prep.ai.mit.edu/pub/gnu/ghostview/ghostview-1.5.tar.gz. (Unfortunately, ghostview is not in executable format so it also requires "tar", "gzip", "make", and a C compiler for you to build it.) The compressed version is about 20% as large as the uncompressed, so is valuable for those with slow links. The user must have "gzip" to uncompress the file. gzip is available from prep.ai.mit.edu/pub/gnu/gzip/gzip-1.2.4.msdos.exe.
Some problems in the USNO-A2.0 catalog are apparent in the plots. These include
© Lowell Observatory 2008
Web Curators: Ted Bowell & Bruce Koehn
|Last modified: 2008 April 29|