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The Classification

The command CLASSIFY/INV performs a classification of listed objects on stars, galaxies, members of a broad class of defects, and unclassified--mostly very faint--objects. Only the table produced by the ANALYSE/INV command can be used as input for the CLASSIFY/INV command. An additional column containing the result of the classification will be appended to this table. The user can have some influence on the resulting classification by setting few keywords. In some cases it is known beforehand what kind of objects predominate at the faintest magnitudes where accurate classification is impossible. Proper setting of keywords CLASSPAR and DISTANCE can accomplish subjective segregation of the faintest objects into stars and galaxies. The results of classification are reliable only for objects that are considerably brighter than the limiting magnitude. The following additional keywords can be set with the help of the SET/INV C command:
-- This keyword controls the division of objects into ``bright'' and ``faint''. Different algorithms are used for classifying these two groups. The value of this keyword is expressed in magnitudes and should be fainter than the saturation limit.
-- This keyword controls convergence of an iterative classification process. Usually the value of 30.0 is sufficiently good. It may be changed when the iterations fail to converge.
-- Sets conditions for terminating iterations. Each iteration usually changes the classification of some objects. The number of such changes is evaluated and compared with the first value of this keyword, and iterations are terminated when the number of changes is not larger than the value of the keyword.

The second value of this keyword gives the allowed number of iterations. Recommended values are 0,20. It happens sometimes that iterations converge initially into a proper solution, but fail to stop and begin to diverge at an accelerating pace. It is good in such cases to increase the first component and to decrease the second value of the keyword ITERATE.

-- This keyword controls the selection of ``seed objects'', which teach the program how typical stars and galaxies look. This selection is done on the isophotal magnitude versus the relative gradient diagram. There are three parameters. The first sets a half-width of a band centered on relative gradient =0.0, and extends from bright objects down to a limit set by the third parameter, which gives the difference of magnitudes between this limit and the detection limit. The second parameter sets an upper limit of the relative gradient for selecting a seed galaxy. Recommended values are 0.05, -0.15, 1.0.

next up previous contents
Next: Description of INVENTORY Keywords Up: Procedures to Follow Previous: In Case of Trouble
Petra Nass