The New AIPS Task FRMAP to Image Spectral Line Radio Sources from VLBI Data
Session 8 -- Radio Surveys and Techniques
Display presentation, Monday, 9, 1995, 9:20am - 6:30pm

## [8.09] The New AIPS Task FRMAP to Image Spectral Line Radio Sources from VLBI Data

L.R. Kogan (NRAO)

The fringe-rate mapping method uses the fact that the fringe rate offset $\omega_f$ for a given feature relatively to the reference feature is given by equation of a straight line on the sky along which the emmission corresponding to the measured $\omega_f$ must locate. If there is only one feature (in space and therefore in fringe rate spectrum) at the given velocity then having used measurement of $\omega_f$ for different baselines we can determine the feature position by finding the point of intersection of the lines.

If there is emission at the same velocity at several locations, then several fringe-rate peaks will be produced for each baseline. As a result we'll receive the set of parrallel line's groups. The positions of the features at this case can be found using criteria of maximum density of the lines crossing the given area.

The multiple-point fringe-rate mapping method has been realized in the new task FRMAP installed in NRAO A stronomical I mage P rocessing S ystem (AIPS ). The task works in wide range of baselines, time intervals, frequency channels. The new algorithm for fringe rates determination has been invented. This algorithm realizes the accuracy which the data allows, and eliminates the erroneous peaks in the fringe rate spectrum which appear for lack of data in some points of FFT intervals. According to the mapping algorithm the rectangular window is divided into many small rectangulars. The rectangulars crossed by the number of lines exceded the given number are selected. If the selected rectangulars organize clusters the program select the rectangular with maximum number of intersections. The picture of the straight lines is drawn on the screen and helps estimate the features positions visually.