centroid -- Compute average rectangular coordinates and observation time.
centroid input intable outtable output
This task reads the encoder positions from an FGS file, computes the average rectangular coordinates and time of observation, creates an output table, and puts the average positions and other information into that table. Centroid defines the columns used by subsequent tasks in the FGS package; some of these columns are filled in with preliminary values from the data file header, while other columns are left with null values. After rejecting outliers, the average rectangular coordinates are derived by fitting a low-order curve by a least squares method, and evaluating the curve at the average time. The curve may be either a quadratic, a linear fit, or just the mean value; the same degree of fit is used for both X vs. time and Y vs. time.
If an input FGS file contains a transfer function (AASTMODE = TRANSFER) then a different type of output table will be produced. Several parameters will be written to the table header rather than to columns in the first row. The data will be written to four columns (X & Y positions, normalized fine error signal for X and for Y) and as many rows as are required. Data flagged as bad in the input file will not be copied to output (i.e., all table entries will be valid, none INDEF), so the X and Y positions may be unequally spaced. Centroid does not identify and reject outliers from transfer-function data. Only 32-Kbps transfer-function data may be processed.
TIME Modified Julian Date of the average time of observation X direction cosine Y direction cosine X_X variance of X (mean of squared deviations) X_Y covariance of X and Y Y_Y variance of Y DXDT slope of fitted curve of X vs time (sec**-1) DXDT2 curvature of fitted curve for X (sec**-2) DYDT slope of fitted curve of Y vs time (sec**-1) DYDT2 curvature of fitted curve for Y (sec**-2) NUMPTC number of points used in computing averages
The columns for transfer-function data are:
X X coordinate relative to reference position FESX normalized fine error signal in the X coordinate Y Y coordinate relative to reference position FESY normalized fine error signal in the Y coordinateThe reference position is an (X,Y) pair along the scan, and the values of X & Y are given by the table header parameters REF_X and REF_Y respectively.
No output data file is produced if the input data are for a transfer function.
This specifies whether the actual encoder positions or the average encoder positions should be used to determine the coordinates. This must agree with whatever times were computed by puttime. If you do not set choice="default", you had better know what you are doing. Choice is ignored for transfer-function data.
1. Compute centroided positions for the file f1m.a1h.
fg>centroid f1m.a1h cdbs.tab f1m.tab resid_f1m.hhh