Calculation of residuals of asteroid positions

This page provides an interface to a program which calculates residuals (differences between observations and calculated positions) for asteroid astrometry. The input form below accepts a file with observations in MPC format, makes the necessary calculations and presents results in a form of tables and graphs.

From 13 November 2005, the asteroid server was changed to recognize and to use also asteroid numbers higher or equal to 100000. For the time being, MPCORB database is used instead of the usual Lowell database.

This service uses several programs and information sources developed by different people. The asteroid database used is computed at the Lowell Observatory by E. Bowell. Propagation of asteroid positions is done by a program called OrbFit, part of a NEO information tool NEODyS. It is developed by the Orbfit consortium. Identification of asteroids with missing or user-assigned designations is made by the asteroid_server, developed by J. Skvarc.

How it works: After you upload the file, a script is invoked at the server side which reads the observations line by line. Only lines of exactly 80 characters are taken into account, others are ignored. The script checks whether an asteroid with a given designation exists in the database. If not, or if no designation is given, the script tries to find whether some asteroid exists within a certain distance (see the form below) from the observed position. The calculation of these positions is based on osculating elements near the current epoch, so the identification may fail for old measurements. It is therefore better to submit observations with correct numbers or designations albeit it is not obligatory. However, if you send incorrectly identified observations, the residuals will be both big and incorrect. Recently discovered asteroids will not be in the database because it is updated on a monthly basis. If you submit new observations for poorly observed asteroids before you send them to the MPC, the resulting residuals may be greater than real because your observations have not yet been used for the orbit determination. It is better to use this service for a review of observations already submitted to the MPC. Future version of this page may include an option to calculate a new orbit on the basis of the submitted observations. For well observed asteroids, especially numbered asteroids from the main belt it should not make any difference if the observations are used for orbit fitting or not.

After all of the measurements have assigned designations, a OrbFit is invoked for each observation and accurate positions are calculated. The difference between the observed and the calculated positions is used for creating graphs and tables.

The resulting page The calculated residuals are used to present the following information:

An example of residuals of all observations made at the Crni Vrh Observatory (106) since 1997 and in 1999 alone.

How to use this form It is very simple. First prepare observations you want to analyze and merge them into a single file. Then select the reslimit. This value separates observations into a group of normal observations which have certain scatter due to different reasons and a group of "big mistakes". Big mistakes may be due to wrong observation times, wrong identifications, typing errors and similar reasons. Observation with residuals larger than reslimit are not included in the statistics. The default value is 10 arcseconds what should be convenient for most cases. Another use of the reslimit value is in the process of the asteroid identification when only user-assigned designation is provided. The program assumes that the nearest asteroid within the reslimit distance from the observed position is the correct one. This may not always be the truth, but in most cases it is. For unidentified observations older than few month you may increase reslimit up to 300 arc seconds.

Some warnings Calculation of residuals takes quite some time and you should be patient. To entertain you while you wait, you will see on the resulting page which observation is being handled at the moment. After that you will click a link to the result page. Although efforts have been made to avoid mistakes, some of the results you get may be wrong for various reasons. If you submit unidentified observations made several years ago, chances are they will not be identified correctly because the identifier routine does not propagate orbital elements and only uses the current ones. If you get residuals of order hundreds or thousands arc seconds, this may mean that the designation has been incorrectly interpreted or that the orbit computation failed for some reason. Also, if you identified an object incorrectly or used the same designation for different objects, you will get incorrect results. Please report such cases to the email address below.

Reslimit arc sec To software index